[clangd] Turn off implicit cancellation based on client capabilities

Capability is in upcoming 3.17: https://microsoft.github.io/language-server-protocol/specifications/specification-3-17/

(This is also useful for C++ embedders)

Differential Revision: https://reviews.llvm.org/D98414

Author: sam-mccall

clang-tools-extra/clangd/ClangdLSPServer.cpp

@@ -518,6 +518,7 @@ void ClangdLSPServer::onInitialize(const InitializeParams &Params,
   if (Params.capabilities.WorkDoneProgress)
     BackgroundIndexProgressState = BackgroundIndexProgress::Empty;
   BackgroundIndexSkipCreate = Params.capabilities.ImplicitProgressCreation;
+  Opts.ImplicitCancellation = !Params.capabilities.CancelsStaleRequests;
 
   llvm::json::Object ServerCaps{
       {"textDocumentSync",

clang-tools-extra/clangd/ClangdServer.cpp

@@ -150,6 +150,8 @@ ClangdServer::ClangdServer(const GlobalCompilationDatabase &CDB,
       DynamicIdx(Opts.BuildDynamicSymbolIndex ? new FileIndex() : nullptr),
       ClangTidyProvider(Opts.ClangTidyProvider),
       WorkspaceRoot(Opts.WorkspaceRoot),
+      Transient(Opts.ImplicitCancellation ? TUScheduler::InvalidateOnUpdate
+                                          : TUScheduler::NoInvalidation),
       DirtyFS(std::make_unique<DraftStoreFS>(TFS, DraftMgr)) {
   // Pass a callback into `WorkScheduler` to extract symbols from a newly
   // parsed file and rebuild the file index synchronously each time an AST
@@ -593,7 +595,7 @@ void ClangdServer::enumerateTweaks(
   };
 
   WorkScheduler->runWithAST("EnumerateTweaks", File, std::move(Action),
-                            TUScheduler::InvalidateOnUpdate);
+                            Transient);
 }
 
 void ClangdServer::applyTweak(PathRef File, Range Sel, StringRef TweakID,
@@ -683,8 +685,7 @@ void ClangdServer::findDocumentHighlights(
         CB(clangd::findDocumentHighlights(InpAST->AST, Pos));
       };
 
-  WorkScheduler->runWithAST("Highlights", File, std::move(Action),
-                            TUScheduler::InvalidateOnUpdate);
+  WorkScheduler->runWithAST("Highlights", File, std::move(Action), Transient);
 }
 
 void ClangdServer::findHover(PathRef File, Position Pos,
@@ -698,8 +699,7 @@ void ClangdServer::findHover(PathRef File, Position Pos,
     CB(clangd::getHover(InpAST->AST, Pos, std::move(Style), Index));
   };
 
-  WorkScheduler->runWithAST("Hover", File, std::move(Action),
-                            TUScheduler::InvalidateOnUpdate);
+  WorkScheduler->runWithAST("Hover", File, std::move(Action), Transient);
 }
 
 void ClangdServer::typeHierarchy(PathRef File, Position Pos, int Resolve,
@@ -771,7 +771,7 @@ void ClangdServer::documentSymbols(llvm::StringRef File,
         CB(clangd::getDocumentSymbols(InpAST->AST));
       };
   WorkScheduler->runWithAST("DocumentSymbols", File, std::move(Action),
-                            TUScheduler::InvalidateOnUpdate);
+                            Transient);
 }
 
 void ClangdServer::foldingRanges(llvm::StringRef File,
@@ -783,7 +783,7 @@ void ClangdServer::foldingRanges(llvm::StringRef File,
         CB(clangd::getFoldingRanges(InpAST->AST));
       };
   WorkScheduler->runWithAST("FoldingRanges", File, std::move(Action),
-                            TUScheduler::InvalidateOnUpdate);
+                            Transient);
 }
 
 void ClangdServer::findImplementations(
@@ -850,7 +850,7 @@ void ClangdServer::documentLinks(PathRef File,
         CB(clangd::getDocumentLinks(InpAST->AST));
       };
   WorkScheduler->runWithAST("DocumentLinks", File, std::move(Action),
-                            TUScheduler::InvalidateOnUpdate);
+                            Transient);
 }
 
 void ClangdServer::semanticHighlights(
@@ -862,7 +862,7 @@ void ClangdServer::semanticHighlights(
         CB(clangd::getSemanticHighlightings(InpAST->AST));
       };
   WorkScheduler->runWithAST("SemanticHighlights", File, std::move(Action),
-                            TUScheduler::InvalidateOnUpdate);
+                            Transient);
 }
 
 void ClangdServer::getAST(PathRef File, Range R,

clang-tools-extra/clangd/ClangdServer.h

@@ -146,6 +146,12 @@ class ClangdServer {
         /*RebuildRatio=*/1,
     };
 
+    /// Cancel certain requests if the file changes before they begin running.
+    /// This is useful for "transient" actions like enumerateTweaks that were
+    /// likely implicitly generated, and avoids redundant work if clients forget
+    /// to cancel. Clients that always cancel stale requests should clear this.
+    bool ImplicitCancellation = true;
+
     /// Clangd will execute compiler drivers matching one of these globs to
     /// fetch system include path.
     std::vector<std::string> QueryDriverGlobs;
@@ -391,6 +397,8 @@ class ClangdServer {
 
   llvm::Optional<std::string> WorkspaceRoot;
   llvm::Optional<TUScheduler> WorkScheduler;
+  // Invalidation policy used for actions that we assume are "transient".
+  TUScheduler::ASTActionInvalidation Transient;
 
   // Store of the current versions of the open documents.
   // Only written from the main thread (despite being threadsafe).

clang-tools-extra/clangd/Protocol.cpp

@@ -414,6 +414,12 @@ bool fromJSON(const llvm::json::Value &Params, ClientCapabilities &R,
     if (auto Implicit = Window->getBoolean("implicitWorkDoneProgressCreate"))
       R.ImplicitProgressCreation = *Implicit;
   }
+  if (auto *General = O->getObject("general")) {
+    if (auto *StaleRequestSupport = General->getObject("staleRequestSupport")) {
+      if (auto Cancel = StaleRequestSupport->getBoolean("cancel"))
+        R.CancelsStaleRequests = *Cancel;
+    }
+  }
   if (auto *OffsetEncoding = O->get("offsetEncoding")) {
     R.offsetEncoding.emplace();
     if (!fromJSON(*OffsetEncoding, *R.offsetEncoding,

clang-tools-extra/clangd/Protocol.h

@@ -475,6 +475,10 @@ struct ClientCapabilities {
   /// window.implicitWorkDoneProgressCreate
   bool ImplicitProgressCreation = false;
 
+  /// Whether the client claims to cancel stale requests.
+  /// general.staleRequestSupport.cancel
+  bool CancelsStaleRequests = false;
+
   /// Whether the client implementation supports a refresh request sent from the
   /// server to the client.
   bool SemanticTokenRefreshSupport = false;