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[clangd] SIGSEGV at clangd: DiagnosticConsumer Is Used After Free
This is a follow-up patch for D148088. The dynamic symbol index (`FileIndex::updatePreamble`) may run in a separate thread, and the `DiagnosticConsumer` that is set up in `buildPreamble` might go out of scope before it is used. This could result in a SIGSEGV when attempting to call any method of the `DiagnosticConsumer` class. The function `buildPreamble` sets up the `DiagnosticConsumer` as follows: ``` ... buildPreamble(...) { ... StoreDiags PreambleDiagnostics; ... llvm::IntrusiveRefCntPtr<DiagnosticsEngine> PreambleDiagsEngine = CompilerInstance::createDiagnostics(&CI.getDiagnosticOpts(), &PreambleDiagnostics, /*ShouldOwnClient=*/false); ... // The call might use the diagnostic consumer in a separate thread PreambleCallback(...) ... } ``` `PreambleDiagnostics` might be out of scope for `buildPreamble` function when we call it inside `PreambleCallback` in a separate thread. The Fix The fix involves replacing the client (DiagnosticConsumer) with an `IgnoringDiagConsumer` instance, which will print messages to the clangd log. Alternatively, we can replace `PreambleDiagnostics` with an object that is owned by `DiagnosticsEngine`. Note There is no corresponding LIT/GTest for this issue, since there is a specific race condition that is difficult to reproduce within a test framework. Test Plan: ``` ninja check-clangd ``` Reviewed By: kadircet, sammccall Differential Revision: https://reviews.llvm.org/D159363
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