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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clangd

Author: kadir çetinkaya (kadircet)

<details>
<summary>Changes</summary>

We started seeing some use-after-frees starting with https://github.com/llvm/llvm-project/pull/125433.

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


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


1 Files Affected:

- (modified) clang-tools-extra/clangd/ClangdServer.cpp (+11-5) 


``````````diff
diff --git a/clang-tools-extra/clangd/ClangdServer.cpp b/clang-tools-extra/clangd/ClangdServer.cpp
index 52be15d3da936..49a97da2bfa42 100644
--- a/clang-tools-extra/clangd/ClangdServer.cpp
+++ b/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.

``````````

</details>


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