[clangd] Explicitly block until async task completes (llvm#130077)

We started seeing some use-after-frees starting with
llvm#125433.

This patch ensures we explicitly block for the async task, if there's
one, before destructing `std::future`, independent of the stdlib
implementation.

Author: kadircet

clang-tools-extra/clangd/ClangdServer.cpp

@@ -460,18 +460,24 @@ void ClangdServer::codeComplete(PathRef File, Position Pos,
     CodeCompleteResult Result = clangd::codeComplete(
         File, Pos, IP->Preamble, ParseInput, CodeCompleteOpts,
         SpecFuzzyFind ? &*SpecFuzzyFind : nullptr);
+    // We don't want `codeComplete` to wait for the async call if it doesn't use
+    // the result (e.g. non-index completion, speculation fails), so that `CB`
+    // is called as soon as results are available.
     {
       clang::clangd::trace::Span Tracer("Completion results callback");
       CB(std::move(Result));
     }
-    if (SpecFuzzyFind && SpecFuzzyFind->NewReq) {
+    if (!SpecFuzzyFind)
+      return;
+    if (SpecFuzzyFind->NewReq) {
       std::lock_guard<std::mutex> Lock(CachedCompletionFuzzyFindRequestMutex);
       CachedCompletionFuzzyFindRequestByFile[File] = *SpecFuzzyFind->NewReq;
     }
-    // SpecFuzzyFind is only destroyed after speculative fuzzy find finishes.
-    // We don't want `codeComplete` to wait for the async call if it doesn't use
-    // the result (e.g. non-index completion, speculation fails), so that `CB`
-    // is called as soon as results are available.
+    // Explicitly block until async task completes, this is fine as we've
+    // already provided reply to the client and running as a preamble task
+    // (i.e. won't block other preamble tasks).
+    if (SpecFuzzyFind->Result.valid())
+      SpecFuzzyFind->Result.wait();
   };
 
   // We use a potentially-stale preamble because latency is critical here.

clang-tools-extra/clangd/CodeComplete.h

@@ -274,7 +274,6 @@ struct SpeculativeFuzzyFind {
   /// Set by `codeComplete()`. This can be used by callers to update cache.
   std::optional<FuzzyFindRequest> NewReq;
   /// The result is consumed by `codeComplete()` if speculation succeeded.
-  /// NOTE: the destructor will wait for the async call to finish.
   std::future<std::pair<bool /*Incomplete*/, SymbolSlab>> Result;
 };