From a3ddeba4a795ef7b125ae21e812c52933fa1796f Mon Sep 17 00:00:00 2001
From: Bill McCloskey <billm@mozilla.com>
Date: Mon, 13 Jul 2015 14:14:13 -0700
Subject: [PATCH] Bug 1191143 - Cancel CPOWs from both sides (r=dvander)

---
 ipc/glue/MessageChannel.cpp | 19 +++++++++++--------
 ipc/glue/MessageChannel.h   |  2 +-
 2 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/ipc/glue/MessageChannel.cpp b/ipc/glue/MessageChannel.cpp
index 9341ba5cdef..87bc69db644 100644
--- a/ipc/glue/MessageChannel.cpp
+++ b/ipc/glue/MessageChannel.cpp
@@ -106,7 +106,7 @@ struct RunnableMethodTraits<mozilla::ipc::MessageChannel>
             DebugAbort(__FILE__, __LINE__, #_cond,## __VA_ARGS__);  \
     } while (0)
 
-static MessageChannel* gParentProcessBlocker;
+static MessageChannel* gMainThreadBlocker;
 
 namespace mozilla {
 namespace ipc {
@@ -404,8 +404,8 @@ MessageChannel::Clear()
     // In practice, mListener owns the channel, so the channel gets deleted
     // before mListener.  But just to be safe, mListener is a weak pointer.
 
-    if (gParentProcessBlocker == this) {
-        gParentProcessBlocker = nullptr;
+    if (gMainThreadBlocker == this) {
+        gMainThreadBlocker = nullptr;
     }
 
     mDequeueOneTask->Cancel();
@@ -1334,7 +1334,7 @@ MessageChannel::DispatchSyncMessage(const Message& aMsg, Message*& aReply)
     MaybeScriptBlocker scriptBlocker(this, prio > IPC::Message::PRIORITY_NORMAL);
 
     MessageChannel* dummy;
-    MessageChannel*& blockingVar = ShouldBlockScripts() ? gParentProcessBlocker : dummy;
+    MessageChannel*& blockingVar = NS_IsMainThread() ? gMainThreadBlocker : dummy;
 
     Result rv;
     if (mTimedOutMessageSeqno && mTimedOutMessagePriority >= prio) {
@@ -2046,22 +2046,25 @@ MessageChannel::CancelCurrentTransactionInternal()
     // see if mCurrentTransaction is 0 before examining DispatchSyncMessage.
 }
 
-void
+bool
 MessageChannel::CancelCurrentTransaction()
 {
     MonitorAutoLock lock(*mMonitor);
-    if (mCurrentTransaction) {
+    if (mCurrentTransaction &&
+        (DispatchingSyncMessagePriority() >= IPC::Message::PRIORITY_HIGH))
+    {
         CancelCurrentTransactionInternal();
         mLink->SendMessage(new CancelMessage());
+        return true;
     }
+    return false;
 }
 
 void
 CancelCPOWs()
 {
-    if (gParentProcessBlocker) {
+    if (gMainThreadBlocker && gMainThreadBlocker->CancelCurrentTransaction()) {
         mozilla::Telemetry::Accumulate(mozilla::Telemetry::IPC_TRANSACTION_CANCEL, true);
-        gParentProcessBlocker->CancelCurrentTransaction();
     }
 }
 
diff --git a/ipc/glue/MessageChannel.h b/ipc/glue/MessageChannel.h
index 17230973340..3aad5d8064c 100644
--- a/ipc/glue/MessageChannel.h
+++ b/ipc/glue/MessageChannel.h
@@ -143,7 +143,7 @@ class MessageChannel : HasResultCodes
         return !mCxxStackFrames.empty();
     }
 
-    void CancelCurrentTransaction();
+    bool CancelCurrentTransaction();
 
     /**
      * This function is used by hang annotation code to determine which IPDL