[clang-tools-extra] [clangd] Explicitly block until async task completes (PR #130077)

[Ilya Biryukov via cfe-commits]

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@@ -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();
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ilya-biryukov wrote:

Let's remove or update [this comment](https://github.com/llvm/llvm-project/blob/156cdcf2ffc9a5f851e1b340891172462938b5ea/clang-tools-extra/clangd/CodeComplete.h#L277)?

It seems to suggest we are still relying on destructor to do the right thing.

https://github.com/llvm/llvm-project/pull/130077