[clangd] Implemented logging using Context

[sammccall]

This is pretty bikesheddy, but I wonder what you think about passing Ctx as the first vs last parameter.
First has some advantages (maybe I just read too much Go though):

• it's a short expr, and F(short, long) tends to format better than F(long, short). Particularly with lambdas but also long strings.
• it doesn't interfere with default args

It would be nice if we could be completely uniform here.

[ilya-biryukov]

• Updated to match the new Context implementation.
• Logger is now stored in a global variable instead of the global Context.
• Removed json-specific RequestContext, now storing all required information in Context.
• reply,replyError and call are freestanding functions now.

[ilya-biryukov]

Additionally:

• All async methods now take Context by-value and pass it to their Callbacks or return them in futures.
• All sync methods take Context by-ref

In D40486#941210, @sammccall wrote:

This is pretty bikesheddy, but I wonder what you think about passing Ctx as the first vs last parameter.
First has some advantages (maybe I just read too much Go though):

• it's a short expr, and F(short, long) tends to format better than F(long, short). Particularly with lambdas but also long strings.
• it doesn't interfere with default args

It would be nice if we could be completely uniform here.

To be fair it was just a random thought and I totally agree with your points. I'll update the implementation and place the Context as the first parameter everywhere instead.
Could you take a look at other changes in the patch while I'm at it?

[sammccall]

Mostly nits - throughout there's going to be lots of judgement calls about where to propagate context and where not.
No need to get all those "exactly right", but trying to get a feel for what these answers are likely to be.

Most of the interesting stuff is around the logger itself of course :-)

clangd/ClangdServer.cpp

30–31

Up to you, but I like the old name better - this class is local so it's OK to be a bit opinionated.

30–31

Yikes, I can see how we got here, but we really don't get to move out of something we received an lvalue-reference to...

I think a safer idiom here is turning a whole lambda into a destructor:

auto Fulfil = MakeDestructor([&]{
   DonePromise.set_value(std::move(Ctx));
});

I'm not sure if LLVM has a shared utility for this, I've seen it in other codebases. Either way we could just define it here.

(Or we could copy the context to avoid the awkwardness)

199

You dropped the end of this comment

200

I don't quite understand what this is saying should be better. Is it saying we do these two things together instead of running them in parallel?

And why not clone the context instead?

clangd/ClangdUnit.cpp

626

why are we dropping the context here?
It's important that in general we don't store the ctx in the object and blindly reuse it, but passing it into the constructor for use *in* the constructor seems fine.

Reading on... OK, it saves a *lot* of plumbing to avoid attributing this one API call. I still don't totally understand how the CppFile API/lifetime works, so up to you.

723

nit: this wrapping has become *really* weird over time - can we change the comment to /* allow changing OldPreamble */ to try to fix it?

clangd/ClangdUnit.h

257

I think this is a bad merge - signatureHelp has moved to CodeComplete.h

clangd/GlobalCompilationDatabase.h

37

Do we want Ctx in this interface?
It's an extension point, and the ability for embedders to track calls flowing between their API calls and their extension points is one of the reasons we have ctx...

clangd/JSONRPCDispatcher.h

29–31

does this class need to be public? Is it staying around for the long haul?

41

Is this temporary?

[sammccall]

clangd/JSONRPCDispatcher.cpp

22

Maybe we don't need the Key suffix, as it's in the type... we should spend our verbosity elsewhere

23

RequestTracer?

24

RequestID

48

why?

clangd/Logger.cpp

16

Any need to have an empty logger, rather than just have log() branch?

22

There are no non-global loggers. Just L, or Current, or something?

26

I actually think this would be clearer without the assert - this can statically never be null

clangd/Logger.h

29

why is this not called log()?

32–33

Why does this need to be public?

38–39

nit: these seem like low SNR. Are we actually worried about people trying to move sessions?

[sammccall]

clangd/JSONRPCDispatcher.cpp

23

actually, RequestSpan I think - "tracer" is pretty confusing at global scope

68

Depending on a span tracer existing seems needlessly brittle. What about

if (auto *Tracer = Ctx->get(TracerKey))
  SPAN_ATTACH(*Tracer, ...)

[ilya-biryukov]

• Pass Context by const-ref instead of by-ref.
• Don't expose global logger, it is only used in log method now.
• Renamed logImpl to log.

[ilya-biryukov]

Udpated the patch after changes in Context

[ilya-biryukov]

• Copy Context in forceReparse's call to scheduleCancelRebuild.
• Renamed Key<> for ID, Out and Span.
• Removed the FIXME
• Got rid of getExisting(RequestSpan)
• Remove EmptyLogger, branch in log instead.
• Renamed GlobalLogger to L
• Addressed review comments.

[ilya-biryukov]

clangd/ClangdServer.cpp

199

Missed that. Thanks.

200

The first implementation did not allow to copy the Context, hence the comment. I've removed it and the context is now copied.

clangd/ClangdUnit.cpp

626

Exactly, plumbing it here is hard, so we're dropping the context here for now. Most of the useful work is still done outside the constructor, so this seems fine.
The threading patch will make it easier to pass proper Context here. I've added a FIXME.

clangd/ClangdUnit.h

257

It is totally a bad merge. Thanks.

clangd/GlobalCompilationDatabase.h

37

I think we do, but I'd add this in a separate patch. (We're currently not changing the behavior, simply plumbing the Context through all the APIs).
Happy to come up with a follow-up patch, though.

clangd/JSONRPCDispatcher.cpp

48

This was a reminder to myself to discuss whether we want it globally in tracing+logging, not only in JSONOutput.
Removed it from the code.

68

Done.
Turned out to be SPAN(**Tracer, .... (Note the double dereference).

clangd/JSONRPCDispatcher.h

29–31

I'm not sure. I'd rather keep it private, but we don't have the right abstractions to hide it yet, right?
I'd say this is not directly related to the current patch and should be addressed separately.

41

Renamed back to log.
It's a leftover from an older version of the patch.

clangd/Logger.cpp

22

This is really just a preference. To me the global prefix is not about distinguishing it from other things in this file, but rather about signalling what is this thing so that I don't have to read the declaration before reading the usages to understand what's going on. Having L or Current requires to be in the context of the file.

Updated the code to use L instead.

clangd/Logger.h

32–33

It shouldn't be. Removed.

38–39

If they do, that will lead to a bug (destructor being called twice), so it feels safer to delete the corresponding constructors.

[ilya-biryukov]

• Remove mention of globalLogger() from the comments.

[ilya-biryukov]

• Use derive(key, val) instead of derive().add(key, val).

[ilya-biryukov]

clangd/ClangdServer.cpp

30–31

Thanks for suggestion. I totally agree, callbacks on scope exit are better.

I couldn't find an utility like that in LLVM, but I think it's a useful one. I'll add this locally to this file, but maybe we should make this helper public and put it into clangd?
What are you thoughts on that?

[sammccall]

In D40486#945193, @ilya-biryukov wrote:

I'll update the implementation and place the Context as the first parameter everywhere instead.
Could you take a look at other changes in the patch while I'm at it?

Just pinging this - there's still places where Context is in a different position (at least in ClangdServer)

clangd/ClangdServer.cpp

30–31

Sounds good. I'm wary of adding lots of tiny utility headers - maybe if we squint it could go in Function.h?

clangd/ProtocolHandlers.h

32–33

We should use one name or the other. Fine to do this to keep the patch small, but leave a FIXME?

[ilya-biryukov]

• Added a helper that runs functions in destructor.
• Fixed compilation after rebase.
• Got rid of FulfilContextPromiseGuard
• Added a FIXME to remove the Ctx typedef.

[ilya-biryukov]

I think all the comments are addressed now and this change is ready for another round of review.
Let me know if I missed something.

clangd/ClangdServer.cpp

30–31

Then Function.h it is :-)

clangd/ProtocolHandlers.h

32–33

Added a FIXME, will send a follow-up change that removes it after this change lands.

[ilya-biryukov]

• Make Context the first parameter everywhere.
• Add a SFINAE check to UniqueFunction's constructor to avoid creating it from non-callable objects.

[ilya-biryukov]

clangd/Function.h

44

We need this check to make ClangdServer::codeComplete overloads work. Without it, overloading thinks UniqueFunction can be constructed from any type, so we end up getting ambig between the two overloads when calling it in a following way: codeComplete(Ctx, File, Pos, CCOpts, OverridenContents).

[ilya-biryukov]

This is now ready for review.

[ilya-biryukov]

• Also check that return type of Callable is correct.

[ilya-biryukov]

• Make Context the first parameter in rename and a few helper methods.

[ilya-biryukov]

These should be the last usages where Context is not a first paramter.

• Make Context the first parameter in findDefinitions and CppFile::rebuild.
• Make Context the first parameter in invokeCodeCompletion.

[sammccall]

Thanks! Just style and comment nits left really.

clangd/ClangdServer.cpp

206–207

this is a pretty gross return type.
Even after (if?) we replace tagged with context, we'll still have completionlist and context.
Should we define a struct here?

(If not, we should consider having the context first, for consistency)

560–561

This wrapping has gone beyond the point of readability for me.
Pull out using RebuildAction = llvm::Optional<...>(const Context &)?

clangd/ClangdServer.h

252

Is this comment obsolete? (and others below)
Ctx is passed by value so I don't understand what it could mean.

259–264

(and this comment)

308

ctx to first param

332

ctx to first param

334–335

ctx to first param

clangd/ClangdUnit.cpp

616

(while here, this should really be make_shared)

clangd/ClangdUnit.h

175–176

ctx to first param

261–262

ctx to first param

clangd/CodeComplete.cpp

594

ctx to first param

clangd/Function.h

143

FWIW I think this assert adds more noise than value - this can clearly only be triggered by someone instantiating the template directly, which is probably better avoided using namespace detail or so.

157

I'm struggling to think of cases where moving these guards is the right solution.
Can we delete the move ops until we need them? It seems to make this class almost trivial.

clangd/GlobalCompilationDatabase.cpp

106–111

Add a fixme to get a context in here?

clangd/JSONRPCDispatcher.cpp

145

There are *lots* of Context::empty() calls in this file.
Maybe conceptually clearer for JSONRPCDispatcher to own a "background" context that it derives from? It'd be empty for now in either case, so not a big deal.

The idea being that explicit Context::empty() calls are a bit suspect, but these ones aren't really "different" from each other - we may want to provide an extension point to customize it, but there'd only be one.

Up to you.

148

I'm not sure this is better than unconditionally attaching the optional ID.
Being able to distinguish between request-with-no-ID and no-request seems like a good thing.

clangd/JSONRPCDispatcher.h

29–31

It's related in that context takes some of the job of JSONOutput, so now we have a messy overlap.
A FIXME to work out how JSONOutput's role should shrink in light of Context is enough, I think.

clangd/Protocol.h

24 ↗

(On Diff #126554)

I think tihs is unused?

unittests/clangd/ClangdTests.cpp

31

nit: revert?

[ilya-biryukov]

• Removed obsolete comments.
• Moved ScopeExitGuard to namespace detail.
• Added FIXMEs to pass context to compilation database.
• Addded a FIXME to JSONOutput.
• Removed unused #include directive.
• Revert accidental whitespace change.

[ilya-biryukov]

clangd/ClangdServer.cpp

206–207

I agree, callback-based APIs look way better though.
Since we'll eventually switch to callback-based versions anyway, I'm gonna leave it as pair for now.

560–561

I'd refrain from doing that as this code will go away in the threading patch anyway. Hope that's ok.

clangd/ClangdUnit.cpp

616

It can't, because CppFile's constructor is private. This code is also going away in the threading patch.

clangd/Function.h

143

Moved it to namespace detail.

157

We need moves as we return the class by value from the onScopeGuard function. Neither of these will compile without move constructors:

auto Guard = onScopeGuard([]() { /*...*/ });
auto Guard2(onScopeGuard([]() { /*...*/ }));
auto Guard3{onScopeGuard([]() { /*...*/ })};

clangd/JSONRPCDispatcher.cpp

145

Since these usages are easy to spot (almost all uses of Context::empty in JSONRPCDispatcher.cpp), I'd leave them as is for now. It's easy to update them later.

148

I like that we can avoid having a two-level unwrap for RequestID. If we're going down that route, maybe we should have a dedicated struct instead:

// name may be bad, just for the sake of example.
struct RequestData { 
  llvm::Optional<json::Expr> RequestID;
  Span RequestSpan;
  JSONOutput Out;
};

Key<RequestData> ReqData;

That would also make it clear that we either attach all of those fields or none of them.
Given that RequestID is not yet public, I'd keep it as is and change it in a separate patch.

[sammccall]

clangd/Function.h

157

Of course :-)
As you pointed out offline, making the member Optional<Func> would let you use the default move operators at least.

[ilya-biryukov]

• Simplify ScopeExitGuard by using llvm::Optional<>.

[ilya-biryukov]

Merged with head

clangd/ClangdLSPServer.cpp

std::vector<TextEdit> replacementsToEdits(StringRef Code,
   for (const auto &R : Repls)
     Edits.push_back(replacementToEdit(Code, R));
   return Edits;
 }
 
 } // namespace
 
 void ClangdLSPServer::onInitialize(Ctx C, InitializeParams &Params) {
-  C.reply(json::obj{
+  reply(C, json::obj{
       {{"capabilities",
         json::obj{
             {"textDocumentSync", 1},
             {"documentFormattingProvider", true},
             {"documentRangeFormattingProvider", true},
             {"documentOnTypeFormattingProvider",
              json::obj{
                  {"firstTriggerCharacter", "}"},
if (Params.rootUri && !Params.rootUri->file.empty())
     Server.setRootPath(Params.rootUri->file);
   else if (Params.rootPath && !Params.rootPath->empty())
     Server.setRootPath(*Params.rootPath);
 }
 
 void ClangdLSPServer::onShutdown(Ctx C, ShutdownParams &Params) {
   // Do essentially nothing, just say we're ready to exit.
   ShutdownRequestReceived = true;
-  C.reply(nullptr);
+  reply(C, nullptr);
 }
 
 void ClangdLSPServer::onExit(Ctx C, ExitParams &Params) { IsDone = true; }
 
 void ClangdLSPServer::onDocumentDidOpen(Ctx C,
                                         DidOpenTextDocumentParams &Params) {
   if (Params.metadata && !Params.metadata->extraFlags.empty())
     CDB.setExtraFlagsForFile(Params.textDocument.uri.file,
                              std::move(Params.metadata->extraFlags));
-  Server.addDocument(Params.textDocument.uri.file, Params.textDocument.text);
+  Server.addDocument(std::move(C), Params.textDocument.uri.file,
+                     Params.textDocument.text);
 }
 
 void ClangdLSPServer::onDocumentDidChange(Ctx C,
                                           DidChangeTextDocumentParams &Params) {
   if (Params.contentChanges.size() != 1)
-    return C.replyError(ErrorCode::InvalidParams,
-                        "can only apply one change at a time");
+    return replyError(C, ErrorCode::InvalidParams,
   // We only support full syncing right now.
-  Server.addDocument(Params.textDocument.uri.file,
+  Server.addDocument(std::move(C), Params.textDocument.uri.file,
                      Params.contentChanges[0].text);
 }
 
 void ClangdLSPServer::onFileEvent(Ctx C, DidChangeWatchedFilesParams &Params) {
   Server.onFileEvent(Params);
 }
 
 void ClangdLSPServer::onCommand(Ctx C, ExecuteCommandParams &Params) {
   if (Params.command == ExecuteCommandParams::CLANGD_APPLY_FIX_COMMAND &&
       Params.workspaceEdit) {
     // The flow for "apply-fix" :
     // 1. We publish a diagnostic, including fixits
     // 2. The user clicks on the diagnostic, the editor asks us for code actions
     // 3. We send code actions, with the fixit embedded as context
     // 4. The user selects the fixit, the editor asks us to apply it
     // 5. We unwrap the changes and send them back to the editor
     // 6. The editor applies the changes (applyEdit), and sends us a reply (but
     // we ignore it)
 
     ApplyWorkspaceEditParams ApplyEdit;
     ApplyEdit.edit = *Params.workspaceEdit;
-    C.reply("Fix applied.");
+    reply(C, "Fix applied.");
     // We don't need the response so id == 1 is OK.
     // Ideally, we would wait for the response and if there is no error, we
     // would reply success/failure to the original RPC.
-    C.call("workspace/applyEdit", ApplyEdit);
+    call(C, "workspace/applyEdit", ApplyEdit);
   } else {
     // We should not get here because ExecuteCommandParams would not have
     // parsed in the first place and this handler should not be called. But if
     // more commands are added, this will be here has a safe guard.
-    C.replyError(
-        ErrorCode::InvalidParams,
+    replyError(
+        C, ErrorCode::InvalidParams,
         llvm::formatv("Unsupported command \"{0}\".", Params.command).str());
   }
 }
 
 void ClangdLSPServer::onRename(Ctx C, RenameParams &Params) {
   auto File = Params.textDocument.uri.file;
-  auto Replacements = Server.rename(File, Params.position, Params.newName);
+  auto Replacements = Server.rename(C, File, Params.position, Params.newName);
   if (!Replacements) {
-    C.replyError(ErrorCode::InternalError,
-                 llvm::toString(Replacements.takeError()));
+    replyError(C, ErrorCode::InternalError,
     return;
   }
   std::string Code = Server.getDocument(File);
   std::vector<TextEdit> Edits = replacementsToEdits(Code, *Replacements);
   WorkspaceEdit WE;
   WE.changes = {{Params.textDocument.uri.uri, Edits}};
-  C.reply(WE);
+  reply(C, WE);
 }
 
 void ClangdLSPServer::onDocumentDidClose(Ctx C,
                                          DidCloseTextDocumentParams &Params) {
-  Server.removeDocument(Params.textDocument.uri.file);
+  Server.removeDocument(std::move(C), Params.textDocument.uri.file);
 }
 
 void ClangdLSPServer::onDocumentOnTypeFormatting(
     Ctx C, DocumentOnTypeFormattingParams &Params) {
   auto File = Params.textDocument.uri.file;
   std::string Code = Server.getDocument(File);
   auto ReplacementsOrError = Server.formatOnType(Code, File, Params.position);
   if (ReplacementsOrError)
-    C.reply(json::ary(replacementsToEdits(Code, ReplacementsOrError.get())));
+    reply(C, json::ary(replacementsToEdits(Code, ReplacementsOrError.get())));
   else
-    C.replyError(ErrorCode::UnknownErrorCode,
-                 llvm::toString(ReplacementsOrError.takeError()));
+    replyError(C, ErrorCode::UnknownErrorCode,
 }
 
 void ClangdLSPServer::onDocumentRangeFormatting(
     Ctx C, DocumentRangeFormattingParams &Params) {
   auto File = Params.textDocument.uri.file;
   std::string Code = Server.getDocument(File);
   auto ReplacementsOrError = Server.formatRange(Code, File, Params.range);
   if (ReplacementsOrError)
-    C.reply(json::ary(replacementsToEdits(Code, ReplacementsOrError.get())));
+    reply(C, json::ary(replacementsToEdits(Code, ReplacementsOrError.get())));
   else
-    C.replyError(ErrorCode::UnknownErrorCode,
-                 llvm::toString(ReplacementsOrError.takeError()));
+    replyError(C, ErrorCode::UnknownErrorCode,
 }
 
 void ClangdLSPServer::onDocumentFormatting(Ctx C,
                                            DocumentFormattingParams &Params) {
   auto File = Params.textDocument.uri.file;
   std::string Code = Server.getDocument(File);
   auto ReplacementsOrError = Server.formatFile(Code, File);
   if (ReplacementsOrError)
-    C.reply(json::ary(replacementsToEdits(Code, ReplacementsOrError.get())));
+    reply(C, json::ary(replacementsToEdits(Code, ReplacementsOrError.get())));
   else
-    C.replyError(ErrorCode::UnknownErrorCode,
-                 llvm::toString(ReplacementsOrError.takeError()));
+    replyError(C, ErrorCode::UnknownErrorCode,
 }
 
 void ClangdLSPServer::onCodeAction(Ctx C, CodeActionParams &Params) {
   // We provide a code action for each diagnostic at the requested location
   // which has FixIts available.
   std::string Code = Server.getDocument(Params.textDocument.uri.file);
   json::ary Commands;
   for (Diagnostic &D : Params.context.diagnostics) {
     auto Edits = getFixIts(Params.textDocument.uri.file, D);
     if (!Edits.empty()) {
       WorkspaceEdit WE;
       WE.changes = {{Params.textDocument.uri.uri, std::move(Edits)}};
       Commands.push_back(json::obj{
           {"title", llvm::formatv("Apply FixIt {0}", D.message)},
           {"command", ExecuteCommandParams::CLANGD_APPLY_FIX_COMMAND},
           {"arguments", {WE}},
       });
     }
   }
-  C.reply(std::move(Commands));
+  reply(C, std::move(Commands));
 }
 
 void ClangdLSPServer::onCompletion(Ctx C, TextDocumentPositionParams &Params) {
-  auto List =
-      Server
-          .codeComplete(
-              Params.textDocument.uri.file,
-              Position{Params.position.line, Params.position.character}, CCOpts)
-          .get() // FIXME(ibiryukov): This could be made async if we
+  auto Reply = Server
+                   .codeComplete(std::move(C), Params.textDocument.uri.file,
+                                 Position{Params.position.line,
+                                          Params.position.character},
+                                 CCOpts)
+                   .get(); // FIXME(ibiryukov): This could be made async if we
                            // had an API that would allow to attach callbacks to
                            // futures returned by ClangdServer.
-          .Value;
-  C.reply(List);
+
+  // We have std::move'd from C, now restore it from response of codeComplete.
+  C = std::move(Reply.first);
+  auto List = std::move(Reply.second.Value);
+  reply(C, List);
 }
 
 void ClangdLSPServer::onSignatureHelp(Ctx C,
                                       TextDocumentPositionParams &Params) {
   auto SignatureHelp = Server.signatureHelp(
-      Params.textDocument.uri.file,
+      C, Params.textDocument.uri.file,
       Position{Params.position.line, Params.position.character});
   if (!SignatureHelp)
-    return C.replyError(ErrorCode::InvalidParams,
-                        llvm::toString(SignatureHelp.takeError()));
-  C.reply(SignatureHelp->Value);
+    return replyError(C, ErrorCode::InvalidParams,
+  reply(C, SignatureHelp->Value);
 }
 
 void ClangdLSPServer::onGoToDefinition(Ctx C,
                                        TextDocumentPositionParams &Params) {
   auto Items = Server.findDefinitions(
-      Params.textDocument.uri.file,
+      C, Params.textDocument.uri.file,
       Position{Params.position.line, Params.position.character});
   if (!Items)
-    return C.replyError(ErrorCode::InvalidParams,
-                        llvm::toString(Items.takeError()));
-  C.reply(json::ary(Items->Value));
+    return replyError(C, ErrorCode::InvalidParams,
+  reply(C, json::ary(Items->Value));
 }
 
 void ClangdLSPServer::onSwitchSourceHeader(Ctx C,
                                            TextDocumentIdentifier &Params) {
   llvm::Optional<Path> Result = Server.switchSourceHeader(Params.uri.file);
   std::string ResultUri;
-  C.reply(Result ? URI::fromFile(*Result).uri : "");
+  reply(C, Result ? URI::fromFile(*Result).uri : "");
 }
 
 void ClangdLSPServer::onDocumentHighlight(Ctx C,
                                           TextDocumentPositionParams &Params) {
 
   auto Highlights = Server.findDocumentHighlights(
-      Params.textDocument.uri.file,
+      C, Params.textDocument.uri.file,
       Position{Params.position.line, Params.position.character});
 
   if (!Highlights) {
-    C.replyError(ErrorCode::InternalError,
-                 llvm::toString(Highlights.takeError()));
+    replyError(C, ErrorCode::InternalError,
     return;
   }
 
-  C.reply(json::ary(Highlights->Value));
+  reply(C, json::ary(Highlights->Value));
 }
 
 ClangdLSPServer::ClangdLSPServer(JSONOutput &Out, unsigned AsyncThreadsCount,
                                  bool StorePreamblesInMemory,
                                  const clangd::CodeCompleteOptions &CCOpts,
                                  llvm::Optional<StringRef> ResourceDir,
                                  llvm::Optional<Path> CompileCommandsDir)
-    : Out(Out), CDB(/*Logger=*/Out, std::move(CompileCommandsDir)),
-      CCOpts(CCOpts), Server(CDB, /*DiagConsumer=*/*this, FSProvider,
-                             AsyncThreadsCount, StorePreamblesInMemory,
-                             /*Logger=*/Out, ResourceDir) {}
+    : Out(Out), CDB(std::move(CompileCommandsDir)), CCOpts(CCOpts),
+      Server(CDB, /*DiagConsumer=*/*this, FSProvider, AsyncThreadsCount,
+             StorePreamblesInMemory, ResourceDir) {}
 
 bool ClangdLSPServer::run(std::istream &In) {
   assert(!IsDone && "Run was called before");
 
   // Set up JSONRPCDispatcher.
-  JSONRPCDispatcher Dispatcher(
-      [](RequestContext Ctx, const json::Expr &Params) {
-        Ctx.replyError(ErrorCode::MethodNotFound, "method not found");
-      });
+  JSONRPCDispatcher Dispatcher([](Context Ctx, const json::Expr &Params) {
+    replyError(Ctx, ErrorCode::MethodNotFound, "method not found");
   registerCallbackHandlers(Dispatcher, Out, /*Callbacks=*/*this);
 
   // Run the Language Server loop.
   runLanguageServerLoop(In, Out, Dispatcher, IsDone);
 
   // Make sure IsDone is set to true after this method exits to ensure assertion
   // at the start of the method fires if it's ever executed again.
   IsDone = true;

clangd/ClangdServer.h

 #include <type_traits>
 #include <utility>
 
 namespace clang {
 class PCHContainerOperations;
 
 namespace clangd {
 
-class Logger;
-
 /// Turn a [line, column] pair into an offset in Code.
 size_t positionToOffset(StringRef Code, Position P);
 
 /// Turn an offset in Code into a [line, column] pair.
 Position offsetToPosition(StringRef Code, size_t Offset);
 
 /// A tag supplied by the FileSytemProvider.
 typedef std::string VFSTag;
public:
   ///
   /// After each parsing request finishes, ClangdServer reports diagnostics to
   /// \p DiagConsumer. Note that a callback to \p DiagConsumer happens on a
   /// worker thread. Therefore, instances of \p DiagConsumer must properly
   /// synchronize access to shared state.
   ///
   /// \p StorePreamblesInMemory defines whether the Preambles generated by
   /// clangd are stored in-memory or on disk.
-  ///
-  /// Various messages are logged using \p Logger.
   ClangdServer(GlobalCompilationDatabase &CDB,
                DiagnosticsConsumer &DiagConsumer,
                FileSystemProvider &FSProvider, unsigned AsyncThreadsCount,
-               bool StorePreamblesInMemory, clangd::Logger &Logger,
+               bool StorePreamblesInMemory,
                llvm::Optional<StringRef> ResourceDir = llvm::None);
 
   /// Set the root path of the workspace.
   void setRootPath(PathRef RootPath);
 
   /// Add a \p File to the list of tracked C++ files or update the contents if
   /// \p File is already tracked. Also schedules parsing of the AST for it on a
   /// separate thread. When the parsing is complete, DiagConsumer passed in
   /// constructor will receive onDiagnosticsReady callback.
   /// \return A future that will become ready when the rebuild (including
   /// diagnostics) is finished.
-  std::future<void> addDocument(PathRef File, StringRef Contents);
+  std::future<Context> addDocument(Context Ctx, PathRef File,
+                                   StringRef Contents);
   /// Remove \p File from list of tracked files, schedule a request to free
   /// resources associated with it.
   /// \return A future that will become ready when the file is removed and all
   /// associated resources are freed.
-  std::future<void> removeDocument(PathRef File);
+  std::future<Context> removeDocument(Context Ctx, PathRef File);
   /// Force \p File to be reparsed using the latest contents.
   /// Will also check if CompileCommand, provided by GlobalCompilationDatabase
   /// for \p File has changed. If it has, will remove currently stored Preamble
   /// and AST and rebuild them from scratch.
-  std::future<void> forceReparse(PathRef File);
+  std::future<Context> forceReparse(Context Ctx, PathRef File);
 
   /// DEPRECATED. Please use a callback-based version, this API is deprecated
   /// and will soon be removed.
   ///
   /// Run code completion for \p File at \p Pos.
   ///
   /// Request is processed asynchronously. You can use the returned future to
   /// wait for the results of the async request.
   ///
   /// If \p OverridenContents is not None, they will used only for code
   /// completion, i.e. no diagnostics update will be scheduled and a draft for
   /// \p File will not be updated. If \p OverridenContents is None, contents of
   /// the current draft for \p File will be used. If \p UsedFS is non-null, it
   /// will be overwritten by vfs::FileSystem used for completion.
   ///
   /// This method should only be called for currently tracked files. However, it
   /// is safe to call removeDocument for \p File after this method returns, even
   /// while returned future is not yet ready.
-  std::future<Tagged<CompletionList>>
-  codeComplete(PathRef File, Position Pos,
+  std::future<std::pair<Context, Tagged<CompletionList>>>
+  codeComplete(Context Ctx, PathRef File, Position Pos,
                const clangd::CodeCompleteOptions &Opts,
                llvm::Optional<StringRef> OverridenContents = llvm::None,
                IntrusiveRefCntPtr<vfs::FileSystem> *UsedFS = nullptr);
 
   /// A version of `codeComplete` that runs \p Callback on the processing thread
   /// when codeComplete results become available.
-  void codeComplete(UniqueFunction<void(Tagged<CompletionList>)> Callback,
-                    PathRef File, Position Pos,
-                    const clangd::CodeCompleteOptions &Opts,
-                    llvm::Optional<StringRef> OverridenContents = llvm::None,
-                    IntrusiveRefCntPtr<vfs::FileSystem> *UsedFS = nullptr);
+  void
+  codeComplete(Context Ctx, PathRef File, Position Pos,
+               UniqueFunction<void(Context, Tagged<CompletionList>)> Callback,
 
   /// Provide signature help for \p File at \p Pos. If \p OverridenContents is
   /// not None, they will used only for signature help, i.e. no diagnostics
   /// update will be scheduled and a draft for \p File will not be updated. If
   /// \p OverridenContents is None, contents of the current draft for \p File
   /// will be used. If \p UsedFS is non-null, it will be overwritten by
   /// vfs::FileSystem used for signature help. This method should only be called
   /// for currently tracked files.
   llvm::Expected<Tagged<SignatureHelp>>
-  signatureHelp(PathRef File, Position Pos,
+  signatureHelp(const Context &Ctx, PathRef File, Position Pos,
                 llvm::Optional<StringRef> OverridenContents = llvm::None,
                 IntrusiveRefCntPtr<vfs::FileSystem> *UsedFS = nullptr);
 
   /// Get definition of symbol at a specified \p Line and \p Column in \p File.
-  llvm::Expected<Tagged<std::vector<Location>>> findDefinitions(PathRef File,
-                                                                Position Pos);
+  llvm::Expected<Tagged<std::vector<Location>>>
+  findDefinitions(const Context &Ctx, PathRef File, Position Pos);
 
   /// Helper function that returns a path to the corresponding source file when
   /// given a header file and vice versa.
   llvm::Optional<Path> switchSourceHeader(PathRef Path);
 
   /// Get document highlights for a given position.
   llvm::Expected<Tagged<std::vector<DocumentHighlight>>>
-  findDocumentHighlights(PathRef File, Position Pos);
+  findDocumentHighlights(const Context &Ctx, PathRef File, Position Pos);
 
   /// Run formatting for \p Rng inside \p File with content \p Code.
   llvm::Expected<tooling::Replacements> formatRange(StringRef Code,
                                                     PathRef File, Range Rng);
 
   /// Run formatting for the whole \p File with content \p Code.
   llvm::Expected<tooling::Replacements> formatFile(StringRef Code,
                                                    PathRef File);
 
   /// Run formatting after a character was typed at \p Pos in \p File with
   /// content \p Code.
   llvm::Expected<tooling::Replacements>
   formatOnType(StringRef Code, PathRef File, Position Pos);
 
   /// Rename all occurrences of the symbol at the \p Pos in \p File to
   /// \p NewName.
-  Expected<std::vector<tooling::Replacement>> rename(PathRef File, Position Pos,
+  Expected<std::vector<tooling::Replacement>> rename(const Context &Ctx,
+                                                     PathRef File, Position Pos,
                                                      llvm::StringRef NewName);
 
   /// Gets current document contents for \p File. \p File must point to a
   /// currently tracked file.
   /// FIXME(ibiryukov): This function is here to allow offset-to-Position
   /// conversions in outside code, maybe there's a way to get rid of it.
   std::string getDocument(PathRef File);
 
   /// Only for testing purposes.
   /// Waits until all requests to worker thread are finished and dumps AST for
   /// \p File. \p File must be in the list of added documents.
   std::string dumpAST(PathRef File);
   /// Called when an event occurs for a watched file in the workspace.
   void onFileEvent(const DidChangeWatchedFilesParams &Params);
 
 private:
-
   /// FIXME: This stats several files to find a .clang-format file. I/O can be
   /// slow. Think of a way to cache this.
   llvm::Expected<tooling::Replacements>
   formatCode(llvm::StringRef Code, PathRef File,
              ArrayRef<tooling::Range> Ranges);
 
-  std::future<void>
-  scheduleReparseAndDiags(PathRef File, VersionedDraft Contents,
+  std::future<Context>
+  scheduleReparseAndDiags(Context Ctx, PathRef File, VersionedDraft Contents,
                           std::shared_ptr<CppFile> Resources,
                           Tagged<IntrusiveRefCntPtr<vfs::FileSystem>> TaggedFS);
 
-  std::future<void> scheduleCancelRebuild(std::shared_ptr<CppFile> Resources);
+  std::future<Context>
+  scheduleCancelRebuild(Context Ctx, std::shared_ptr<CppFile> Resources);
 
-  clangd::Logger &Logger;
   GlobalCompilationDatabase &CDB;
   DiagnosticsConsumer &DiagConsumer;
   FileSystemProvider &FSProvider;
   DraftStore DraftMgr;
   CppFileCollection Units;
   std::string ResourceDir;
   // If set, this represents the workspace path.
   llvm::Optional<std::string> RootPath;

clangd/ClangdServer.cpp

 #include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
 #include <future>
 
 using namespace clang;
 using namespace clang::clangd;
 
 namespace {
 
-class FulfillPromiseGuard {
-public:
-  FulfillPromiseGuard(std::promise<void> &Promise) : Promise(Promise) {}
-
-  ~FulfillPromiseGuard() { Promise.set_value(); }
-
-private:
-  std::promise<void> &Promise;
-};
-
 std::string getStandardResourceDir() {
   static int Dummy; // Just an address in this process.
   return CompilerInvocation::GetResourcesPath("clangd", (void *)&Dummy);
 }
 
 class RefactoringResultCollector final
     : public tooling::RefactoringResultConsumer {
 public:
   void handleError(llvm::Error Err) override {
ClangdScheduler::~ClangdScheduler() {
   for (auto &Worker : Workers)
     Worker.join();
 }
 
 ClangdServer::ClangdServer(GlobalCompilationDatabase &CDB,
                            DiagnosticsConsumer &DiagConsumer,
                            FileSystemProvider &FSProvider,
                            unsigned AsyncThreadsCount,
-                           bool StorePreamblesInMemory, clangd::Logger &Logger,
+                           bool StorePreamblesInMemory,
                            llvm::Optional<StringRef> ResourceDir)
-    : Logger(Logger), CDB(CDB), DiagConsumer(DiagConsumer),
-      FSProvider(FSProvider),
+    : CDB(CDB), DiagConsumer(DiagConsumer), FSProvider(FSProvider),
       ResourceDir(ResourceDir ? ResourceDir->str() : getStandardResourceDir()),
       PCHs(std::make_shared<PCHContainerOperations>()),
       StorePreamblesInMemory(StorePreamblesInMemory),
       WorkScheduler(AsyncThreadsCount) {}
 
 void ClangdServer::setRootPath(PathRef RootPath) {
   std::string NewRootPath = llvm::sys::path::convert_to_slash(
       RootPath, llvm::sys::path::Style::posix);
   if (llvm::sys::fs::is_directory(NewRootPath))
     this->RootPath = NewRootPath;
 }
 
-std::future<void> ClangdServer::addDocument(PathRef File, StringRef Contents) {
+std::future<Context> ClangdServer::addDocument(Context Ctx, PathRef File,
+                                               StringRef Contents) {
   DocVersion Version = DraftMgr.updateDraft(File, Contents);
 
   auto TaggedFS = FSProvider.getTaggedFileSystem(File);
   std::shared_ptr<CppFile> Resources = Units.getOrCreateFile(
-      File, ResourceDir, CDB, StorePreamblesInMemory, PCHs, Logger);
-  return scheduleReparseAndDiags(File, VersionedDraft{Version, Contents.str()},
+      File, ResourceDir, CDB, StorePreamblesInMemory, PCHs);
+  return scheduleReparseAndDiags(std::move(Ctx), File,
+                                 VersionedDraft{Version, Contents.str()},
                                  std::move(Resources), std::move(TaggedFS));
 }
 
-std::future<void> ClangdServer::removeDocument(PathRef File) {
+std::future<Context> ClangdServer::removeDocument(Context Ctx, PathRef File) {
   DraftMgr.removeDraft(File);
   std::shared_ptr<CppFile> Resources = Units.removeIfPresent(File);
-  return scheduleCancelRebuild(std::move(Resources));
+  return scheduleCancelRebuild(std::move(Ctx), std::move(Resources));
 }
 
-std::future<void> ClangdServer::forceReparse(PathRef File) {
+std::future<Context> ClangdServer::forceReparse(Context Ctx, PathRef File) {
   auto FileContents = DraftMgr.getDraft(File);
   assert(FileContents.Draft &&
          "forceReparse() was called for non-added document");
 
   auto TaggedFS = FSProvider.getTaggedFileSystem(File);
   auto Recreated = Units.recreateFileIfCompileCommandChanged(
-      File, ResourceDir, CDB, StorePreamblesInMemory, PCHs, Logger);
+      File, ResourceDir, CDB, StorePreamblesInMemory, PCHs);
 
   // Note that std::future from this cleanup action is ignored.
-  scheduleCancelRebuild(std::move(Recreated.RemovedFile));
+  scheduleCancelRebuild(Ctx.clone(), std::move(Recreated.RemovedFile));
   // Schedule a reparse.
-  return scheduleReparseAndDiags(File, std::move(FileContents),
+  return scheduleReparseAndDiags(std::move(Ctx), File, std::move(FileContents),
                                  std::move(Recreated.FileInCollection),
                                  std::move(TaggedFS));
 }
 
-std::future<Tagged<CompletionList>>
-ClangdServer::codeComplete(PathRef File, Position Pos,
+std::future<std::pair<Context, Tagged<CompletionList>>>
+ClangdServer::codeComplete(Context Ctx, PathRef File, Position Pos,
                            const clangd::CodeCompleteOptions &Opts,
                            llvm::Optional<StringRef> OverridenContents,
                            IntrusiveRefCntPtr<vfs::FileSystem> *UsedFS) {
-  using ResultType = Tagged<CompletionList>;
+  using ResultType = std::pair<Context, Tagged<CompletionList>>;
 
   std::promise<ResultType> ResultPromise;
 
-  auto Callback = [](std::promise<ResultType> ResultPromise,
-                     ResultType Result) -> void {
-    ResultPromise.set_value(std::move(Result));
+  auto Callback = [](std::promise<ResultType> ResultPromise, Context Ctx,
+                     Tagged<CompletionList> Result) -> void {
+    ResultPromise.set_value({std::move(Ctx), std::move(Result)});
   };
 
   std::future<ResultType> ResultFuture = ResultPromise.get_future();
-  codeComplete(BindWithForward(Callback, std::move(ResultPromise)), File, Pos,
-               Opts, OverridenContents, UsedFS);
+  codeComplete(std::move(Ctx), File, Pos, Opts,
+               BindWithForward(Callback, std::move(ResultPromise)),
+               OverridenContents, UsedFS);
   return ResultFuture;
 }
 
 void ClangdServer::codeComplete(
-    UniqueFunction<void(Tagged<CompletionList>)> Callback, PathRef File,
-    Position Pos, const clangd::CodeCompleteOptions &Opts,
+    Context Ctx, PathRef File, Position Pos,
+    const clangd::CodeCompleteOptions &Opts,
+    UniqueFunction<void(Context, Tagged<CompletionList>)> Callback,
     llvm::Optional<StringRef> OverridenContents,
     IntrusiveRefCntPtr<vfs::FileSystem> *UsedFS) {
-  using CallbackType = UniqueFunction<void(Tagged<CompletionList>)>;
+  using CallbackType = UniqueFunction<void(Context, Tagged<CompletionList>)>;
 
   std::string Contents;
   if (OverridenContents) {
     Contents = *OverridenContents;
   } else {
     auto FileContents = DraftMgr.getDraft(File);
     assert(FileContents.Draft &&
            "codeComplete is called for non-added document");
void ClangdServer::codeComplete(
   // is reusable in completion more often.
   std::shared_ptr<const PreambleData> Preamble =
       Resources->getPossiblyStalePreamble();
   // Copy completion options for passing them to async task handler.
   auto CodeCompleteOpts = Opts;
   // A task that will be run asynchronously.
   auto Task =
       // 'mutable' to reassign Preamble variable.
-      [=](CallbackType Callback) mutable {
+      [=](Context Ctx, CallbackType Callback) mutable {
         if (!Preamble) {
           // Maybe we built some preamble before processing this request.
           Preamble = Resources->getPossiblyStalePreamble();
         }
         // FIXME(ibiryukov): even if Preamble is non-null, we may want to check
         // both the old and the new version in case only one of them matches.
 
         CompletionList Result = clangd::codeComplete(
-            File, Resources->getCompileCommand(),
+            Ctx, File, Resources->getCompileCommand(),
             Preamble ? &Preamble->Preamble : nullptr, Contents, Pos,
-            TaggedFS.Value, PCHs, CodeCompleteOpts, Logger);
+            TaggedFS.Value, PCHs, CodeCompleteOpts);
 
-        Callback(make_tagged(std::move(Result), std::move(TaggedFS.Tag)));
+        Callback(std::move(Ctx),
+                 make_tagged(std::move(Result), std::move(TaggedFS.Tag)));
       };
 
-  WorkScheduler.addToFront(std::move(Task), std::move(Callback));
+  WorkScheduler.addToFront(std::move(Task), std::move(Ctx),
+                           std::move(Callback));
 }
 
 llvm::Expected<Tagged<SignatureHelp>>
-ClangdServer::signatureHelp(PathRef File, Position Pos,
+ClangdServer::signatureHelp(const Context &Ctx, PathRef File, Position Pos,
                             llvm::Optional<StringRef> OverridenContents,
                             IntrusiveRefCntPtr<vfs::FileSystem> *UsedFS) {
   std::string DraftStorage;
   if (!OverridenContents) {
     auto FileContents = DraftMgr.getDraft(File);
     if (!FileContents.Draft)
       return llvm::make_error<llvm::StringError>(
           "signatureHelp is called for non-added document",
ClangdServer::signatureHelp(const Context &Ctx, PathRef File, Position Pos,
 
   std::shared_ptr<CppFile> Resources = Units.getFile(File);
   if (!Resources)
     return llvm::make_error<llvm::StringError>(
         "signatureHelp is called for non-added document",
         llvm::errc::invalid_argument);
 
   auto Preamble = Resources->getPossiblyStalePreamble();
-  auto Result = clangd::signatureHelp(File, Resources->getCompileCommand(),
-                                      Preamble ? &Preamble->Preamble : nullptr,
-                                      *OverridenContents, Pos, TaggedFS.Value,
-                                      PCHs, Logger);
+  auto Result =
+      clangd::signatureHelp(Ctx, File, Resources->getCompileCommand(),
+                            *OverridenContents, Pos, TaggedFS.Value, PCHs);
   return make_tagged(std::move(Result), TaggedFS.Tag);
 }
 
 llvm::Expected<tooling::Replacements>
 ClangdServer::formatRange(StringRef Code, PathRef File, Range Rng) {
   size_t Begin = positionToOffset(Code, Rng.start);
   size_t Len = positionToOffset(Code, Rng.end) - Begin;
   return formatCode(Code, File, {tooling::Range(Begin, Len)});
ClangdServer::formatOnType(StringRef Code, PathRef File, Position Pos) {
   if (PreviousLBracePos == StringRef::npos)
     PreviousLBracePos = CursorPos;
   size_t Len = 1 + CursorPos - PreviousLBracePos;
 
   return formatCode(Code, File, {tooling::Range(PreviousLBracePos, Len)});
 }
 
 Expected<std::vector<tooling::Replacement>>
-ClangdServer::rename(PathRef File, Position Pos, llvm::StringRef NewName) {
+ClangdServer::rename(const Context &Ctx, PathRef File, Position Pos,
+                     llvm::StringRef NewName) {
   std::string Code = getDocument(File);
   std::shared_ptr<CppFile> Resources = Units.getFile(File);
   RefactoringResultCollector ResultCollector;
   Resources->getAST().get()->runUnderLock([&](ParsedAST *AST) {
     const SourceManager &SourceMgr = AST->getASTContext().getSourceManager();
     const FileEntry *FE =
         SourceMgr.getFileEntryForID(SourceMgr.getMainFileID());
     if (!FE)
if (AST) {
       ResultOS << "<no-ast>";
     }
     ResultOS.flush();
   });
   return Result;
 }
 
 llvm::Expected<Tagged<std::vector<Location>>>
-ClangdServer::findDefinitions(PathRef File, Position Pos) {
+ClangdServer::findDefinitions(const Context &Ctx, PathRef File, Position Pos) {
   auto TaggedFS = FSProvider.getTaggedFileSystem(File);
 
   std::shared_ptr<CppFile> Resources = Units.getFile(File);
   if (!Resources)
     return llvm::make_error<llvm::StringError>(
         "findDefinitions called on non-added file",
         llvm::errc::invalid_argument);
 
   std::vector<Location> Result;
-  Resources->getAST().get()->runUnderLock([Pos, &Result, this](ParsedAST *AST) {
+  Resources->getAST().get()->runUnderLock([Pos, &Result, &Ctx](ParsedAST *AST) {
     if (!AST)
       return;
-    Result = clangd::findDefinitions(*AST, Pos, Logger);
+    Result = clangd::findDefinitions(Ctx, *AST, Pos);
   });
   return make_tagged(std::move(Result), TaggedFS.Tag);
 }
 
 llvm::Optional<Path> ClangdServer::switchSourceHeader(PathRef Path) {
 
   StringRef SourceExtensions[] = {".cpp", ".c", ".cc", ".cxx",
                                   ".c++", ".m", ".mm"};
ClangdServer::formatCode(llvm::StringRef Code, PathRef File,
   if (!StyleOrError) {
     return StyleOrError.takeError();
   } else {
     return format::reformat(StyleOrError.get(), Code, Ranges, File);
   }
 }
 
 llvm::Expected<Tagged<std::vector<DocumentHighlight>>>
-ClangdServer::findDocumentHighlights(PathRef File, Position Pos) {
+ClangdServer::findDocumentHighlights(const Context &Ctx, PathRef File,
+                                     Position Pos) {
   auto FileContents = DraftMgr.getDraft(File);
   if (!FileContents.Draft)
     return llvm::make_error<llvm::StringError>(
         "findDocumentHighlights called on non-added file",
         llvm::errc::invalid_argument);
 
   auto TaggedFS = FSProvider.getTaggedFileSystem(File);
 
   std::shared_ptr<CppFile> Resources = Units.getFile(File);
   if (!Resources)
     return llvm::make_error<llvm::StringError>(
         "findDocumentHighlights called on non-added file",
         llvm::errc::invalid_argument);
 
   std::vector<DocumentHighlight> Result;
   llvm::Optional<llvm::Error> Err;
-  Resources->getAST().get()->runUnderLock([Pos, &Result, &Err,
-                                           this](ParsedAST *AST) {
+  Resources->getAST().get()->runUnderLock([Pos, &Ctx, &Err,
+                                           &Result](ParsedAST *AST) {
     if (!AST) {
       Err = llvm::make_error<llvm::StringError>("Invalid AST",
                                                 llvm::errc::invalid_argument);
       return;
     }
-    Result = clangd::findDocumentHighlights(*AST, Pos, Logger);
+    Result = clangd::findDocumentHighlights(Ctx, *AST, Pos);
   });
 
   if (Err)
     return std::move(*Err);
   return make_tagged(Result, TaggedFS.Tag);
 }
 
-std::future<void> ClangdServer::scheduleReparseAndDiags(
-    PathRef File, VersionedDraft Contents, std::shared_ptr<CppFile> Resources,
+std::future<Context> ClangdServer::scheduleReparseAndDiags(
+    Context Ctx, PathRef File, VersionedDraft Contents,
+    std::shared_ptr<CppFile> Resources,
     Tagged<IntrusiveRefCntPtr<vfs::FileSystem>> TaggedFS) {
 
   assert(Contents.Draft && "Draft must have contents");
-  UniqueFunction<llvm::Optional<std::vector<DiagWithFixIts>>()>
+  UniqueFunction<llvm::Optional<std::vector<DiagWithFixIts>>(const Context &)>
       DeferredRebuild =
           Resources->deferRebuild(*Contents.Draft, TaggedFS.Value);
-  std::promise<void> DonePromise;
-  std::future<void> DoneFuture = DonePromise.get_future();
+  std::promise<Context> DonePromise;
+  std::future<Context> DoneFuture = DonePromise.get_future();
 
   DocVersion Version = Contents.Version;
   Path FileStr = File;
   VFSTag Tag = TaggedFS.Tag;
   auto ReparseAndPublishDiags =
       [this, FileStr, Version,
-       Tag](UniqueFunction<llvm::Optional<std::vector<DiagWithFixIts>>()>
+       Tag](UniqueFunction<llvm::Optional<std::vector<DiagWithFixIts>>(
+                const Context &)>
                 DeferredRebuild,
-            std::promise<void> DonePromise) -> void {
-    FulfillPromiseGuard Guard(DonePromise);
+            std::promise<Context> DonePromise, Context Ctx) -> void {
+    auto Guard = onScopeExit([&]() { DonePromise.set_value(std::move(Ctx)); });
 
     auto CurrentVersion = DraftMgr.getVersion(FileStr);
     if (CurrentVersion != Version)
       return; // This request is outdated
 
-    auto Diags = DeferredRebuild();
+    auto Diags = DeferredRebuild(Ctx);
     if (!Diags)
       return; // A new reparse was requested before this one completed.
 
     // We need to serialize access to resulting diagnostics to avoid calling
     // `onDiagnosticsReady` in the wrong order.
     std::lock_guard<std::mutex> DiagsLock(DiagnosticsMutex);
     DocVersion &LastReportedDiagsVersion = ReportedDiagnosticVersions[FileStr];
     // FIXME(ibiryukov): get rid of '<' comparison here. In the current
     // implementation diagnostics will not be reported after version counters'
     // overflow. This should not happen in practice, since DocVersion is a
     // 64-bit unsigned integer.
     if (Version < LastReportedDiagsVersion)
       return;
     LastReportedDiagsVersion = Version;
 
     DiagConsumer.onDiagnosticsReady(FileStr,
                                     make_tagged(std::move(*Diags), Tag));
   };
 
   WorkScheduler.addToFront(std::move(ReparseAndPublishDiags),
-                           std::move(DeferredRebuild), std::move(DonePromise));
+                           std::move(DeferredRebuild), std::move(DonePromise),
+                           std::move(Ctx));
   return DoneFuture;
 }
 
-std::future<void>
-ClangdServer::scheduleCancelRebuild(std::shared_ptr<CppFile> Resources) {
-  std::promise<void> DonePromise;
-  std::future<void> DoneFuture = DonePromise.get_future();
+std::future<Context>
+ClangdServer::scheduleCancelRebuild(Context Ctx,
+                                    std::shared_ptr<CppFile> Resources) {
+  std::promise<Context> DonePromise;
+  std::future<Context> DoneFuture = DonePromise.get_future();
   if (!Resources) {
     // No need to schedule any cleanup.
-    DonePromise.set_value();
+    DonePromise.set_value(std::move(Ctx));
     return DoneFuture;
   }
 
   UniqueFunction<void()> DeferredCancel = Resources->deferCancelRebuild();
-  auto CancelReparses = [Resources](std::promise<void> DonePromise,
-                                    UniqueFunction<void()> DeferredCancel) {
-    FulfillPromiseGuard Guard(DonePromise);
+  auto CancelReparses = [Resources](std::promise<Context> DonePromise,
+                                    UniqueFunction<void()> DeferredCancel,
+                                    Context Ctx) {
     DeferredCancel();
+    DonePromise.set_value(std::move(Ctx));
   };
   WorkScheduler.addToFront(std::move(CancelReparses), std::move(DonePromise),
-                           std::move(DeferredCancel));
+                           std::move(DeferredCancel), std::move(Ctx));
   return DoneFuture;
 }
 
 void ClangdServer::onFileEvent(const DidChangeWatchedFilesParams &Params) {
   // FIXME: Do nothing for now. This will be used for indexing and potentially
   // invalidating other caches.
 }

clangd/ClangdUnit.h

 //===--- ClangdUnit.h -------------------------------------------*- C++-*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===---------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_CLANGDUNIT_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_CLANGDUNIT_H
 
+#include "Context.h"
 #include "Function.h"
 #include "Path.h"
 #include "Protocol.h"
 #include "clang/Frontend/FrontendAction.h"
 #include "clang/Frontend/PrecompiledPreamble.h"
 #include "clang/Serialization/ASTBitCodes.h"
 #include "clang/Tooling/CompilationDatabase.h"
 #include "clang/Tooling/Core/Replacement.h"

 }
 
 namespace tooling {
 struct CompileCommand;
 }
 
 namespace clangd {
 
-class Logger;
-
 /// A diagnostic with its FixIts.
 struct DiagWithFixIts {
   clangd::Diagnostic Diag;
   llvm::SmallVector<TextEdit, 1> FixIts;
 };
 
 // Stores Preamble and associated data.
 struct PreambleData {
   PreambleData(PrecompiledPreamble Preamble,
                std::vector<serialization::DeclID> TopLevelDeclIDs,
                std::vector<DiagWithFixIts> Diags);
 
   PrecompiledPreamble Preamble;
   std::vector<serialization::DeclID> TopLevelDeclIDs;
   std::vector<DiagWithFixIts> Diags;
 };
 
 /// Stores and provides access to parsed AST.
 class ParsedAST {
 public:
   /// Attempts to run Clang and store parsed AST. If \p Preamble is non-null
   /// it is reused during parsing.
   static llvm::Optional<ParsedAST>
-  Build(std::unique_ptr<clang::CompilerInvocation> CI,
+  Build(const Context &Ctx, std::unique_ptr<clang::CompilerInvocation> CI,
         std::shared_ptr<const PreambleData> Preamble,
         std::unique_ptr<llvm::MemoryBuffer> Buffer,
         std::shared_ptr<PCHContainerOperations> PCHs,
-        IntrusiveRefCntPtr<vfs::FileSystem> VFS, clangd::Logger &Logger);
+        IntrusiveRefCntPtr<vfs::FileSystem> VFS);
 
   ParsedAST(ParsedAST &&Other);
   ParsedAST &operator=(ParsedAST &&Other);
 
   ~ParsedAST();
 
   ASTContext &getASTContext();
   const ASTContext &getASTContext() const;

 /// contents, and provides AST and Preamble for it.
 class CppFile : public std::enable_shared_from_this<CppFile> {
 public:
   // We only allow to create CppFile as shared_ptr, because a future returned by
   // deferRebuild will hold references to it.
   static std::shared_ptr<CppFile>
   Create(PathRef FileName, tooling::CompileCommand Command,
          bool StorePreamblesInMemory,
-         std::shared_ptr<PCHContainerOperations> PCHs, clangd::Logger &Logger);
+         std::shared_ptr<PCHContainerOperations> PCHs);
 
 private:
   CppFile(PathRef FileName, tooling::CompileCommand Command,
           bool StorePreamblesInMemory,
-          std::shared_ptr<PCHContainerOperations> PCHs, clangd::Logger &Logger);
+          std::shared_ptr<PCHContainerOperations> PCHs);
 
 public:
   CppFile(CppFile const &) = delete;
   CppFile(CppFile &&) = delete;
 
   /// Cancels a scheduled rebuild, if any, and sets AST and Preamble to nulls.
   /// If a rebuild is in progress, will wait for it to finish.
   void cancelRebuild();
 
   /// Similar to deferRebuild, but sets both Preamble and AST to nulls instead
   /// of doing an actual parsing. Returned function is a deferred computation
   /// that will wait for any ongoing rebuilds to finish and actually set the AST
   /// and Preamble to nulls. It can be run on a different thread. This function
   /// is useful to cancel ongoing rebuilds, if any, before removing CppFile.
   UniqueFunction<void()> deferCancelRebuild();
 
   /// Rebuild AST and Preamble synchronously on the calling thread.
   /// Returns a list of diagnostics or a llvm::None, if another rebuild was
   /// requested in parallel (effectively cancelling this rebuild) before
   /// diagnostics were produced.
   llvm::Optional<std::vector<DiagWithFixIts>>
-  rebuild(StringRef NewContents, IntrusiveRefCntPtr<vfs::FileSystem> VFS);
+  rebuild(const Context &Ctx, StringRef NewContents,
+          IntrusiveRefCntPtr<vfs::FileSystem> VFS);
 
   /// Schedule a rebuild and return a deferred computation that will finish the
   /// rebuild, that can be called on a different thread.
   /// After calling this method, resources, available via futures returned by
   /// getPreamble() and getAST(), will be waiting for rebuild to finish. A
   /// continuation fininshing rebuild, returned by this function, must be
   /// computed(i.e., operator() must be called on it) in order to make those
   /// resources ready. If deferRebuild is called again before the rebuild is
   /// finished (either because returned future had not been called or because it
   /// had not returned yet), the previous rebuild request is cancelled and the
   /// resource futures (returned by getPreamble() or getAST()) that were not
   /// ready will be waiting for the last rebuild to finish instead.
   /// The future to finish rebuild returns a list of diagnostics built during
   /// reparse, or None, if another deferRebuild was called before this
   /// rebuild was finished.
-  UniqueFunction<llvm::Optional<std::vector<DiagWithFixIts>>()>
+  UniqueFunction<llvm::Optional<std::vector<DiagWithFixIts>>(const Context &)>
   deferRebuild(StringRef NewContents, IntrusiveRefCntPtr<vfs::FileSystem> VFS);
 
   /// Returns a future to get the most fresh PreambleData for a file. The
   /// future will wait until the Preamble is rebuilt.
   std::shared_future<std::shared_ptr<const PreambleData>> getPreamble() const;
   /// Return some preamble for a file. It might be stale, but won't wait for
   /// rebuild to finish.
   std::shared_ptr<const PreambleData> getPossiblyStalePreamble() const;
private:
   /// Promise and future for the latests Preamble. Fulfilled during rebuild.
   std::promise<std::shared_ptr<const PreambleData>> PreamblePromise;
   std::shared_future<std::shared_ptr<const PreambleData>> PreambleFuture;
   /// Latest preamble that was built. May be stale, but always available without
   /// waiting for rebuild to finish.
   std::shared_ptr<const PreambleData> LatestAvailablePreamble;
   /// Utility class, required by clang.
   std::shared_ptr<PCHContainerOperations> PCHs;
-  /// Used for logging various messages.
-  clangd::Logger &Logger;
 };
 
 /// Get the beginning SourceLocation at a specified \p Pos.
 SourceLocation getBeginningOfIdentifier(ParsedAST &Unit, const Position &Pos,
                                         const FileEntry *FE);
 
 /// Get definition of symbol at a specified \p Pos.
-std::vector<Location> findDefinitions(ParsedAST &AST, Position Pos,
-                                      clangd::Logger &Logger);
+std::vector<Location> findDefinitions(const Context &Ctx, ParsedAST &AST,
+                                      Position Pos);
 
 std::vector<DocumentHighlight>
-findDocumentHighlights(ParsedAST &AST, Position Pos, clangd::Logger &Logger);
+findDocumentHighlights(const Context &Ctx, ParsedAST &AST, Position Pos);
 
 /// For testing/debugging purposes. Note that this method deserializes all
 /// unserialized Decls, so use with care.
 void dumpAST(ParsedAST &AST, llvm::raw_ostream &OS);
 
 } // namespace clangd
 } // namespace clang
 #endif

clangd/ClangdUnit.cpp

 
 } // namespace
 
 void clangd::dumpAST(ParsedAST &AST, llvm::raw_ostream &OS) {
   AST.getASTContext().getTranslationUnitDecl()->dump(OS, true);
 }
 
 llvm::Optional<ParsedAST>
-ParsedAST::Build(std::unique_ptr<clang::CompilerInvocation> CI,
+ParsedAST::Build(const Context &Ctx,
+                 std::unique_ptr<clang::CompilerInvocation> CI,
                  std::shared_ptr<const PreambleData> Preamble,
                  std::unique_ptr<llvm::MemoryBuffer> Buffer,
                  std::shared_ptr<PCHContainerOperations> PCHs,
-                 IntrusiveRefCntPtr<vfs::FileSystem> VFS,
-                 clangd::Logger &Logger) {
+                 IntrusiveRefCntPtr<vfs::FileSystem> VFS) {
 
   std::vector<DiagWithFixIts> ASTDiags;
   StoreDiagsConsumer UnitDiagsConsumer(/*ref*/ ASTDiags);
 
   const PrecompiledPreamble *PreamblePCH =
       Preamble ? &Preamble->Preamble : nullptr;
   auto Clang = prepareCompilerInstance(
       std::move(CI), PreamblePCH, std::move(Buffer), std::move(PCHs),
       std::move(VFS), /*ref*/ UnitDiagsConsumer);
 
   // Recover resources if we crash before exiting this method.
   llvm::CrashRecoveryContextCleanupRegistrar<CompilerInstance> CICleanup(
       Clang.get());
 
   auto Action = llvm::make_unique<ClangdFrontendAction>();
   const FrontendInputFile &MainInput = Clang->getFrontendOpts().Inputs[0];
   if (!Action->BeginSourceFile(*Clang, MainInput)) {
-    Logger.log("BeginSourceFile() failed when building AST for " +
+    log(Ctx, "BeginSourceFile() failed when building AST for " +
                  MainInput.getFile());
     return llvm::None;
   }
   if (!Action->Execute())
-    Logger.log("Execute() failed when building AST for " + MainInput.getFile());
+    log(Ctx, "Execute() failed when building AST for " + MainInput.getFile());
 
   // UnitDiagsConsumer is local, we can not store it in CompilerInstance that
   // has a longer lifetime.
   Clang->getDiagnostics().setClient(new IgnoreDiagnostics);
 
   std::vector<const Decl *> ParsedDecls = Action->takeTopLevelDecls();
   return ParsedAST(std::move(Preamble), std::move(Clang), std::move(Action),
                    std::move(ParsedDecls), std::move(ASTDiags));
getDeclarationLocation(ParsedAST &AST, const SourceRange &ValSourceRange) {
   StringRef FilePath = F->tryGetRealPathName();
   if (FilePath.empty())
     FilePath = F->getName();
   L.uri = URI::fromFile(FilePath);
   L.range = R;
   return L;
 }
 
-std::vector<Location> clangd::findDefinitions(ParsedAST &AST, Position Pos,
-                                              clangd::Logger &Logger) {
+std::vector<Location> clangd::findDefinitions(const Context &Ctx,
+                                              ParsedAST &AST, Position Pos) {
   const SourceManager &SourceMgr = AST.getASTContext().getSourceManager();
   const FileEntry *FE = SourceMgr.getFileEntryForID(SourceMgr.getMainFileID());
   if (!FE)
     return {};
 
   SourceLocation SourceLocationBeg = getBeginningOfIdentifier(AST, Pos, FE);
 
   auto DeclMacrosFinder = std::make_shared<DeclarationAndMacrosFinder>(
for (auto Item : MacroInfos) {
     if (L)
       Result.push_back(*L);
   }
 
   return Result;
 }
 
 std::vector<DocumentHighlight>
-clangd::findDocumentHighlights(ParsedAST &AST, Position Pos,
-                               clangd::Logger &Logger) {
+clangd::findDocumentHighlights(const Context &Ctx, ParsedAST &AST,
+                               Position Pos) {
   const SourceManager &SourceMgr = AST.getASTContext().getSourceManager();
   const FileEntry *FE = SourceMgr.getFileEntryForID(SourceMgr.getMainFileID());
   if (!FE)
     return {};
 
   SourceLocation SourceLocationBeg = getBeginningOfIdentifier(AST, Pos, FE);
 
   auto DeclMacrosFinder = std::make_shared<DeclarationAndMacrosFinder>(
ParsedASTWrapper::ParsedASTWrapper(ParsedASTWrapper &&Wrapper)
     : AST(std::move(Wrapper.AST)) {}
 
 ParsedASTWrapper::ParsedASTWrapper(llvm::Optional<ParsedAST> AST)
     : AST(std::move(AST)) {}
 
 std::shared_ptr<CppFile>
 CppFile::Create(PathRef FileName, tooling::CompileCommand Command,
                 bool StorePreamblesInMemory,
-                std::shared_ptr<PCHContainerOperations> PCHs,
-                clangd::Logger &Logger) {
-  return std::shared_ptr<CppFile>(new CppFile(FileName, std::move(Command),
-                                              StorePreamblesInMemory,
-                                              std::move(PCHs), Logger));
+                std::shared_ptr<PCHContainerOperations> PCHs) {
+  return std::shared_ptr<CppFile>(new CppFile(
+      FileName, std::move(Command), StorePreamblesInMemory, std::move(PCHs)));
 }
 
 CppFile::CppFile(PathRef FileName, tooling::CompileCommand Command,
                  bool StorePreamblesInMemory,
-                 std::shared_ptr<PCHContainerOperations> PCHs,
-                 clangd::Logger &Logger)
+                 std::shared_ptr<PCHContainerOperations> PCHs)
     : FileName(FileName), Command(std::move(Command)),
       StorePreamblesInMemory(StorePreamblesInMemory), RebuildCounter(0),
-      RebuildInProgress(false), PCHs(std::move(PCHs)), Logger(Logger) {
-  Logger.log("Opened file " + FileName + " with command [" +
+      RebuildInProgress(false), PCHs(std::move(PCHs)) {
+  // FIXME(ibiryukov): we should pass a proper Context here.
+  log(Context::empty(), "Opened file " + FileName + " with command [" +
                             this->Command.Directory + "] " +
                             llvm::join(this->Command.CommandLine, " "));
 
   std::lock_guard<std::mutex> Lock(Mutex);
   LatestAvailablePreamble = nullptr;
   PreamblePromise.set_value(nullptr);
   PreambleFuture = PreamblePromise.get_future();
 
   ASTPromise.set_value(std::make_shared<ParsedASTWrapper>(llvm::None));
   ASTFuture = ASTPromise.get_future();
return [That, RequestRebuildCounter]() {
 
     // Set empty results for Promises.
     That->PreamblePromise.set_value(nullptr);
     That->ASTPromise.set_value(std::make_shared<ParsedASTWrapper>(llvm::None));
   };
 }
 
 llvm::Optional<std::vector<DiagWithFixIts>>
-CppFile::rebuild(StringRef NewContents,
+CppFile::rebuild(const Context &Ctx, StringRef NewContents,
                  IntrusiveRefCntPtr<vfs::FileSystem> VFS) {
-  return deferRebuild(NewContents, std::move(VFS))();
+  return deferRebuild(NewContents, std::move(VFS))(Ctx);
 }
 
-UniqueFunction<llvm::Optional<std::vector<DiagWithFixIts>>()>
+UniqueFunction<llvm::Optional<std::vector<DiagWithFixIts>>(const Context &)>
 CppFile::deferRebuild(StringRef NewContents,
                       IntrusiveRefCntPtr<vfs::FileSystem> VFS) {
   std::shared_ptr<const PreambleData> OldPreamble;
   std::shared_ptr<PCHContainerOperations> PCHs;
   unsigned RequestRebuildCounter;
   {
     std::unique_lock<std::mutex> Lock(Mutex);
     // Increase RebuildCounter to cancel all ongoing FinishRebuild operations.
CppFile::deferRebuild(StringRef NewContents,
   // Notify about changes to RebuildCounter.
   RebuildCond.notify_all();
 
   // A helper to function to finish the rebuild. May be run on a different
   // thread.
 
   // Don't let this CppFile die before rebuild is finished.
   std::shared_ptr<CppFile> That = shared_from_this();
-  auto FinishRebuild = [OldPreamble, VFS, RequestRebuildCounter, PCHs,
-                        That](std::string NewContents) mutable // 'mutable' to
-                                                               // allow changing
-                                                               // OldPreamble.
+  auto FinishRebuild =
+      [OldPreamble, VFS, RequestRebuildCounter, PCHs,
+       That](std::string NewContents,
+             const Context &Ctx) mutable /* to allow changing OldPreamble. */
       -> llvm::Optional<std::vector<DiagWithFixIts>> {
     // Only one execution of this method is possible at a time.
     // RebuildGuard will wait for any ongoing rebuilds to finish and will put us
     // into a state for doing a rebuild.
     RebuildGuard Rebuild(*That, RequestRebuildCounter);
     if (Rebuild.wasCancelledBeforeConstruction())
       return llvm::None;
 
if (NewPreamble) {
                          NewPreamble->Diags.end());
     }
 
     // Compute updated AST.
     llvm::Optional<ParsedAST> NewAST;
     {
       trace::Span Tracer("Build");
       SPAN_ATTACH(Tracer, "File", That->FileName);
-      NewAST =
-          ParsedAST::Build(std::move(CI), std::move(NewPreamble),
-                           std::move(ContentsBuffer), PCHs, VFS, That->Logger);
+      NewAST = ParsedAST::Build(Ctx, std::move(CI), std::move(NewPreamble),
+                                std::move(ContentsBuffer), PCHs, VFS);
     }
 
     if (NewAST) {
       Diagnostics.insert(Diagnostics.end(), NewAST->getDiagnostics().begin(),
                          NewAST->getDiagnostics().end());
     } else {
       // Don't report even Preamble diagnostics if we coulnd't build AST.
       Diagnostics.clear();

clangd/ClangdUnitStore.h

 //===--- ClangdUnitStore.h - A container of CppFiles -------------*-C++-*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===---------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_CLANGDUNITSTORE_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_CLANGDUNITSTORE_H
 
 #include <mutex>
 
 #include "ClangdUnit.h"
 #include "GlobalCompilationDatabase.h"
+#include "Logger.h"
 #include "Path.h"
 #include "clang/Tooling/CompilationDatabase.h"
 
 namespace clang {
 namespace clangd {
 
 class Logger;
 
 /// Thread-safe mapping from FileNames to CppFile.
 class CppFileCollection {
 public:
   std::shared_ptr<CppFile>
   getOrCreateFile(PathRef File, PathRef ResourceDir,
                   GlobalCompilationDatabase &CDB, bool StorePreamblesInMemory,
-                  std::shared_ptr<PCHContainerOperations> PCHs,
-                  clangd::Logger &Logger) {
+                  std::shared_ptr<PCHContainerOperations> PCHs) {
     std::lock_guard<std::mutex> Lock(Mutex);
 
     auto It = OpenedFiles.find(File);
     if (It == OpenedFiles.end()) {
       auto Command = getCompileCommand(CDB, File, ResourceDir);
 
       It = OpenedFiles
                .try_emplace(File, CppFile::Create(File, std::move(Command),
                                                   StorePreamblesInMemory,
-                                                  std::move(PCHs), Logger))
+                                                  std::move(PCHs)))
                .first;
     }
     return It->second;
   }
 
   struct RecreateResult {
     /// A CppFile, stored in this CppFileCollection for the corresponding
     /// filepath after calling recreateFileIfCompileCommandChanged.
     std::shared_ptr<CppFile> FileInCollection;
     /// If a new CppFile had to be created to account for changed
     /// CompileCommand, a previous CppFile instance will be returned in this
     /// field.
     std::shared_ptr<CppFile> RemovedFile;
   };
 
   /// Similar to getOrCreateFile, but will replace a current CppFile for \p File
   /// with a new one if CompileCommand, provided by \p CDB has changed.
   /// If a currently stored CppFile had to be replaced, the previous instance
   /// will be returned in RecreateResult.RemovedFile.
   RecreateResult recreateFileIfCompileCommandChanged(
       PathRef File, PathRef ResourceDir, GlobalCompilationDatabase &CDB,
-      bool StorePreamblesInMemory, std::shared_ptr<PCHContainerOperations> PCHs,
-      clangd::Logger &Logger);
+      bool StorePreamblesInMemory,
+      std::shared_ptr<PCHContainerOperations> PCHs);
 
   std::shared_ptr<CppFile> getFile(PathRef File) {
     std::lock_guard<std::mutex> Lock(Mutex);
 
     auto It = OpenedFiles.find(File);
     if (It == OpenedFiles.end())
       return nullptr;
     return It->second;

clangd/ClangdUnitStore.cpp

std::shared_ptr<CppFile> CppFileCollection::removeIfPresent(PathRef File) {
   std::shared_ptr<CppFile> Result = It->second;
   OpenedFiles.erase(It);
   return Result;
 }
 
 CppFileCollection::RecreateResult
 CppFileCollection::recreateFileIfCompileCommandChanged(
     PathRef File, PathRef ResourceDir, GlobalCompilationDatabase &CDB,
-    bool StorePreamblesInMemory, std::shared_ptr<PCHContainerOperations> PCHs,
-    clangd::Logger &Logger) {
+    bool StorePreamblesInMemory, std::shared_ptr<PCHContainerOperations> PCHs) {
   auto NewCommand = getCompileCommand(CDB, File, ResourceDir);
 
   std::lock_guard<std::mutex> Lock(Mutex);
 
   RecreateResult Result;
 
   auto It = OpenedFiles.find(File);
   if (It == OpenedFiles.end()) {
     It = OpenedFiles
              .try_emplace(File, CppFile::Create(File, std::move(NewCommand),
                                                 StorePreamblesInMemory,
-                                                std::move(PCHs), Logger))
+                                                std::move(PCHs)))
              .first;
   } else if (!compileCommandsAreEqual(It->second->getCompileCommand(),
                                       NewCommand)) {
     Result.RemovedFile = std::move(It->second);
-    It->second =
-        CppFile::Create(File, std::move(NewCommand), StorePreamblesInMemory,
-                        std::move(PCHs), Logger);
+    It->second = CppFile::Create(File, std::move(NewCommand),
+                                 StorePreamblesInMemory, std::move(PCHs));
   }
   Result.FileInCollection = It->second;
   return Result;
 }
 
 tooling::CompileCommand
 CppFileCollection::getCompileCommand(GlobalCompilationDatabase &CDB,
                                      PathRef File, PathRef ResourceDir) {

clangd/CodeComplete.h

 // Code completion provides suggestions for what the user might type next.
 // After "std::string S; S." we might suggest members of std::string.
 // Signature help describes the parameters of a function as you type them.
 //
 //===---------------------------------------------------------------------===//
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_CODECOMPLETE_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_CODECOMPLETE_H
 
+#include "Context.h"
 #include "Logger.h"
 #include "Path.h"
 #include "Protocol.h"
 #include "clang/Frontend/PrecompiledPreamble.h"
 #include "clang/Sema/CodeCompleteOptions.h"
 #include "clang/Tooling/CompilationDatabase.h"
 
 namespace clang {
struct CodeCompleteOptions {
   bool IncludeIneligibleResults = false;
 
   /// Limit the number of results returned (0 means no limit).
   /// If more results are available, we set CompletionList.isIncomplete.
   size_t Limit = 0;
 };
 
 /// Get code completions at a specified \p Pos in \p FileName.
-CompletionList codeComplete(PathRef FileName,
+CompletionList codeComplete(const Context &Ctx, PathRef FileName,
                             const tooling::CompileCommand &Command,
                             PrecompiledPreamble const *Preamble,
                             StringRef Contents, Position Pos,
                             IntrusiveRefCntPtr<vfs::FileSystem> VFS,
                             std::shared_ptr<PCHContainerOperations> PCHs,
-                            CodeCompleteOptions Opts, Logger &Logger);
+                            CodeCompleteOptions Opts);
 
 /// Get signature help at a specified \p Pos in \p FileName.
-SignatureHelp
-signatureHelp(PathRef FileName, const tooling::CompileCommand &Command,
-              PrecompiledPreamble const *Preamble, StringRef Contents,
-              Position Pos, IntrusiveRefCntPtr<vfs::FileSystem> VFS,
-              std::shared_ptr<PCHContainerOperations> PCHs, Logger &Logger);
+SignatureHelp signatureHelp(const Context &Ctx, PathRef FileName,
+                            const tooling::CompileCommand &Command,
+                            PrecompiledPreamble const *Preamble,
+                            StringRef Contents, Position Pos,
+                            IntrusiveRefCntPtr<vfs::FileSystem> VFS,
+                            std::shared_ptr<PCHContainerOperations> PCHs);
 
 } // namespace clangd
 } // namespace clang
 
 #endif

clangd/CodeComplete.cpp

private:
   }
 
   SignatureHelp &SigHelp;
   std::shared_ptr<clang::GlobalCodeCompletionAllocator> Allocator;
   CodeCompletionTUInfo CCTUInfo;
 
 }; // SignatureHelpCollector
 
-bool invokeCodeComplete(std::unique_ptr<CodeCompleteConsumer> Consumer,
+bool invokeCodeComplete(const Context &Ctx,
+                        std::unique_ptr<CodeCompleteConsumer> Consumer,
                         const clang::CodeCompleteOptions &Options,
                         PathRef FileName,
                         const tooling::CompileCommand &Command,
                         PrecompiledPreamble const *Preamble, StringRef Contents,
                         Position Pos, IntrusiveRefCntPtr<vfs::FileSystem> VFS,
-                        std::shared_ptr<PCHContainerOperations> PCHs,
-                        Logger &Logger) {
+                        std::shared_ptr<PCHContainerOperations> PCHs) {
   std::vector<const char *> ArgStrs;
   for (const auto &S : Command.CommandLine)
     ArgStrs.push_back(S.c_str());
 
   VFS->setCurrentWorkingDirectory(Command.Directory);
 
   IgnoreDiagnostics DummyDiagsConsumer;
   auto CI = createInvocationFromCommandLine(
bool invokeCodeComplete(const Context &Ctx,
   FrontendOpts.CodeCompletionAt.FileName = FileName;
   FrontendOpts.CodeCompletionAt.Line = Pos.line + 1;
   FrontendOpts.CodeCompletionAt.Column = Pos.character + 1;
 
   Clang->setCodeCompletionConsumer(Consumer.release());
 
   SyntaxOnlyAction Action;
   if (!Action.BeginSourceFile(*Clang, Clang->getFrontendOpts().Inputs[0])) {
-    Logger.log("BeginSourceFile() failed when running codeComplete for " +
-               FileName);
+    log(Ctx,
+        "BeginSourceFile() failed when running codeComplete for " + FileName);
     return false;
   }
   if (!Action.Execute()) {
-    Logger.log("Execute() failed when running codeComplete for " + FileName);
+    log(Ctx, "Execute() failed when running codeComplete for " + FileName);
     return false;
   }
 
   Action.EndSourceFile();
 
   return true;
 }
 
 } // namespace
 
 clang::CodeCompleteOptions CodeCompleteOptions::getClangCompleteOpts() const {
   clang::CodeCompleteOptions Result;
   Result.IncludeCodePatterns = EnableSnippets && IncludeCodePatterns;
   Result.IncludeMacros = IncludeMacros;
   Result.IncludeGlobals = IncludeGlobals;
   Result.IncludeBriefComments = IncludeBriefComments;
 
   return Result;
 }
 
-CompletionList codeComplete(PathRef FileName,
+CompletionList codeComplete(const Context &Ctx, PathRef FileName,
                             const tooling::CompileCommand &Command,
                             PrecompiledPreamble const *Preamble,
                             StringRef Contents, Position Pos,
                             IntrusiveRefCntPtr<vfs::FileSystem> VFS,
                             std::shared_ptr<PCHContainerOperations> PCHs,
-                            CodeCompleteOptions Opts, Logger &Logger) {
+                            CodeCompleteOptions Opts) {
   CompletionList Results;
   std::unique_ptr<CodeCompleteConsumer> Consumer;
   if (Opts.EnableSnippets) {
     Consumer =
         llvm::make_unique<SnippetCompletionItemsCollector>(Opts, Results);
   } else {
     Consumer =
         llvm::make_unique<PlainTextCompletionItemsCollector>(Opts, Results);
   }
-  invokeCodeComplete(std::move(Consumer), Opts.getClangCompleteOpts(), FileName,
-                     Command, Preamble, Contents, Pos, std::move(VFS),
-                     std::move(PCHs), Logger);
+  invokeCodeComplete(Ctx, std::move(Consumer), Opts.getClangCompleteOpts(),
+                     FileName, Command, Preamble, Contents, Pos, std::move(VFS),
+                     std::move(PCHs));
   return Results;
 }
 
-SignatureHelp
-signatureHelp(PathRef FileName, const tooling::CompileCommand &Command,
-              PrecompiledPreamble const *Preamble, StringRef Contents,
-              Position Pos, IntrusiveRefCntPtr<vfs::FileSystem> VFS,
-              std::shared_ptr<PCHContainerOperations> PCHs, Logger &Logger) {
+SignatureHelp signatureHelp(const Context &Ctx, PathRef FileName,
+                            const tooling::CompileCommand &Command,
+                            PrecompiledPreamble const *Preamble,
+                            StringRef Contents, Position Pos,
+                            IntrusiveRefCntPtr<vfs::FileSystem> VFS,
+                            std::shared_ptr<PCHContainerOperations> PCHs) {
   SignatureHelp Result;
   clang::CodeCompleteOptions Options;
   Options.IncludeGlobals = false;
   Options.IncludeMacros = false;
   Options.IncludeCodePatterns = false;
   Options.IncludeBriefComments = true;
-  invokeCodeComplete(llvm::make_unique<SignatureHelpCollector>(Options, Result),
+  invokeCodeComplete(Ctx,
+                     llvm::make_unique<SignatureHelpCollector>(Options, Result),
                      Options, FileName, Command, Preamble, Contents, Pos,
-                     std::move(VFS), std::move(PCHs), Logger);
+                     std::move(VFS), std::move(PCHs));
   return Result;
 }
 
 } // namespace clangd
 } // namespace clang

clangd/Function.h

public:
   UniqueFunction(std::nullptr_t) : UniqueFunction(){};
 
   UniqueFunction(UniqueFunction const &) = delete;
   UniqueFunction &operator=(UniqueFunction const &) = delete;
 
   UniqueFunction(UniqueFunction &&) noexcept = default;
   UniqueFunction &operator=(UniqueFunction &&) noexcept = default;
 
-  template <class Callable>
+  template <class Callable,
+            /// A sfinae-check that Callable can be called with Args... and
+            class = typename std::enable_if<std::is_convertible<
+                decltype(std::declval<Callable>()(std::declval<Args>()...)),
+                Ret>::value>::type>
   UniqueFunction(Callable &&Func)
       : CallablePtr(llvm::make_unique<
                     FunctionCallImpl<typename std::decay<Callable>::type>>(
             std::forward<Callable>(Func))) {}
 
   operator bool() { return CallablePtr; }
 
   Ret operator()(Args... As) {

 /// The returned object must be called no more than once, as \p As are
 /// std::forwarded'ed (therefore can be moved) into \p F during the call.
 template <class Func, class... Args>
 ForwardBinder<Func, Args...> BindWithForward(Func F, Args &&... As) {
   return ForwardBinder<Func, Args...>(
       std::make_tuple(std::forward<Func>(F), std::forward<Args>(As)...));
 }
 
+namespace detail {
+/// Runs provided callback in destructor. Use onScopeExit helper function to
+/// create this object.
+template <class Func> struct ScopeExitGuard {
+  static_assert(std::is_same<typename std::decay<Func>::type, Func>::value,
+                "Func must be decayed");
+
+  ScopeExitGuard(Func F) : F(std::move(F)) {}
+  ~ScopeExitGuard() {
+    if (!F)
+      return;
+    (*F)();
+  }
+
+  // Move-only.
+  ScopeExitGuard(const ScopeExitGuard &) = delete;
+  ScopeExitGuard &operator=(const ScopeExitGuard &) = delete;
+
+  ScopeExitGuard(ScopeExitGuard &&Other) = default;
+  ScopeExitGuard &operator=(ScopeExitGuard &&Other) = default;
+
+private:
+  llvm::Optional<Func> F;
+};
+} // namespace detail
+
+/// Creates a RAII object that will run \p F in its destructor.
+template <class Func>
+auto onScopeExit(Func &&F)
+    -> detail::ScopeExitGuard<typename std::decay<Func>::type> {
+  return detail::ScopeExitGuard<typename std::decay<Func>::type>(
+      std::forward<Func>(F));
+}
+
 } // namespace clangd
 } // namespace clang
 
 #endif

clangd/GlobalCompilationDatabase.h

 
 /// Provides compilation arguments used for parsing C and C++ files.
 class GlobalCompilationDatabase {
 public:
   virtual ~GlobalCompilationDatabase() = default;
 
   /// If there are any known-good commands for building this file, returns one.
   virtual llvm::Optional<tooling::CompileCommand>
   getCompileCommand(PathRef File) const = 0;
 
   /// Makes a guess at how to build a file.
   /// The default implementation just runs clang on the file.
   /// Clangd should treat the results as unreliable.
   virtual tooling::CompileCommand getFallbackCommand(PathRef File) const;
 
   /// FIXME(ibiryukov): add facilities to track changes to compilation flags of
   /// existing targets.
 };
 
 /// Gets compile args from tooling::CompilationDatabases built for parent
 /// directories.
 class DirectoryBasedGlobalCompilationDatabase
     : public GlobalCompilationDatabase {
 public:
   DirectoryBasedGlobalCompilationDatabase(
-      clangd::Logger &Logger, llvm::Optional<Path> CompileCommandsDir);
+      llvm::Optional<Path> CompileCommandsDir);
 
   /// Scans File's parents looking for compilation databases.
   /// Any extra flags will be added.
   llvm::Optional<tooling::CompileCommand>
   getCompileCommand(PathRef File) const override;
 
   /// Uses the default fallback command, adding any extra flags.
   tooling::CompileCommand getFallbackCommand(PathRef File) const override;
private:
   mutable std::mutex Mutex;
   /// Caches compilation databases loaded from directories(keys are
   /// directories).
   mutable llvm::StringMap<std::unique_ptr<clang::tooling::CompilationDatabase>>
       CompilationDatabases;
 
   /// Stores extra flags per file.
   llvm::StringMap<std::vector<std::string>> ExtraFlagsForFile;
-  /// Used for logging.
-  clangd::Logger &Logger;
   /// Used for command argument pointing to folder where compile_commands.json
   /// is located.
   llvm::Optional<Path> CompileCommandsDir;
 };
 } // namespace clangd
 } // namespace clang
 
 #endif

clangd/GlobalCompilationDatabase.cpp

GlobalCompilationDatabase::getFallbackCommand(PathRef File) const {
   return tooling::CompileCommand(llvm::sys::path::parent_path(File),
                                  llvm::sys::path::filename(File),
                                  {"clang", File.str()},
                                  /*Output=*/"");
 }
 
 DirectoryBasedGlobalCompilationDatabase::
     DirectoryBasedGlobalCompilationDatabase(
-        clangd::Logger &Logger, llvm::Optional<Path> CompileCommandsDir)
-    : Logger(Logger), CompileCommandsDir(std::move(CompileCommandsDir)) {}
+        llvm::Optional<Path> CompileCommandsDir)
+    : CompileCommandsDir(std::move(CompileCommandsDir)) {}
 
 llvm::Optional<tooling::CompileCommand>
 DirectoryBasedGlobalCompilationDatabase::getCompileCommand(PathRef File) const {
   if (auto CDB = getCompilationDatabase(File)) {
     auto Candidates = CDB->getCompileCommands(File);
     if (!Candidates.empty()) {
       addExtraFlags(File, Candidates.front());
       return std::move(Candidates.front());

 DirectoryBasedGlobalCompilationDatabase::getCompilationDatabase(
     PathRef File) const {
   std::lock_guard<std::mutex> Lock(Mutex);
 
   namespace path = llvm::sys::path;
   if (CompileCommandsDir.hasValue()) {
     tooling::CompilationDatabase *ReturnValue =
         tryLoadDatabaseFromPath(CompileCommandsDir.getValue());
-    if (ReturnValue == nullptr)
-      Logger.log("Failed to find compilation database for " + Twine(File) +
-                 "in overriden directory " + CompileCommandsDir.getValue());
+    if (ReturnValue == nullptr) {
+      // FIXME(ibiryukov): pass a proper Context here.
+      log(Context::empty(), "Failed to find compilation database for " +
+                                Twine(File) + "in overriden directory " +
+                                CompileCommandsDir.getValue());
+    }
     return ReturnValue;
   }
 
   for (auto Path = path::parent_path(File); !Path.empty();
        Path = path::parent_path(Path)) {
     auto CDB = tryLoadDatabaseFromPath(Path);
     if (!CDB)
       continue;
     // FIXME(ibiryukov): Invalidate cached compilation databases on changes
     return CDB;
   }
 
-  Logger.log("Failed to find compilation database for " + Twine(File));
+  // FIXME(ibiryukov): pass a proper Context here.
+  log(Context::empty(),
+      "Failed to find compilation database for " + Twine(File));
   return nullptr;
 }
 
 } // namespace clangd
 } // namespace clang

clangd/JSONRPCDispatcher.h

 //===--- JSONRPCDispatcher.h - Main JSON parser entry point -----*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_JSONRPCDISPATCHER_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_JSONRPCDISPATCHER_H
 
+#include "Context.h"
 #include "JSONExpr.h"
 #include "Logger.h"
 #include "Protocol.h"
 #include "Trace.h"
 #include "clang/Basic/LLVM.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringMap.h"
 #include <iosfwd>
 #include <mutex>
 
 namespace clang {
 namespace clangd {
 
 /// Encapsulates output and logs streams and provides thread-safe access to
 /// them.
 class JSONOutput : public Logger {
+  // FIXME(ibiryukov): figure out if we can shrink the public interface of
+  // JSONOutput now that we pass Context everywhere.
 public:
   JSONOutput(llvm::raw_ostream &Outs, llvm::raw_ostream &Logs,
              llvm::raw_ostream *InputMirror = nullptr, bool Pretty = false)
       : Pretty(Pretty), Outs(Outs), Logs(Logs), InputMirror(InputMirror) {}
 
   /// Emit a JSONRPC message.
   void writeMessage(const json::Expr &Result);
 
   /// Write a line to the logging stream.
-  void log(const Twine &Message) override;
+  void log(const Context &Ctx, const Twine &Message) override;
 
   /// Mirror \p Message into InputMirror stream. Does nothing if InputMirror is
   /// null.
   /// Unlike other methods of JSONOutput, mirrorInput is not thread-safe.
   void mirrorInput(const Twine &Message);
 
   // Whether output should be pretty-printed.
   const bool Pretty;
 
 private:
   llvm::raw_ostream &Outs;
   llvm::raw_ostream &Logs;
   llvm::raw_ostream *InputMirror;
 
   std::mutex StreamMutex;
 };
 
-/// Context object passed to handlers to allow replies.
-class RequestContext {
-public:
-  RequestContext(JSONOutput &Out, StringRef Method,
-                 llvm::Optional<json::Expr> ID)
-      : Out(Out), ID(std::move(ID)),
-        Tracer(llvm::make_unique<trace::Span>(Method)) {
-    if (this->ID)
-      SPAN_ATTACH(tracer(), "ID", *this->ID);
-  }
-
-  /// Sends a successful reply.
-  void reply(json::Expr &&Result);
-  /// Sends an error response to the client, and logs it.
-  void replyError(ErrorCode code, const llvm::StringRef &Message);
-  /// Sends a request to the client.
-  void call(llvm::StringRef Method, json::Expr &&Params);
-
-  trace::Span &tracer() { return *Tracer; }
-
-private:
-  JSONOutput &Out;
-  llvm::Optional<json::Expr> ID;
-  std::unique_ptr<trace::Span> Tracer;
-};
+/// Sends a successful reply. \p Ctx must either be the Context accepted by
+/// JSONRPCDispatcher::Handler or be derived from it.
+void reply(const Context &Ctx, json::Expr &&Result);
+/// Sends an error response to the client, and logs it. \p Ctx must either be
+/// the Context accepted by JSONRPCDispatcher::Handler or be derived from it.
+void replyError(const Context &Ctx, ErrorCode code,
+                const llvm::StringRef &Message);
+/// Sends a request to the client. \p Ctx must either be the Context accepted by
+/// JSONRPCDispatcher::Handler or be derived from it.
+void call(const Context &Ctx, llvm::StringRef Method, json::Expr &&Params);
 
 /// Main JSONRPC entry point. This parses the JSONRPC "header" and calls the
 /// registered Handler for the method received.
 class JSONRPCDispatcher {
 public:
   // A handler responds to requests for a particular method name.
-  using Handler = std::function<void(RequestContext, const json::Expr &)>;
+  using Handler = std::function<void(Context, const json::Expr &)>;
 
   /// Create a new JSONRPCDispatcher. UnknownHandler is called when an unknown
   /// method is received.
   JSONRPCDispatcher(Handler UnknownHandler)
       : UnknownHandler(std::move(UnknownHandler)) {}
 
   /// Registers a Handler for the specified Method.
   void registerHandler(StringRef Method, Handler H);

clangd/JSONRPCDispatcher.cpp

 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/SourceMgr.h"
 #include <istream>
 
 using namespace clang;
 using namespace clangd;
 
+namespace {
+static Key<std::unique_ptr<trace::Span>> RequestSpan;
+static Key<json::Expr> RequestID;
+static Key<JSONOutput *> RequestOut;
+} // namespace
+
 void JSONOutput::writeMessage(const json::Expr &Message) {
   std::string S;
   llvm::raw_string_ostream OS(S);
   if (Pretty)
     OS << llvm::formatv("{0:2}", Message);
   else
     OS << Message;
   OS.flush();
 
   std::lock_guard<std::mutex> Guard(StreamMutex);
   // Log without headers.
   Logs << "--> " << S << '\n';
   Logs.flush();
 
   // Emit message with header.
   Outs << "Content-Length: " << S.size() << "\r\n\r\n" << S;
   Outs.flush();
 }
 
-void JSONOutput::log(const Twine &Message) {
+void JSONOutput::log(const Context &Ctx, const Twine &Message) {
   trace::log(Message);
   std::lock_guard<std::mutex> Guard(StreamMutex);
   Logs << Message << '\n';
   Logs.flush();
 }
 
 void JSONOutput::mirrorInput(const Twine &Message) {
   if (!InputMirror)
     return;
 
   *InputMirror << Message;
   InputMirror->flush();
 }
 
-void RequestContext::reply(json::Expr &&Result) {
+void clangd::reply(const Context &Ctx, json::Expr &&Result) {
+  auto ID = Ctx.get(RequestID);
   if (!ID) {
-    Out.log("Attempted to reply to a notification!");
+    log(Ctx, "Attempted to reply to a notification!");
     return;
   }
-  SPAN_ATTACH(tracer(), "Reply", Result);
-  Out.writeMessage(json::obj{
+
+  if (auto *Span = Ctx.get(RequestSpan))
+    SPAN_ATTACH(**Span, "Reply", Result);
+
+  Ctx.getExisting(RequestOut)
+      ->writeMessage(json::obj{
           {"jsonrpc", "2.0"},
           {"id", *ID},
           {"result", std::move(Result)},
       });
 }
 
-void RequestContext::replyError(ErrorCode code,
-                                const llvm::StringRef &Message) {
-  Out.log("Error " + Twine(static_cast<int>(code)) + ": " + Message);
-  SPAN_ATTACH(tracer(), "Error",
+void clangd::replyError(const Context &Ctx, ErrorCode code,
+  log(Ctx, "Error " + Twine(static_cast<int>(code)) + ": " + Message);
+  if (auto *Span = Ctx.get(RequestSpan))
+    SPAN_ATTACH(**Span, "Error",
                 (json::obj{{"code", static_cast<int>(code)},
                            {"message", Message.str()}}));
-  if (ID) {
-    Out.writeMessage(json::obj{
+
+  if (auto ID = Ctx.get(RequestID)) {
+    Ctx.getExisting(RequestOut)
+        ->writeMessage(json::obj{
             {"jsonrpc", "2.0"},
             {"id", *ID},
             {"error",
              json::obj{{"code", static_cast<int>(code)}, {"message", Message}}},
         });
   }
 }
 
-void RequestContext::call(StringRef Method, json::Expr &&Params) {
+void clangd::call(const Context &Ctx, StringRef Method, json::Expr &&Params) {
   // FIXME: Generate/Increment IDs for every request so that we can get proper
   // replies once we need to.
-  SPAN_ATTACH(tracer(), "Call",
+  if (auto *Span = Ctx.get(RequestSpan))
+    SPAN_ATTACH(**Span, "Call",
                 (json::obj{{"method", Method.str()}, {"params", Params}}));
-  Out.writeMessage(json::obj{
+  Ctx.getExisting(RequestOut)
+      ->writeMessage(json::obj{
           {"jsonrpc", "2.0"},
           {"id", 1},
           {"method", Method},
           {"params", std::move(Params)},
       });
 }
 
 void JSONRPCDispatcher::registerHandler(StringRef Method, Handler H) {
   assert(!Handlers.count(Method) && "Handler already registered!");
   Handlers[Method] = std::move(H);
 }
 
 bool JSONRPCDispatcher::call(const json::Expr &Message, JSONOutput &Out) const {
if (!Method)
     return false;
   // Params should be given, use null if not.
   json::Expr Params = nullptr;
   if (auto *P = Object->get("params"))
     Params = std::move(*P);
 
   auto I = Handlers.find(*Method);
   auto &Handler = I != Handlers.end() ? I->second : UnknownHandler;
-  RequestContext Ctx(Out, *Method, std::move(ID));
-  SPAN_ATTACH(Ctx.tracer(), "Params", Params);
+
+  auto Tracer = llvm::make_unique<trace::Span>(*Method);
+  if (ID)
+    SPAN_ATTACH(*Tracer, "ID", *ID);
+  SPAN_ATTACH(*Tracer, "Params", Params);
+
+  auto Ctx = Context::empty()
+                 .derive(RequestOut, &Out)
+                 .derive(RequestSpan, std::move(Tracer));
+  if (ID)
+    Ctx = std::move(Ctx).derive(RequestID, *ID);
+
   Handler(std::move(Ctx), std::move(Params));
   return true;
 }
 
 void clangd::runLanguageServerLoop(std::istream &In, JSONOutput &Out,
                                    JSONRPCDispatcher &Dispatcher,
                                    bool &IsDone) {
   while (In.good()) {
while (In.good()) {
       // Content-Type is a specified header, but does nothing.
       // Content-Length is a mandatory header. It specifies the length of the
       // following JSON.
       // It is unspecified what sequence headers must be supplied in, so we
       // allow any sequence.
       // The end of headers is signified by an empty line.
       if (LineRef.consume_front("Content-Length: ")) {
         if (ContentLength != 0) {
-          Out.log("Warning: Duplicate Content-Length header received. "
-                  "The previous value for this message (" +
-                  std::to_string(ContentLength) + ") was ignored.");
+          log(Context::empty(),
+              "Warning: Duplicate Content-Length header received. "
+                  std::to_string(ContentLength) + ") was ignored.\n");
         }
 
         llvm::getAsUnsignedInteger(LineRef.trim(), 0, ContentLength);
         continue;
       } else if (!LineRef.trim().empty()) {
         // It's another header, ignore it.
         continue;
       } else {
         // An empty line indicates the end of headers.
         // Go ahead and read the JSON.
         break;
       }
     }
 
     // Guard against large messages. This is usually a bug in the client code
     // and we don't want to crash downstream because of it.
     if (ContentLength > 1 << 30) { // 1024M
       In.ignore(ContentLength);
-      Out.log("Skipped overly large message of " + Twine(ContentLength) +
-              " bytes.");
+      log(Context::empty(), "Skipped overly large message of " +
+                                Twine(ContentLength) + " bytes.\n");
       continue;
     }
 
     if (ContentLength > 0) {
       std::vector<char> JSON(ContentLength);
       llvm::StringRef JSONRef;
       {
         In.read(JSON.data(), ContentLength);
         Out.mirrorInput(StringRef(JSON.data(), In.gcount()));
 
         // If the stream is aborted before we read ContentLength bytes, In
         // will have eofbit and failbit set.
         if (!In) {
-          Out.log("Input was aborted. Read only " +
-                  std::to_string(In.gcount()) + " bytes of expected " +
-                  std::to_string(ContentLength) + ".");
+          log(Context::empty(), "Input was aborted. Read only " +
+                                    std::to_string(In.gcount()) +
+                                    " bytes of expected " +
+                                    std::to_string(ContentLength) + ".\n");
           break;
         }
 
         JSONRef = StringRef(JSON.data(), ContentLength);
       }
 
       if (auto Doc = json::parse(JSONRef)) {
         // Log the formatted message.
-        Out.log(llvm::formatv(Out.Pretty ? "<-- {0:2}" : "<-- {0}", *Doc));
+        log(Context::empty(),
+            llvm::formatv(Out.Pretty ? "<-- {0:2}\n" : "<-- {0}\n", *Doc));
         // Finally, execute the action for this JSON message.
         if (!Dispatcher.call(*Doc, Out))
-          Out.log("JSON dispatch failed!");
+          log(Context::empty(), "JSON dispatch failed!\n");
       } else {
         // Parse error. Log the raw message.
-        Out.log("<-- " + JSONRef);
-        Out.log(llvm::Twine("JSON parse error: ") +
-                llvm::toString(Doc.takeError()));
+        log(Context::empty(), "<-- " + JSONRef + "\n");
+        log(Context::empty(), llvm::Twine("JSON parse error: ") +
+                                  llvm::toString(Doc.takeError()) + "\n");
       }
 
       // If we're done, exit the loop.
       if (IsDone)
         break;
     } else {
-      Out.log("Warning: Missing Content-Length header, or message has zero "
-              "length.");
+      log(Context::empty(),
+          "Warning: Missing Content-Length header, or message has zero "
+          "length.\n");
     }
   }
 }

clangd/Logger.h

 //===--- Logger.h - Logger interface for clangd ------------------*- C++-*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_LOGGER_H
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_LOGGER_H
 
+#include "Context.h"
 #include "llvm/ADT/Twine.h"
 
 namespace clang {
 namespace clangd {
 
+/// Main logging function. Logs messages to a global logger, which can be set up
+/// by LoggingSesssion.
+void log(const Context &Ctx, const llvm::Twine &Message);
+
 /// Interface to allow custom logging in clangd.
 class Logger {
 public:
   virtual ~Logger() = default;
 
   /// Implementations of this method must be thread-safe.
-  virtual void log(const llvm::Twine &Message) = 0;
+  virtual void log(const Context &Ctx, const llvm::Twine &Message) = 0;
 };
 
-/// Logger implementation that ignores all messages.
-class EmptyLogger : public Logger {
+/// Only one LoggingSession can be active at a time.
+class LoggingSession {
 public:
-  static EmptyLogger &getInstance();
+  LoggingSession(clangd::Logger &Instance);
+  ~LoggingSession();
 
-  void log(const llvm::Twine &Message) override;
+  LoggingSession(LoggingSession &&) = delete;
+  LoggingSession &operator=(LoggingSession &&) = delete;
 
-private:
-  EmptyLogger() = default;
+  LoggingSession(LoggingSession const &) = delete;
+  LoggingSession &operator=(LoggingSession const &) = delete;
 };
 
 } // namespace clangd
 } // namespace clang
 
 #endif

clangd/Logger.cpp

 //===--- Logger.cpp - Logger interface for clangd -------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "Logger.h"
 
-using namespace clang::clangd;
+namespace clang {
+namespace clangd {
 
-EmptyLogger &EmptyLogger::getInstance() {
-  static EmptyLogger Logger;
-  return Logger;
+namespace {
+Logger *L = nullptr;
+} // namespace
+
+LoggingSession::LoggingSession(clangd::Logger &Instance) {
+  assert(!L);
+  L = &Instance;
+}
+
+LoggingSession::~LoggingSession() { L = nullptr; }
+
+void log(const Context &Ctx, const llvm::Twine &Message) {
+  if (!L)
+    return;
+  L->log(Ctx, Message);
 }
 
-void EmptyLogger::log(const llvm::Twine &Message) {}
+} // namespace clangd
+} // namespace clang

clangd/ProtocolHandlers.h

 #include "llvm/Support/raw_ostream.h"
 
 namespace clang {
 namespace clangd {
 
 // The interface implemented by ClangLSPServer to handle incoming requests.
 class ProtocolCallbacks {
 public:
-  using Ctx = RequestContext;
+  // FIXME(ibiryukov): remove this typedef, inline its usages.
+  using Ctx = Context;
   virtual ~ProtocolCallbacks() = default;
 
   virtual void onInitialize(Ctx C, InitializeParams &Params) = 0;
   virtual void onShutdown(Ctx C, ShutdownParams &Params) = 0;
   virtual void onExit(Ctx C, ExitParams &Params) = 0;
   virtual void onDocumentDidOpen(Ctx C, DidOpenTextDocumentParams &Params) = 0;
   virtual void onDocumentDidChange(Ctx C,
                                    DidChangeTextDocumentParams &Params) = 0;

clangd/ProtocolHandlers.cpp

 
 // Helper for attaching ProtocolCallbacks methods to a JSONRPCDispatcher.
 // Invoke like: Registerer("foo", &ProtocolCallbacks::onFoo)
 // onFoo should be: void onFoo(Ctx &C, FooParams &Params)
 // FooParams should have a fromJSON function.
 struct HandlerRegisterer {
   template <typename Param>
   void operator()(StringRef Method,
-                  void (ProtocolCallbacks::*Handler)(RequestContext, Param)) {
+                  void (ProtocolCallbacks::*Handler)(Context, Param)) {
     // Capture pointers by value, as the lambda will outlive this object.
-    auto *Out = this->Out;
     auto *Callbacks = this->Callbacks;
     Dispatcher.registerHandler(
-        Method, [=](RequestContext C, const json::Expr &RawParams) {
+        Method, [=](Context C, const json::Expr &RawParams) {
           typename std::remove_reference<Param>::type P;
           if (fromJSON(RawParams, P)) {
             (Callbacks->*Handler)(std::move(C), P);
           } else {
-            Out->log("Failed to decode " + Method + " request.");
+            log(C, "Failed to decode " + Method + " request.");
           }
         });
   }
 
   JSONRPCDispatcher &Dispatcher;
   JSONOutput *Out;
   ProtocolCallbacks *Callbacks;
 };

clangd/tool/ClangdMain.cpp

int main(int argc, char *argv[]) {
   }
 
   llvm::raw_ostream &Outs = llvm::outs();
   llvm::raw_ostream &Logs = llvm::errs();
   JSONOutput Out(Outs, Logs,
                  InputMirrorStream ? InputMirrorStream.getPointer() : nullptr,
                  PrettyPrint);
 
+  clangd::LoggingSession LoggingSession(Out);
+
   // If --compile-commands-dir arg was invoked, check value and override default
   // path.
   llvm::Optional<Path> CompileCommandsDirPath;
 
   if (CompileCommandsDir.empty()) {
     CompileCommandsDirPath = llvm::None;
   } else if (!llvm::sys::path::is_absolute(CompileCommandsDir) ||
              !llvm::sys::fs::exists(CompileCommandsDir)) {

unittests/clangd/ClangdTests.cpp

 //===-- ClangdTests.cpp - Clangd unit tests ---------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "ClangdLSPServer.h"
 #include "ClangdServer.h"
-#include "Logger.h"
+#include "Context.h"
 #include "TestFS.h"
 #include "clang/Config/config.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/Support/Errc.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Regex.h"
 #include "gtest/gtest.h"
 #include <algorithm>
 #include <chrono>
 #include <iostream>
 #include <random>
 #include <string>
 #include <thread>
 #include <vector>
 
 namespace clang {
 namespace clangd {
 namespace {
 
 // Don't wait for async ops in clangd test more than that to avoid blocking
 // indefinitely in case of bugs.
 static const std::chrono::seconds DefaultFutureTimeout =
     std::chrono::seconds(10);
 
 static bool diagsContainErrors(ArrayRef<DiagWithFixIts> Diagnostics) {
   for (const auto &DiagAndFixIts : Diagnostics) {
protected:
   std::string parseSourceAndDumpAST(
       PathRef SourceFileRelPath, StringRef SourceContents,
       std::vector<std::pair<PathRef, StringRef>> ExtraFiles = {},
       bool ExpectErrors = false) {
     MockFSProvider FS;
     ErrorCheckingDiagConsumer DiagConsumer;
     MockCompilationDatabase CDB;
     ClangdServer Server(CDB, DiagConsumer, FS, getDefaultAsyncThreadsCount(),
-                        /*StorePreamblesInMemory=*/true,
-                        EmptyLogger::getInstance());
+                        /*StorePreamblesInMemory=*/true);
     for (const auto &FileWithContents : ExtraFiles)
       FS.Files[getVirtualTestFilePath(FileWithContents.first)] =
           FileWithContents.second;
 
     auto SourceFilename = getVirtualTestFilePath(SourceFileRelPath);
 
     FS.ExpectedFile = SourceFilename;
 
     // Have to sync reparses because requests are processed on the calling
     // thread.
-    auto AddDocFuture = Server.addDocument(SourceFilename, SourceContents);
+    auto AddDocFuture =
+        Server.addDocument(Context::empty(), SourceFilename, SourceContents);
 
     auto Result = dumpASTWithoutMemoryLocs(Server, SourceFilename);
 
     // Wait for reparse to finish before checking for errors.
     EXPECT_EQ(AddDocFuture.wait_for(DefaultFutureTimeout),
               std::future_status::ready);
     EXPECT_EQ(ExpectErrors, DiagConsumer.hadErrorInLastDiags());
     return Result;
parseSourceAndDumpAST("foo.cpp", SourceContents, {{"foo.h", "int a;"}},
                         /*ExpectErrors=*/false);
 }
 
 TEST_F(ClangdVFSTest, Reparse) {
   MockFSProvider FS;
   ErrorCheckingDiagConsumer DiagConsumer;
   MockCompilationDatabase CDB;
   ClangdServer Server(CDB, DiagConsumer, FS, getDefaultAsyncThreadsCount(),
-                      /*StorePreamblesInMemory=*/true,
-                      EmptyLogger::getInstance());
+                      /*StorePreamblesInMemory=*/true);
 
   const auto SourceContents = R"cpp(
 #include "foo.h"
 int b = a;
 )cpp";
 
   auto FooCpp = getVirtualTestFilePath("foo.cpp");
   auto FooH = getVirtualTestFilePath("foo.h");
 
   FS.Files[FooH] = "int a;";
   FS.Files[FooCpp] = SourceContents;
   FS.ExpectedFile = FooCpp;
 
   // To sync reparses before checking for errors.
-  std::future<void> ParseFuture;
+  std::future<Context> ParseFuture;
 
-  ParseFuture = Server.addDocument(FooCpp, SourceContents);
+  ParseFuture = Server.addDocument(Context::empty(), FooCpp, SourceContents);
   auto DumpParse1 = dumpASTWithoutMemoryLocs(Server, FooCpp);
   ASSERT_EQ(ParseFuture.wait_for(DefaultFutureTimeout),
             std::future_status::ready);
   EXPECT_FALSE(DiagConsumer.hadErrorInLastDiags());
 
-  ParseFuture = Server.addDocument(FooCpp, "");
+  ParseFuture = Server.addDocument(Context::empty(), FooCpp, "");
   auto DumpParseEmpty = dumpASTWithoutMemoryLocs(Server, FooCpp);
   ASSERT_EQ(ParseFuture.wait_for(DefaultFutureTimeout),
             std::future_status::ready);
   EXPECT_FALSE(DiagConsumer.hadErrorInLastDiags());
 
-  ParseFuture = Server.addDocument(FooCpp, SourceContents);
+  ParseFuture = Server.addDocument(Context::empty(), FooCpp, SourceContents);
   auto DumpParse2 = dumpASTWithoutMemoryLocs(Server, FooCpp);
   ASSERT_EQ(ParseFuture.wait_for(DefaultFutureTimeout),
             std::future_status::ready);
   EXPECT_FALSE(DiagConsumer.hadErrorInLastDiags());
 
   EXPECT_EQ(DumpParse1, DumpParse2);
   EXPECT_NE(DumpParse1, DumpParseEmpty);
 }
 
 TEST_F(ClangdVFSTest, ReparseOnHeaderChange) {
   MockFSProvider FS;
   ErrorCheckingDiagConsumer DiagConsumer;
   MockCompilationDatabase CDB;
 
   ClangdServer Server(CDB, DiagConsumer, FS, getDefaultAsyncThreadsCount(),
-                      /*StorePreamblesInMemory=*/true,
-                      EmptyLogger::getInstance());
+                      /*StorePreamblesInMemory=*/true);
 
   const auto SourceContents = R"cpp(
 #include "foo.h"
 int b = a;
 )cpp";
 
   auto FooCpp = getVirtualTestFilePath("foo.cpp");
   auto FooH = getVirtualTestFilePath("foo.h");
 
   FS.Files[FooH] = "int a;";
   FS.Files[FooCpp] = SourceContents;
   FS.ExpectedFile = FooCpp;
 
   // To sync reparses before checking for errors.
-  std::future<void> ParseFuture;
+  std::future<Context> ParseFuture;
 
-  ParseFuture = Server.addDocument(FooCpp, SourceContents);
+  ParseFuture = Server.addDocument(Context::empty(), FooCpp, SourceContents);
   auto DumpParse1 = dumpASTWithoutMemoryLocs(Server, FooCpp);
   ASSERT_EQ(ParseFuture.wait_for(DefaultFutureTimeout),
             std::future_status::ready);
   EXPECT_FALSE(DiagConsumer.hadErrorInLastDiags());
 
   FS.Files[FooH] = "";
-  ParseFuture = Server.forceReparse(FooCpp);
+  ParseFuture = Server.forceReparse(Context::empty(), FooCpp);
   auto DumpParseDifferent = dumpASTWithoutMemoryLocs(Server, FooCpp);
   ASSERT_EQ(ParseFuture.wait_for(DefaultFutureTimeout),
             std::future_status::ready);
   EXPECT_TRUE(DiagConsumer.hadErrorInLastDiags());
 
   FS.Files[FooH] = "int a;";
-  ParseFuture = Server.forceReparse(FooCpp);
+  ParseFuture = Server.forceReparse(Context::empty(), FooCpp);
   auto DumpParse2 = dumpASTWithoutMemoryLocs(Server, FooCpp);
   EXPECT_EQ(ParseFuture.wait_for(DefaultFutureTimeout),
             std::future_status::ready);
   EXPECT_FALSE(DiagConsumer.hadErrorInLastDiags());
 
   EXPECT_EQ(DumpParse1, DumpParse2);
   EXPECT_NE(DumpParse1, DumpParseDifferent);
 }
 
 TEST_F(ClangdVFSTest, CheckVersions) {
   MockFSProvider FS;
   ErrorCheckingDiagConsumer DiagConsumer;
   MockCompilationDatabase CDB;
   // Run ClangdServer synchronously.
   ClangdServer Server(CDB, DiagConsumer, FS,
                       /*AsyncThreadsCount=*/0,
-                      /*StorePreamblesInMemory=*/true,
-                      EmptyLogger::getInstance());
+                      /*StorePreamblesInMemory=*/true);
 
   auto FooCpp = getVirtualTestFilePath("foo.cpp");
   const auto SourceContents = "int a;";
   FS.Files[FooCpp] = SourceContents;
   FS.ExpectedFile = FooCpp;
 
   // Use default completion options.
   clangd::CodeCompleteOptions CCOpts;
 
   // No need to sync reparses, because requests are processed on the calling
   // thread.
   FS.Tag = "123";
-  Server.addDocument(FooCpp, SourceContents);
-  EXPECT_EQ(Server.codeComplete(FooCpp, Position{0, 0}, CCOpts).get().Tag,
-            FS.Tag);
+  Server.addDocument(Context::empty(), FooCpp, SourceContents);
+  EXPECT_EQ(
+      Server.codeComplete(Context::empty(), FooCpp, Position{0, 0}, CCOpts)
+          .get()
+          .second.Tag,
   EXPECT_EQ(DiagConsumer.lastVFSTag(), FS.Tag);
 
   FS.Tag = "321";
-  Server.addDocument(FooCpp, SourceContents);
+  Server.addDocument(Context::empty(), FooCpp, SourceContents);
   EXPECT_EQ(DiagConsumer.lastVFSTag(), FS.Tag);
-  EXPECT_EQ(Server.codeComplete(FooCpp, Position{0, 0}, CCOpts).get().Tag,
-            FS.Tag);
+  EXPECT_EQ(
+      Server.codeComplete(Context::empty(), FooCpp, Position{0, 0}, CCOpts)
+          .get()
+          .second.Tag,
 }
 
 // Only enable this test on Unix
 #ifdef LLVM_ON_UNIX
 TEST_F(ClangdVFSTest, SearchLibDir) {
   // Checks that searches for GCC installation is done through vfs.
   MockFSProvider FS;
   ErrorCheckingDiagConsumer DiagConsumer;
   MockCompilationDatabase CDB;
   CDB.ExtraClangFlags.insert(CDB.ExtraClangFlags.end(),
                              {"-xc++", "-target", "x86_64-linux-unknown",
                               "-m64", "--gcc-toolchain=/randomusr",
                               "-stdlib=libstdc++"});
   // Run ClangdServer synchronously.
   ClangdServer Server(CDB, DiagConsumer, FS,
                       /*AsyncThreadsCount=*/0,
-                      /*StorePreamblesInMemory=*/true,
-                      EmptyLogger::getInstance());
+                      /*StorePreamblesInMemory=*/true);
 
   // Just a random gcc version string
   SmallString<8> Version("4.9.3");
 
   // A lib dir for gcc installation
   SmallString<64> LibDir("/randomusr/lib/gcc/x86_64-linux-gnu");
   llvm::sys::path::append(LibDir, Version);
 
TEST_F(ClangdVFSTest, SearchLibDir) {
   const auto SourceContents = R"cpp(
 #include <string>
 mock_string x;
 )cpp";
   FS.Files[FooCpp] = SourceContents;
 
   // No need to sync reparses, because requests are processed on the calling
   // thread.
-  Server.addDocument(FooCpp, SourceContents);
+  Server.addDocument(Context::empty(), FooCpp, SourceContents);
   EXPECT_FALSE(DiagConsumer.hadErrorInLastDiags());
 
   const auto SourceContentsWithError = R"cpp(
 #include <string>
 std::string x;
 )cpp";
-  Server.addDocument(FooCpp, SourceContentsWithError);
+  Server.addDocument(Context::empty(), FooCpp, SourceContentsWithError);
   EXPECT_TRUE(DiagConsumer.hadErrorInLastDiags());
 }
 #endif // LLVM_ON_UNIX
 
 TEST_F(ClangdVFSTest, ForceReparseCompileCommand) {
   MockFSProvider FS;
   ErrorCheckingDiagConsumer DiagConsumer;
   MockCompilationDatabase CDB;
   ClangdServer Server(CDB, DiagConsumer, FS,
                       /*AsyncThreadsCount=*/0,
-                      /*StorePreamblesInMemory=*/true,
-                      EmptyLogger::getInstance());
-  // No need to sync reparses, because reparses are performed on the calling
-  // thread to true.
+                      /*StorePreamblesInMemory=*/true);
 
+  // No need to sync reparses, because reparses are performed on the calling
+  // thread.
   auto FooCpp = getVirtualTestFilePath("foo.cpp");
   const auto SourceContents1 = R"cpp(
 template <class T>
 struct foo { T x; };
 )cpp";
   const auto SourceContents2 = R"cpp(
 template <class T>
 struct bar { T x; };
 )cpp";
 
   FS.Files[FooCpp] = "";
   FS.ExpectedFile = FooCpp;
 
   // First parse files in C mode and check they produce errors.
   CDB.ExtraClangFlags = {"-xc"};
-  Server.addDocument(FooCpp, SourceContents1);
+  Server.addDocument(Context::empty(), FooCpp, SourceContents1);
   EXPECT_TRUE(DiagConsumer.hadErrorInLastDiags());
-  Server.addDocument(FooCpp, SourceContents2);
+  Server.addDocument(Context::empty(), FooCpp, SourceContents2);
   EXPECT_TRUE(DiagConsumer.hadErrorInLastDiags());
 
   // Now switch to C++ mode.
   CDB.ExtraClangFlags = {"-xc++"};
   // Currently, addDocument never checks if CompileCommand has changed, so we
   // expect to see the errors.
-  Server.addDocument(FooCpp, SourceContents1);
+  Server.addDocument(Context::empty(), FooCpp, SourceContents1);
   EXPECT_TRUE(DiagConsumer.hadErrorInLastDiags());
-  Server.addDocument(FooCpp, SourceContents2);
+  Server.addDocument(Context::empty(), FooCpp, SourceContents2);
   EXPECT_TRUE(DiagConsumer.hadErrorInLastDiags());
   // But forceReparse should reparse the file with proper flags.
-  Server.forceReparse(FooCpp);
+  Server.forceReparse(Context::empty(), FooCpp);
   EXPECT_FALSE(DiagConsumer.hadErrorInLastDiags());
   // Subsequent addDocument calls should finish without errors too.
-  Server.addDocument(FooCpp, SourceContents1);
+  Server.addDocument(Context::empty(), FooCpp, SourceContents1);
   EXPECT_FALSE(DiagConsumer.hadErrorInLastDiags());
-  Server.addDocument(FooCpp, SourceContents2);
+  Server.addDocument(Context::empty(), FooCpp, SourceContents2);
   EXPECT_FALSE(DiagConsumer.hadErrorInLastDiags());
 }
 
 class ClangdThreadingTest : public ClangdVFSTest {};
 
 TEST_F(ClangdThreadingTest, StressTest) {
   // Without 'static' clang gives an error for a usage inside TestDiagConsumer.
   static const unsigned FilesCount = 5;
private:
     std::vector<FileStat> Stats;
   };
 
   struct RequestStats {
     unsigned RequestsWithoutErrors = 0;
     unsigned RequestsWithErrors = 0;
     bool LastContentsHadErrors = false;
     bool FileIsRemoved = true;
-    std::future<void> LastRequestFuture;
+    std::future<Context> LastRequestFuture;
   };
 
   std::vector<RequestStats> ReqStats;
   ReqStats.reserve(FilesCount);
   for (unsigned FileIndex = 0; FileIndex < FilesCount; ++FileIndex)
     ReqStats.emplace_back();
 
   TestDiagConsumer DiagConsumer;
   {
     MockCompilationDatabase CDB;
     ClangdServer Server(CDB, DiagConsumer, FS, getDefaultAsyncThreadsCount(),
-                        /*StorePreamblesInMemory=*/true,
-                        EmptyLogger::getInstance());
+                        /*StorePreamblesInMemory=*/true);
 
     // Prepare some random distributions for the test.
     std::random_device RandGen;
 
     std::uniform_int_distribution<unsigned> FileIndexDist(0, FilesCount - 1);
     // Pass a text that contains compiler errors to addDocument in about 20% of
     // all requests.
     std::bernoulli_distribution ShouldHaveErrorsDist(0.2);
     // Line and Column numbers for requests that need them.
     std::uniform_int_distribution<int> LineDist(0, MaxLineForFileRequests);
     std::uniform_int_distribution<int> ColumnDist(0, MaxColumnForFileRequests);
 
     // Some helpers.
     auto UpdateStatsOnAddDocument = [&](unsigned FileIndex, bool HadErrors,
-                                        std::future<void> Future) {
+                                        std::future<Context> Future) {
       auto &Stats = ReqStats[FileIndex];
 
       if (HadErrors)
         ++Stats.RequestsWithErrors;
       else
         ++Stats.RequestsWithoutErrors;
       Stats.LastContentsHadErrors = HadErrors;
       Stats.FileIsRemoved = false;
       Stats.LastRequestFuture = std::move(Future);
     };
 
     auto UpdateStatsOnRemoveDocument = [&](unsigned FileIndex,
-                                           std::future<void> Future) {
+                                           std::future<Context> Future) {
       auto &Stats = ReqStats[FileIndex];
 
       Stats.FileIsRemoved = true;
       Stats.LastRequestFuture = std::move(Future);
     };
 
     auto UpdateStatsOnForceReparse = [&](unsigned FileIndex,
-                                         std::future<void> Future) {
+                                         std::future<Context> Future) {
       auto &Stats = ReqStats[FileIndex];
 
       Stats.LastRequestFuture = std::move(Future);
       if (Stats.LastContentsHadErrors)
         ++Stats.RequestsWithErrors;
       else
         ++Stats.RequestsWithoutErrors;
     };
 
     auto AddDocument = [&](unsigned FileIndex) {
       bool ShouldHaveErrors = ShouldHaveErrorsDist(RandGen);
-      auto Future = Server.addDocument(
-          FilePaths[FileIndex], ShouldHaveErrors ? SourceContentsWithErrors
-                                                 : SourceContentsWithoutErrors);
+      auto Future =
+          Server.addDocument(Context::empty(), FilePaths[FileIndex],
+                             ShouldHaveErrors ? SourceContentsWithErrors
       UpdateStatsOnAddDocument(FileIndex, ShouldHaveErrors, std::move(Future));
     };
 
     // Various requests that we would randomly run.
     auto AddDocumentRequest = [&]() {
       unsigned FileIndex = FileIndexDist(RandGen);
       AddDocument(FileIndex);
     };
 
     auto ForceReparseRequest = [&]() {
       unsigned FileIndex = FileIndexDist(RandGen);
       // Make sure we don't violate the ClangdServer's contract.
       if (ReqStats[FileIndex].FileIsRemoved)
         AddDocument(FileIndex);
 
-      auto Future = Server.forceReparse(FilePaths[FileIndex]);
+      auto Future = Server.forceReparse(Context::empty(), FilePaths[FileIndex]);
       UpdateStatsOnForceReparse(FileIndex, std::move(Future));
     };
 
     auto RemoveDocumentRequest = [&]() {
       unsigned FileIndex = FileIndexDist(RandGen);
       // Make sure we don't violate the ClangdServer's contract.
       if (ReqStats[FileIndex].FileIsRemoved)
         AddDocument(FileIndex);
 
-      auto Future = Server.removeDocument(FilePaths[FileIndex]);
+      auto Future =
+          Server.removeDocument(Context::empty(), FilePaths[FileIndex]);
       UpdateStatsOnRemoveDocument(FileIndex, std::move(Future));
     };
 
     auto CodeCompletionRequest = [&]() {
       unsigned FileIndex = FileIndexDist(RandGen);
       // Make sure we don't violate the ClangdServer's contract.
       if (ReqStats[FileIndex].FileIsRemoved)
         AddDocument(FileIndex);
 
       Position Pos{LineDist(RandGen), ColumnDist(RandGen)};
       // FIXME(ibiryukov): Also test async completion requests.
       // Simply putting CodeCompletion into async requests now would make
       // tests slow, since there's no way to cancel previous completion
       // requests as opposed to AddDocument/RemoveDocument, which are implicitly
       // cancelled by any subsequent AddDocument/RemoveDocument request to the
       // same file.
       Server
-          .codeComplete(FilePaths[FileIndex], Pos,
+          .codeComplete(Context::empty(), FilePaths[FileIndex], Pos,
                         clangd::CodeCompleteOptions())
           .wait();
     };
 
     auto FindDefinitionsRequest = [&]() {
       unsigned FileIndex = FileIndexDist(RandGen);
       // Make sure we don't violate the ClangdServer's contract.
       if (ReqStats[FileIndex].FileIsRemoved)
         AddDocument(FileIndex);
 
       Position Pos{LineDist(RandGen), ColumnDist(RandGen)};
-      ASSERT_TRUE(!!Server.findDefinitions(FilePaths[FileIndex], Pos));
+      ASSERT_TRUE(!!Server.findDefinitions(Context::empty(),
+                                           FilePaths[FileIndex], Pos));
     };
 
     std::vector<std::function<void()>> AsyncRequests = {
         AddDocumentRequest, ForceReparseRequest, RemoveDocumentRequest};
     std::vector<std::function<void()>> BlockingRequests = {
         CodeCompletionRequest, FindDefinitionsRequest};
 
     // Bash requests to ClangdServer in a loop.

 }
 
 TEST_F(ClangdVFSTest, CheckSourceHeaderSwitch) {
   MockFSProvider FS;
   ErrorCheckingDiagConsumer DiagConsumer;
   MockCompilationDatabase CDB;
 
   ClangdServer Server(CDB, DiagConsumer, FS, getDefaultAsyncThreadsCount(),
-                      /*StorePreamblesInMemory=*/true,
-                      EmptyLogger::getInstance());
+                      /*StorePreamblesInMemory=*/true);
 
   auto SourceContents = R"cpp(
   #include "foo.h"
   int b = a;
   )cpp";
 
   auto FooCpp = getVirtualTestFilePath("foo.cpp");
   auto FooH = getVirtualTestFilePath("foo.h");
)cpp";
 
   std::promise<void> StartSecondReparsePromise;
   std::future<void> StartSecondReparse = StartSecondReparsePromise.get_future();
 
   NoConcurrentAccessDiagConsumer DiagConsumer(
       std::move(StartSecondReparsePromise));
 
   MockCompilationDatabase CDB;
-  ClangdServer Server(CDB, DiagConsumer, FS, 4, /*StorePreamblesInMemory=*/true,
-                      EmptyLogger::getInstance());
-  Server.addDocument(FooCpp, SourceContentsWithErrors);
+  ClangdServer Server(CDB, DiagConsumer, FS, 4,
+                      /*StorePreamblesInMemory=*/true);
+  Server.addDocument(Context::empty(), FooCpp, SourceContentsWithErrors);
   StartSecondReparse.wait();
 
-  auto Future = Server.addDocument(FooCpp, SourceContentsWithoutErrors);
+  auto Future =
+      Server.addDocument(Context::empty(), FooCpp, SourceContentsWithoutErrors);
   Future.wait();
 }
 
 } // namespace clangd
 } // namespace clang

unittests/clangd/CodeCompleteTests.cpp

 //===-- CodeCompleteTests.cpp -----------------------------------*- C++ -*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 #include "ClangdServer.h"
 #include "Compiler.h"
 #include "Matchers.h"
+#include "Context.h"
 #include "Protocol.h"
 #include "TestFS.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
 namespace clang {
 namespace clangd {
 // Let GMock print completion items.

 MATCHER(IsDocumented, "") { return !arg.documentation.empty(); }
 
 CompletionList completions(StringRef Text,
                            clangd::CodeCompleteOptions Opts = {}) {
   MockFSProvider FS;
   MockCompilationDatabase CDB;
   IgnoreDiagnostics DiagConsumer;
   ClangdServer Server(CDB, DiagConsumer, FS, getDefaultAsyncThreadsCount(),
-                      /*StorePreamblesInMemory=*/true,
-                      EmptyLogger::getInstance());
+                      /*StorePreamblesInMemory=*/true);
   auto File = getVirtualTestFilePath("foo.cpp");
   auto Test = parseTextMarker(Text);
-  Server.addDocument(File, Test.Text);
-  return Server.codeComplete(File, Test.MarkerPos, Opts).get().Value;
+  Server.addDocument(Context::empty(), File, Test.Text);
+  return Server.codeComplete(Context::empty(), File, Test.MarkerPos, Opts)
+      .get()
+      .second.Value;
 }
 
 TEST(CompletionTest, Limit) {
   clangd::CodeCompleteOptions Opts;
   Opts.Limit = 2;
   auto Results = completions(R"cpp(
 struct ClassWithMembers {
   int AAA();
std::string Body = R"cpp(
     int Abracadabra;
     int Alakazam;
     struct S {
       int FooBar;
       int FooBaz;
       int Qux;
     };
   )cpp";
-
   EXPECT_THAT(completions(Body + "int main() { S().Foba^ }").items,
               AllOf(Has("FooBar"), Has("FooBaz"), Not(Has("Qux"))));
 
   EXPECT_THAT(completions(Body + "int main() { S().FR^ }").items,
               AllOf(Has("FooBar"), Not(Has("FooBaz")), Not(Has("Qux"))));
 
   EXPECT_THAT(completions(Body + "int main() { S().opr^ }").items,
               Has("operator="));

 }
 
 // Check code completion works when the file contents are overridden.
 TEST(CompletionTest, CheckContentsOverride) {
   MockFSProvider FS;
   IgnoreDiagnostics DiagConsumer;
   MockCompilationDatabase CDB;
   ClangdServer Server(CDB, DiagConsumer, FS, getDefaultAsyncThreadsCount(),
-                      /*StorePreamblesInMemory=*/true,
-                      EmptyLogger::getInstance());
+                      /*StorePreamblesInMemory=*/true);
   auto File = getVirtualTestFilePath("foo.cpp");
-  Server.addDocument(File, "ignored text!");
+  Server.addDocument(Context::empty(), File, "ignored text!");
 
   auto Example = parseTextMarker("int cbc; int b = ^;");
   auto Results =
       Server
-          .codeComplete(File, Example.MarkerPos, clangd::CodeCompleteOptions(),
-                        StringRef(Example.Text))
+          .codeComplete(Context::empty(), File, Example.MarkerPos,
+                        clangd::CodeCompleteOptions(), StringRef(Example.Text))
           .get()
-          .Value;
+          .second.Value;
   EXPECT_THAT(Results.items, Contains(Named("cbc")));
 }
 
 TEST(CompletionTest, Priorities) {
   auto Internal = completions(R"cpp(
       class Foo {
         public: void pub();
         protected: void prot();