[clangd] Also accept flags from CLANGD_FLAGS variable.

[sammccall]

Summary

This simplifies various workflows, particularly in debugging/development.
e.g. editors will tend to propagate flags, so you can run
env CLANGD_FLAGS=-input-mirror-file=/tmp/mirror vim foo.cc rather than change
the configuration in a persistent way.
(This also gives us a generic lever when we don't know how to customize the
flags in some particular LSP client).

While here, add a test for this and other startup logging, and fix a couple
of direct writes to errs() that should have been logs.

[hokein]

clang-tools-extra/clangd/test/log.test

9 ↗

(On Diff #211307)

nit: remove the extra trailing blank line?

clang-tools-extra/trunk/clangd/index/Serialization.cpp

llvm::Expected<IndexFileIn> readIndexFile(llvm::StringRef Data) {
   }
 }
 
 std::unique_ptr<SymbolIndex> loadIndex(llvm::StringRef SymbolFilename,
                                        bool UseDex) {
   trace::Span OverallTracer("LoadIndex");
   auto Buffer = llvm::MemoryBuffer::getFile(SymbolFilename);
   if (!Buffer) {
-    llvm::errs() << "Can't open " << SymbolFilename << "\n";
+    elog("Can't open {0}", SymbolFilename);
     return nullptr;
   }
 
   SymbolSlab Symbols;
   RefSlab Refs;
   RelationSlab Relations;
   {
     trace::Span Tracer("ParseIndex");
     if (auto I = readIndexFile(Buffer->get()->getBuffer())) {
       if (I->Symbols)
         Symbols = std::move(*I->Symbols);
       if (I->Refs)
         Refs = std::move(*I->Refs);
       if (I->Relations)
         Relations = std::move(*I->Relations);
     } else {
-      llvm::errs() << "Bad Index: " << llvm::toString(I.takeError()) << "\n";
+      elog("Bad Index: {0}", I.takeError());
       return nullptr;
     }
   }
 
   size_t NumSym = Symbols.size();
   size_t NumRefs = Refs.numRefs();
   size_t NumRelations = Relations.size();
 

clang-tools-extra/trunk/clangd/test/log.test

+# RUN: not env CLANGD_FLAGS=-index-file=no-such-index clangd -lit-test </dev/null 2>&1 >/dev/null | FileCheck %s
+CHECK: I[{{.*}}] clangd version {{.*}}
+CHECK: Working directory: {{.*}}
+CHECK: argv[0]: clangd
+CHECK: argv[1]: -lit-test
+CHECK: CLANGD_FLAGS: -index-file=no-such-index
+CHECK: E[{{.*}}] Can't open no-such-index
+CHECK: Starting LSP over stdin/stdout
+

clang-tools-extra/trunk/clangd/tool/ClangdMain.cpp

 #include "index/Serialization.h"
 #include "clang/Basic/Version.h"
 #include "clang/Format/Format.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/Process.h"
 #include "llvm/Support/Program.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/TargetSelect.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cstdlib>
 #include <iostream>
 #include <memory>
 #include <mutex>
int main(int argc, char *argv[]) {
   using namespace clang;
   using namespace clang::clangd;
 
   llvm::InitializeAllTargetInfos();
   llvm::sys::PrintStackTraceOnErrorSignal(argv[0]);
   llvm::cl::SetVersionPrinter([](llvm::raw_ostream &OS) {
     OS << clang::getClangToolFullVersion("clangd") << "\n";
   });
+  const char *FlagsEnvVar = "CLANGD_FLAGS";
+  const char *Overview =
+      R"(clangd is a language server that provides IDE-like features to editors.
+
+It should be used via an editor plugin rather than invoked directly. For more information, see:
+  https://clang.llvm.org/extra/clangd/
+  https://microsoft.github.io/language-server-protocol/
+
+clangd accepts flags on the commandline, and in the CLANGD_FLAGS environment variable.
+)";
   llvm::cl::HideUnrelatedOptions(ClangdCategories);
-  llvm::cl::ParseCommandLineOptions(
-      argc, argv,
-      "clangd is a language server that provides IDE-like features to editors. "
-      "\n\nIt should be used via an editor plugin rather than invoked "
-      "directly. "
-      "For more information, see:"
-      "\n\thttps://clang.llvm.org/extra/clangd.html"
-      "\n\thttps://microsoft.github.io/language-server-protocol/");
+  llvm::cl::ParseCommandLineOptions(argc, argv, Overview,
+                                    /*Errs=*/nullptr, FlagsEnvVar);
   if (Test) {
     Sync = true;
     InputStyle = JSONStreamStyle::Delimited;
     LogLevel = Logger::Verbose;
     PrettyPrint = true;
     // Disable background index on lit tests by default to prevent disk writes.
     if (!EnableBackgroundIndex.getNumOccurrences())
       EnableBackgroundIndex = false;
log("{0}", clang::getClangToolFullVersion("clangd"));
     SmallString<128> CWD;
     if (auto Err = llvm::sys::fs::current_path(CWD))
       log("Working directory unknown: {0}", Err.message());
     else
       log("Working directory: {0}", CWD);
   }
   for (int I = 0; I < argc; ++I)
     log("argv[{0}]: {1}", I, argv[I]);
+  if (auto EnvFlags = llvm::sys::Process::GetEnv(FlagsEnvVar))
+    log("{0}: {1}", FlagsEnvVar, *EnvFlags);
 
   // If --compile-commands-dir arg was invoked, check value and override default
   // path.
   llvm::Optional<Path> CompileCommandsDirPath;
   if (!CompileCommandsDir.empty()) {
     if (llvm::sys::fs::exists(CompileCommandsDir)) {
       // We support passing both relative and absolute paths to the
       // --compile-commands-dir argument, but we assume the path is absolute in