From c77444b2ef4d1362da054a36bf44d971719d027d Mon Sep 17 00:00:00 2001
From: Bill McCloskey <billm@mozilla.com>
Date: Sun, 24 Jan 2016 17:31:11 -0800
Subject: [PATCH] Bug 1240985 - Back out "Stop returning MsgNotAllowed" patch.

---
 ipc/glue/MessageChannel.cpp | 19 ++++++++++++++++++-
 ipc/glue/MessageChannel.h   |  1 +
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/ipc/glue/MessageChannel.cpp b/ipc/glue/MessageChannel.cpp
index 26b1a8a061c..0cd2bb16a0b 100644
--- a/ipc/glue/MessageChannel.cpp
+++ b/ipc/glue/MessageChannel.cpp
@@ -324,6 +324,7 @@ MessageChannel::MessageChannel(MessageListener *aListener)
     mDispatchingAsyncMessagePriority(0),
     mCurrentTransaction(0),
     mTimedOutMessageSeqno(0),
+    mTimedOutMessagePriority(0),
     mRecvdErrors(0),
     mRemoteStackDepthGuess(false),
     mSawInterruptOutMsg(false),
@@ -1039,6 +1040,7 @@ MessageChannel::Send(Message* aMsg, Message* aReply)
             }
 
             mTimedOutMessageSeqno = seqno;
+            mTimedOutMessagePriority = prio;
             return false;
         }
     }
@@ -1408,7 +1410,22 @@ MessageChannel::DispatchSyncMessage(const Message& aMsg, Message*& aReply)
     MessageChannel*& blockingVar = ShouldBlockScripts() ? gParentProcessBlocker : dummy;
 
     Result rv;
-    {
+    if (mTimedOutMessageSeqno && mTimedOutMessagePriority >= prio) {
+        // If the other side sends a message in response to one of our messages
+        // that we've timed out, then we reply with an error.
+        //
+        // We do this because want to avoid a situation where we process an
+        // incoming message from the child here while it simultaneously starts
+        // processing our timed-out CPOW. It's very bad for both sides to
+        // be processing sync messages concurrently.
+        //
+        // The only exception is if the incoming message has urgent priority and
+        // our timed-out message had only high priority. In that case it's safe
+        // to process the incoming message because we know that the child won't
+        // process anything (the child will defer incoming messages when waiting
+        // for a response to its urgent message).
+        rv = MsgNotAllowed;
+    } else {
         AutoSetValue<MessageChannel*> blocked(blockingVar, this);
         AutoSetValue<bool> sync(mDispatchingSyncMessage, true);
         AutoSetValue<int> prioSet(mDispatchingSyncMessagePriority, prio);
diff --git a/ipc/glue/MessageChannel.h b/ipc/glue/MessageChannel.h
index f05eed9338d..72c25e12eae 100644
--- a/ipc/glue/MessageChannel.h
+++ b/ipc/glue/MessageChannel.h
@@ -630,6 +630,7 @@ class MessageChannel : HasResultCodes
     // hitting a lot of corner cases with message nesting that we don't really
     // care about.
     int32_t mTimedOutMessageSeqno;
+    int mTimedOutMessagePriority;
 
     // If waiting for the reply to a sync out-message, it will be saved here
     // on the I/O thread and then read and cleared by the worker thread.