MessagePort: don't post repeated message dispatch tasks.

third_party/WebKit/Source/core/dom/MessagePort.cpp

Index: third_party/WebKit/Source/core/dom/MessagePort.cpp
diff --git a/third_party/WebKit/Source/core/dom/MessagePort.cpp b/third_party/WebKit/Source/core/dom/MessagePort.cpp
index 99d3222bc316012403fcf1cc22c7b1b58c019419..4e4a711cb3ab4bb7aee8fdb7b9b9f0cae920bbe6 100644
--- a/third_party/WebKit/Source/core/dom/MessagePort.cpp
+++ b/third_party/WebKit/Source/core/dom/MessagePort.cpp
@@ -39,7 +39,7 @@
 #include "core/frame/LocalDOMWindow.h"
 #include "core/workers/WorkerGlobalScope.h"
 #include "public/platform/WebString.h"
-#include "wtf/Functional.h"
+#include "wtf/Atomics.h"
 #include "wtf/PtrUtil.h"
 #include "wtf/text/AtomicString.h"
 
@@ -51,6 +51,7 @@ MessagePort* MessagePort::create(ExecutionContext& executionContext) {
 
 MessagePort::MessagePort(ExecutionContext& executionContext)
     : ContextLifecycleObserver(&executionContext),
+      m_pendingDispatchTask(0),
       m_started(false),
       m_closed(false) {}
 
@@ -118,6 +119,10 @@ WebMessagePortChannelUniquePtr MessagePort::disentangle() {
 // non-threadsafe APIs (i.e. should not call into the entangled channel or
 // access mutable variables).
 void MessagePort::messageAvailable() {
+  // Don't post another task if there's an identical one pending.
+  if (atomicTestAndSetToOne(&m_pendingDispatchTask))

[dcheng, 2017/02/27 08:06:37]

It really feels like we should have a better way of doing this (since there were
no callers of this function before this CL), but maybe it's not a very common
situation.

[sof, 2017/02/27 08:21:37]

"this" == cross-thread prevention of task re-posts..? I haven't come across
anything similar in Blink, fwiw.

(Are we allowed to use std::atomic in WebKit/Source/, btw?)

+    return;
+
   DCHECK(getExecutionContext());
   // TODO(tzik): Use ParentThreadTaskRunners instead of ExecutionContext here to
   // avoid touching foreign thread GCed object.
@@ -184,6 +189,11 @@ bool MessagePort::tryGetMessage(RefPtr<SerializedScriptValue>& message,
 }
 
 void MessagePort::dispatchMessages() {
+  // Signal to |messageAvailable()| that there are no ongoing
+  // dispatches of messages. This can cause redundantly posted
+  // tasks, but safely avoids messages languishing.
+  releaseStore(&m_pendingDispatchTask, 0);
+
   // Messages for contexts that are not fully active get dispatched too, but
   // JSAbstractEventListener::handleEvent() doesn't call handlers for these.
   // The HTML5 spec specifies that any messages sent to a document that is not