Make ChildThread::current() and ChildProcess::current() only work on the main thread of the child...

content/child/child_process.cc

 // Copyright (c) 2012 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 "content/child/child_process.h"
 
 #if defined(OS_POSIX) && !defined(OS_ANDROID)
 #include <signal.h>  // For SigUSR1Handler below.
 #endif
 
+#include "base/lazy_instance.h"
 #include "base/message_loop.h"
 #include "base/metrics/statistics_recorder.h"
 #include "base/process_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/threading/thread.h"
+#include "base/threading/thread_local.h"
 #include "content/child/child_thread.h"
 
 #if defined(OS_ANDROID)
 #include "base/debug/debugger.h"
 #endif
 
 #if defined(OS_POSIX) && !defined(OS_ANDROID)
 static void SigUSR1Handler(int signal) { }
 #endif
 
 namespace content {
-// The singleton instance for this process.
-ChildProcess* child_process = NULL;
+
+namespace {
+
+base::LazyInstance<base::ThreadLocalPointer<ChildProcess> > g_lazy_tls =
+    LAZY_INSTANCE_INITIALIZER;
+}
 
 ChildProcess::ChildProcess()
     : ref_count_(0),
       shutdown_event_(true, false),
       io_thread_("Chrome_ChildIOThread") {
-  DCHECK(!child_process);
-  child_process = this;
+  DCHECK(!g_lazy_tls.Pointer()->Get());
+  g_lazy_tls.Pointer()->Set(this);
 
   base::StatisticsRecorder::Initialize();
 
   // We can't recover from failing to start the IO thread.
   CHECK(io_thread_.StartWithOptions(
       base::Thread::Options(base::MessageLoop::TYPE_IO, 0)));
 
 #if defined(OS_ANDROID)
   io_thread_.SetPriority(base::kThreadPriority_Display);
 #endif
 }
 
 ChildProcess::~ChildProcess() {
-  DCHECK(child_process == this);
+  DCHECK(g_lazy_tls.Pointer()->Get() == this);
 
   // Signal this event before destroying the child process.  That way all
   // background threads can cleanup.
   // For example, in the renderer the RenderThread instances will be able to
   // notice shutdown before the render process begins waiting for them to exit.
   shutdown_event_.Signal();
 
   // Kill the main thread object before nulling child_process, since
   // destruction code might depend on it.
   if (main_thread_) {  // null in unittests.
     main_thread_->Shutdown();
     main_thread_.reset();
   }
 
-  child_process = NULL;
+  g_lazy_tls.Pointer()->Set(NULL);
 }
 
 ChildThread* ChildProcess::main_thread() {
   return main_thread_.get();
 }
 
 void ChildProcess::set_main_thread(ChildThread* thread) {
   main_thread_.reset(thread);
 }
 
 void ChildProcess::AddRefProcess() {
   DCHECK(!main_thread_.get() ||  // null in unittests.
          base::MessageLoop::current() == main_thread_->message_loop());
   ref_count_++;
 }
 
 void ChildProcess::ReleaseProcess() {
   DCHECK(!main_thread_.get() ||  // null in unittests.
          base::MessageLoop::current() == main_thread_->message_loop());
   DCHECK(ref_count_);
-  DCHECK(child_process);
   if (--ref_count_)
     return;
 
   if (main_thread_)  // null in unittests.
     main_thread_->OnProcessFinalRelease();
 }
 
 ChildProcess* ChildProcess::current() {
-  return child_process;
+  return g_lazy_tls.Pointer()->Get();

[scottmg, 2013/06/26 15:21:50]

is this used in any hot functions? it's probably small in absolute terms but
could be a relatively a lot more expensive than before.

[jam, 2013/06/26 17:23:08]

Not that I know of, and would be doubtful if so.

 }
 
 base::WaitableEvent* ChildProcess::GetShutDownEvent() {
-  DCHECK(child_process);
-  return &child_process->shutdown_event_;
+  return &shutdown_event_;
 }
 
 void ChildProcess::WaitForDebugger(const std::string& label) {
 #if defined(OS_WIN)
 #if defined(GOOGLE_CHROME_BUILD)
   std::string title = "Google Chrome";
 #else  // CHROMIUM_BUILD
   std::string title = "Chromium";
 #endif  // CHROMIUM_BUILD
   title += " ";
@@ skipping 22 matching lines @@
   memset(&sa, 0, sizeof(sa));
   sa.sa_handler = SigUSR1Handler;
   sigaction(SIGUSR1, &sa, NULL);
 
   pause();
 #endif  // defined(OS_ANDROID)
 #endif  // defined(OS_POSIX)
 }
 
 }  // namespace content