Thread IO safety: file_util annotated, IO thread blocked from doing IO

base/thread_restrictions.cc

 // Copyright (c) 2010 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/thread_restrictions.h"
 
 // This entire file is compiled out in Release mode.
 #ifndef NDEBUG
 
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/thread_local.h"
 
 namespace base {
 
 namespace {
 
 LazyInstance<ThreadLocalBoolean, LeakyLazyInstanceTraits<ThreadLocalBoolean> >
     g_io_disallowed(LINKER_INITIALIZED);
 
 }  // anonymous namespace
 
 // static
-void ThreadRestrictions::SetIOAllowed(bool allowed) {
+bool ThreadRestrictions::SetIOAllowed(bool allowed) {
+  bool previous_allowed = g_io_disallowed.Get().Get();
   g_io_disallowed.Get().Set(!allowed);
+  return !previous_allowed;
 }
 
 // static
 void ThreadRestrictions::AssertIOAllowed() {
   if (g_io_disallowed.Get().Get()) {
     LOG(FATAL) <<
         "Function marked as IO-only was called from a thread that "
         "disallows IO!  If this thread really should be allowed to "
         "make IO calls, adjust the call to "
         "base::ThreadRestrictions::SetIOAllowed() in this thread's "
         "startup.";
   }
 }
 
 }  // namespace base
 
 #endif  // NDEBUG