Add Some Comments to the Timeout Checks on Condition Variables

base/synchronization/condition_variable_win.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/synchronization/condition_variable.h"
 
 #include "base/synchronization/lock.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/time/time.h"
 
@@ skipping 17 matching lines @@
 
 void ConditionVariable::TimedWait(const TimeDelta& max_time) {
   base::ThreadRestrictions::AssertWaitAllowed();
   DWORD timeout = static_cast<DWORD>(max_time.InMilliseconds());
 
 #if DCHECK_IS_ON()
   user_lock_->CheckHeldAndUnmark();
 #endif
 
   if (!SleepConditionVariableSRW(&cv_, srwlock_, timeout, 0)) {
+    // On failure, we only expect the CV to timeout. Any other error value means
+    // that we've unexpectedly woken up.
+    // Note that WAIT_TIMEOUT != ERROR_TIMEOUT. WAIT_TIMEOUT is used with the
+    // WaitFor* family of functions as a direct return value. ERROR_TIMEOUT is
+    // used with GetLastError().
     DCHECK_EQ(static_cast<DWORD>(ERROR_TIMEOUT), GetLastError());
   }
 
 #if DCHECK_IS_ON()
   user_lock_->CheckUnheldAndMark();
 #endif
 }
 
 void ConditionVariable::Broadcast() {
   WakeAllConditionVariable(&cv_);
 }
 
 void ConditionVariable::Signal() {
   WakeConditionVariable(&cv_);
 }
 
 }  // namespace base