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

[Michael Spencer via cfe-commits]

================
@@ -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;
+
+  ModuleCacheMutexLockManager(std::shared_mutex &Mutex)
+      : OwningLock(Mutex, std::try_to_lock_t{}) {}
+
+  operator LockResult() const override {
+    return OwningLock ? LockResult::Owned : LockResult::Shared;
+  }
+
+  WaitForUnlockResult waitForUnlock() override {
+    assert(!OwningLock);
+    std::shared_lock Lock(*OwningLock.mutex());
+    return WaitForUnlockResult::Success;
+  }
+
+  void unsafeRemoveLock() override {
+    llvm_unreachable("ModuleCacheMutexLockManager cannot remove locks");
+  }
+
+  std::string getErrorMessage() const override {
+    llvm_unreachable("ModuleCacheMutexLockManager cannot fail");
+  }
+};
+
+struct ModuleCacheMutexLock : ModuleCacheLock {
+  ModuleCacheMutexes &Mutexes;
+
+  ModuleCacheMutexLock(ModuleCacheMutexes &Mutexes) : Mutexes(Mutexes) {}
+
+  void prepareLock(StringRef Filename) override {}
+
+  std::unique_ptr<ModuleCacheLockManager> tryLock(StringRef Filename) override {
+    auto &Mutex = [&]() -> std::shared_mutex & {
+      std::lock_guard Lock(Mutexes.Mutex);
+      auto &Mutex = Mutexes.Map[Filename];
+      if (!Mutex)
+        Mutex = std::make_unique<std::shared_mutex>();
+      return *Mutex;
+    }();
----------------
Bigcheese wrote:

I don't think you need a lambda here, the only difference in lifetime without it is that the return value will be constructed before unlock.

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