Ensure that BrowserMessageFilter isn't used to process a sync message on the UI thread.

content/browser/browser_message_filter.cc

Index: content/browser/browser_message_filter.cc
===================================================================
--- content/browser/browser_message_filter.cc	(revision 79364)
+++ content/browser/browser_message_filter.cc	(working copy)
@@ -9,6 +9,7 @@
 #include "base/process_util.h"
 #include "chrome/browser/metrics/user_metrics.h"
 #include "content/common/result_codes.h"
+#include "ipc/ipc_sync_message.h"
 
 BrowserMessageFilter::BrowserMessageFilter()
     : channel_(NULL), peer_handle_(base::kNullProcessHandle) {
@@ -67,6 +68,11 @@
   if (thread == BrowserThread::IO)
     return DispatchMessage(message);
 
+  if (thread == BrowserThread::UI &&
+      !MessageCanBeDispatchedOnUI(message, this)) {
+    return true;
+  }
+
   BrowserThread::PostTask(
       thread, FROM_HERE,
       NewRunnableMethod(
@@ -90,3 +96,28 @@
 void BrowserMessageFilter::BadMessageReceived() {
   base::KillProcess(peer_handle(), ResultCodes::KILLED_BAD_MESSAGE, false);
 }
+
+bool BrowserMessageFilter::MessageCanBeDispatchedOnUI(
+    const IPC::Message& message, IPC::Message::Sender* sender) {
+#if defined(OS_WIN)
+  // On Windows there's a potential deadlock with sync messsages going in
+  // a circle from browser -> plugin -> renderer -> browser.
+  // On Linux we can avoid this by avoiding sync messages from browser->plugin.
+  // On Mac we avoid this by not supporting windowed plugins.
+  if (message.is_sync() && !message.is_caller_pumping_messages()) {
+    // NOTE: IF YOU HIT THIS ASSERT, THE SOLUTION IS ALMOST NEVER TO RUN A
+    // NESTED MESSAGE LOOP IN THE RENDERER!!!
+    // That introduces reentrancy which causes hard to track bugs.  You should
+    // find a way to either turn this into an asynchronous message, or one
+    // that can be answered on the IO thread.
+    NOTREACHED() << "Can't send sync messages to UI thread without pumping "
+        "messages in the renderer or else deadlocks can occur if the page "
+        "has windowed plugins! (message type " << message.type() << ")";
+    IPC::Message* reply = IPC::SyncMessage::GenerateReply(&message);
+    reply->set_reply_error();
+    sender->Send(reply);
+    return false;
+  }
+#endif
+  return true;
+}