[clang] [clang][modules][deps] Add mutex as an alternative to file lock (PR #129751)

[Michael Spencer via cfe-commits]

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@@ -0,0 +1,66 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/Tooling/DependencyScanning/ModuleCacheMutexLock.h"
+
+#include <mutex>
+
+using namespace clang;
+using namespace tooling;
+using namespace dependencies;
+
+namespace {
+struct ModuleCacheMutexLockManager : ModuleCacheLockManager {
+  std::unique_lock<std::shared_mutex> OwningLock;
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Bigcheese wrote:

Can you add a comment here saying something like:

TODO: Consider using std::atomic::{wait, notify_all} when LLVM moves to C++20.

This is the actual operation we want to use, which lowers to futex/ulock.

https://github.com/llvm/llvm-project/pull/129751