[clangd] Allow to get vfs::FileSystem used inside codeComplete.

[ilya-biryukov]

Summary

This is useful for managing lifetime of VFS-based caches.

[bkramer]

lg (it could use a test case though)

[ilya-biryukov]

In D36095#826275, @bkramer wrote:

lg (it could use a test case though)

Will add a test case in one of further commits.

clangd/ClangdServer.h

public:
   /// Force \p File to be reparsed using the latest contents.
   void forceReparse(PathRef File);
 
   /// Run code completion for \p File at \p Pos. 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.
-  /// This method should only be called for currently tracked files.
+  /// 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.
   Tagged<std::vector<CompletionItem>>
   codeComplete(PathRef File, Position Pos,
-               llvm::Optional<StringRef> OverridenContents = llvm::None);
+               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.
   Tagged<std::vector<Location>> findDefinitions(PathRef File, Position Pos);
 
   /// Run formatting for \p Rng inside \p File.
   std::vector<tooling::Replacement> formatRange(PathRef File, Range Rng);
   /// Run formatting for the whole \p File.
   std::vector<tooling::Replacement> formatFile(PathRef File);
   /// Run formatting after a character was typed at \p Pos in \p File.

clangd/ClangdServer.cpp

 void ClangdServer::forceReparse(PathRef File) {
   // The addDocument schedules the reparse even if the contents of the file
   // never changed, so we just call it here.
   addDocument(File, getDocument(File));
 }
 
 Tagged<std::vector<CompletionItem>>
 ClangdServer::codeComplete(PathRef File, Position Pos,
-                           llvm::Optional<StringRef> OverridenContents) {
+                           llvm::Optional<StringRef> OverridenContents,
+                           IntrusiveRefCntPtr<vfs::FileSystem> *UsedFS) {
   std::string DraftStorage;
   if (!OverridenContents) {
     auto FileContents = DraftMgr.getDraft(File);
     assert(FileContents.Draft &&
            "codeComplete is called for non-added document");
 
     DraftStorage = std::move(*FileContents.Draft);
     OverridenContents = DraftStorage;
   }
 
-  std::vector<CompletionItem> Result;
   auto TaggedFS = FSProvider.getTaggedFileSystem(File);
+  if (UsedFS)
+    *UsedFS = TaggedFS.Value;
+
+  std::vector<CompletionItem> Result;
   Units.runOnUnitWithoutReparse(File, *OverridenContents, ResourceDir, CDB,
                                 PCHs, TaggedFS.Value, [&](ClangdUnit &Unit) {
                                   Result = Unit.codeComplete(
                                       *OverridenContents, Pos, TaggedFS.Value);
                                 });
   return make_tagged(std::move(Result), TaggedFS.Tag);
 }