PPAPI: Fix GetBrowserInterface race conditions

ppapi/proxy/plugin_globals.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 "ppapi/proxy/plugin_globals.h"
 
 #include "base/task_runner.h"
 #include "base/threading/thread.h"
 #include "ipc/ipc_message.h"
 #include "ipc/ipc_sender.h"
@@ skipping 174 matching lines @@
     GetMainThreadMessageLoop()->PostDelayedTask(
         FROM_HERE,
         RunWhileLocked(base::Bind(&PluginGlobals::OnReleaseKeepaliveThrottle,
                                   weak_factory_.GetWeakPtr())),
         base::TimeDelta::FromMilliseconds(
             keepalive_throttle_interval_milliseconds_));
   }
 }
 
 IPC::Sender* PluginGlobals::GetBrowserSender() {
-  if (!browser_sender_.get()) {
-    browser_sender_.reset(
-        new BrowserSender(plugin_proxy_delegate_->GetBrowserSender()));
-  }
-
+  // CAUTION: This function is called without the ProxyLock. See also
+  // InterfaceList::GetInterfaceForPPB.
+  //
+  // See also SetPluginProxyDelegate. That initializes browser_sender_ before
+  // the plugin can start threads, and it may be cleared after the
+  // plugin has torn down threads. So this pointer is expected to remain valid
+  // during the lifetime of the plugin.
   return browser_sender_.get();
 }
 
 std::string PluginGlobals::GetUILanguage() {
   return plugin_proxy_delegate_->GetUILanguage();
 }
 
 void PluginGlobals::SetActiveURL(const std::string& url) {
   plugin_proxy_delegate_->SetActiveURL(url);
 }
 
 PP_Resource PluginGlobals::CreateBrowserFont(
     Connection connection,
     PP_Instance instance,
     const PP_BrowserFont_Trusted_Description& desc,
     const ppapi::Preferences& prefs) {
   return plugin_proxy_delegate_->CreateBrowserFont(
       connection, instance, desc, prefs);
 }
 
+void PluginGlobals::SetPluginProxyDelegate(PluginProxyDelegate* delegate) {
+  DCHECK(delegate && !plugin_proxy_delegate_);
+  plugin_proxy_delegate_ = delegate;
+  browser_sender_.reset(
+      new BrowserSender(plugin_proxy_delegate_->GetBrowserSender()));
+}
+
+void PluginGlobals::ResetPluginProxyDelegate() {
+  DCHECK(plugin_proxy_delegate_);
+  plugin_proxy_delegate_ = NULL;
+  browser_sender_.reset();
+}
+
 MessageLoopResource* PluginGlobals::loop_for_main_thread() {
   return loop_for_main_thread_.get();
 }
 
 void PluginGlobals::set_keepalive_throttle_interval_milliseconds(unsigned i) {
   keepalive_throttle_interval_milliseconds_ = i;
 }
 
 bool PluginGlobals::IsPluginGlobals() const {
   return true;
 }
 
 void PluginGlobals::OnReleaseKeepaliveThrottle() {
   ppapi::ProxyLock::AssertAcquiredDebugOnly();
   plugin_recently_active_ = false;
 }
 
 }  // namespace proxy
 }  // namespace ppapi