[clang] [llvm] [clang][deps] Add in-flight query caching to `DependencyScanningFilesystemSharedCache` (PR #199680)

[Artem Chikin via cfe-commits]

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@@ -152,19 +154,54 @@ using CachedRealPath = llvm::ErrorOr<std::string>;
 /// the worker threads.
 class DependencyScanningFilesystemSharedCache {
 public:
+  /// Tracks a cache entry whose value is currently being computed by one
+  /// worker so that other workers arriving at the same key can wait for the
+  /// result rather than producing it in parallel. The producer publishes the
+  /// resolved entry into \c Result, sets \c Done, and notifies waiters via
+  /// \c CondVar. Waiters synchronize through the owning shard's \c CacheLock.
+  struct InProgressEntry {
+    std::condition_variable CondVar;
+    bool Done = false;
+    const CachedFileSystemEntry *Result = nullptr;
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artemcm wrote:

`Result` does duplicate the value, but it's the one copy a woken waiter can read without re-taking the shard lock, which I think is still net beneficial, especially in combination with the above refactor to move away from using the shard lock on the wait.


https://github.com/llvm/llvm-project/pull/199680