Revert "Delete unused class GlobalMutex"

This reverts commit 5831afb8328cbecdbdbee882280ef907a09d9f31.

Reason for revert: The code is still in use: https://ci.chromium.org/ui/p/webrtc/builders/ci/Win32%20Builder%20(Clang)/10243/overview. It wasn't detected for some reasons.

Original change's description:
> Delete unused class GlobalMutex
>
> Bug: webrtc:13869
> Change-Id: Id3655bdd24630b78e83b606291605a37129fd4bc
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/258131
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Commit-Queue: Niels Moller <nisse@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#36478}

Bug: webrtc:13869
Change-Id: Id44dd4f8ec31ac9e05fc0ee75b55916e627fd7bf
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/258135
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Artem Titov <titovartem@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36480}

rtc_base/synchronization/BUILD.gn

@@ -23,6 +23,7 @@ rtc_library("yield") {
 
 rtc_library("mutex") {
   sources = [
+    "mutex.cc",
     "mutex.h",
     "mutex_critical_section.h",
     "mutex_pthread.h",

rtc_base/synchronization/mutex.cc

@@ -0,0 +1,39 @@
+/*
+ *  Copyright 2020 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "rtc_base/synchronization/mutex.h"
+
+#include "rtc_base/checks.h"
+#include "rtc_base/synchronization/yield.h"
+
+namespace webrtc {
+
+#if !defined(WEBRTC_ABSL_MUTEX)
+void GlobalMutex::Lock() {
+  while (mutex_locked_.exchange(1)) {
+    YieldCurrentThread();
+  }
+}
+
+void GlobalMutex::Unlock() {
+  int old = mutex_locked_.exchange(0);
+  RTC_DCHECK_EQ(old, 1) << "Unlock called without calling Lock first";
+}
+
+GlobalMutexLock::GlobalMutexLock(GlobalMutex* mutex) : mutex_(mutex) {
+  mutex_->Lock();
+}
+
+GlobalMutexLock::~GlobalMutexLock() {
+  mutex_->Unlock();
+}
+#endif  // #if !defined(WEBRTC_ABSL_MUTEX)
+
+}  // namespace webrtc

rtc_base/synchronization/mutex.h

@@ -72,6 +72,41 @@ class RTC_SCOPED_LOCKABLE MutexLock final {
   Mutex* mutex_;
 };
 
+// A mutex used to protect global variables. Do NOT use for other purposes.
+#if defined(WEBRTC_ABSL_MUTEX)
+using GlobalMutex = absl::Mutex;
+using GlobalMutexLock = absl::MutexLock;
+#else
+class RTC_LOCKABLE GlobalMutex final {
+ public:
+  GlobalMutex(const GlobalMutex&) = delete;
+  GlobalMutex& operator=(const GlobalMutex&) = delete;
+
+  constexpr explicit GlobalMutex(absl::ConstInitType /*unused*/)
+      : mutex_locked_(0) {}
+
+  void Lock() RTC_EXCLUSIVE_LOCK_FUNCTION();
+  void Unlock() RTC_UNLOCK_FUNCTION();
+
+ private:
+  std::atomic<int> mutex_locked_;  // 0 means lock not taken, 1 means taken.
+};
+
+// GlobalMutexLock, for serializing execution through a scope.
+class RTC_SCOPED_LOCKABLE GlobalMutexLock final {
+ public:
+  GlobalMutexLock(const GlobalMutexLock&) = delete;
+  GlobalMutexLock& operator=(const GlobalMutexLock&) = delete;
+
+  explicit GlobalMutexLock(GlobalMutex* mutex)
+      RTC_EXCLUSIVE_LOCK_FUNCTION(mutex_);
+  ~GlobalMutexLock() RTC_UNLOCK_FUNCTION();
+
+ private:
+  GlobalMutex* mutex_;
+};
+#endif  // if defined(WEBRTC_ABSL_MUTEX)
+
 }  // namespace webrtc
 
 #endif  // RTC_BASE_SYNCHRONIZATION_MUTEX_H_

rtc_base/synchronization/mutex_unittest.cc

@@ -177,5 +177,30 @@ TEST(MutexTest, ProtectsSharedResourceWithMutexAndMutexLocker) {
   EXPECT_EQ(0, runner.shared_value());
 }
 
+TEST(MutexTest, ProtectsSharedResourceWithGlobalMutexAndRawMutexLocker) {
+  std::vector<std::unique_ptr<Thread>> threads;
+  LockRunner<GlobalMutex, RawMutexLocker<GlobalMutex>> runner(absl::kConstInit);
+  StartThreads(threads, &runner);
+  runner.SetExpectedThreadCount(kNumThreads);
+  EXPECT_TRUE(runner.Run());
+  EXPECT_EQ(0, runner.shared_value());
+}
+
+TEST(MutexTest, ProtectsSharedResourceWithGlobalMutexAndMutexLocker) {
+  std::vector<std::unique_ptr<Thread>> threads;
+  LockRunner<GlobalMutex, MutexLockLocker<GlobalMutex, GlobalMutexLock>> runner(
+      absl::kConstInit);
+  StartThreads(threads, &runner);
+  runner.SetExpectedThreadCount(kNumThreads);
+  EXPECT_TRUE(runner.Run());
+  EXPECT_EQ(0, runner.shared_value());
+}
+
+TEST(MutexTest, GlobalMutexCanHaveStaticStorageDuration) {
+  ABSL_CONST_INIT static GlobalMutex global_lock(absl::kConstInit);
+  global_lock.Lock();
+  global_lock.Unlock();
+}
+
 }  // namespace
 }  // namespace webrtc