Add Some Comments to the Timeout Checks on Condition Variables

base/synchronization/condition_variable_posix.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 <errno.h>
 #include <stdint.h>
 #include <sys/time.h>
 
@@ skipping 100 matching lines @@
   DCHECK_GE(absolute_time.tv_sec, now.tv_sec);  // Overflow paranoia
 
 #if defined(OS_ANDROID) && defined(HAVE_PTHREAD_COND_TIMEDWAIT_MONOTONIC)
   int rv = pthread_cond_timedwait_monotonic_np(
       &condition_, user_mutex_, &absolute_time);
 #else
   int rv = pthread_cond_timedwait(&condition_, user_mutex_, &absolute_time);
 #endif  // OS_ANDROID && HAVE_PTHREAD_COND_TIMEDWAIT_MONOTONIC
 #endif  // OS_MACOSX
 
+  // On failure, we only expect the CV to timeout. Any other error value means
+  // that we've unexpectedly woken up.
   DCHECK(rv == 0 || rv == ETIMEDOUT);
 #if DCHECK_IS_ON()
   user_lock_->CheckUnheldAndMark();
 #endif
 }
 
 void ConditionVariable::Broadcast() {
   int rv = pthread_cond_broadcast(&condition_);
   DCHECK_EQ(0, rv);
 }
 
 void ConditionVariable::Signal() {
   int rv = pthread_cond_signal(&condition_);
   DCHECK_EQ(0, rv);
 }
 
 }  // namespace base