Bug 968335 - Add an API to determine if a given AutoCxPusher corresponds to the stack-top cx push. r=bz

This patch, and those following, are part of an epic quest to make this API
work properly despite the fact that we don't yet have GetEntryGlobal. See
the comment a few patches forward.

js/xpconnect/src/nsCxPusher.cpp

@@ -115,6 +115,7 @@ AutoCxPusher::AutoCxPusher(JSContext* cx, bool allowNull)
   if (!stack->Push(cx)) {
     MOZ_CRASH();
   }
+  mStackDepthAfterPush = stack->Count();
 
 #ifdef DEBUG
   mPushedContext = cx;
@@ -163,6 +164,14 @@ AutoCxPusher::~AutoCxPusher()
   mScx = nullptr;
 }
 
+bool
+AutoCxPusher::IsStackTop()
+{
+  uint32_t currentDepth = XPCJSRuntime::Get()->GetJSContextStack()->Count();
+  MOZ_ASSERT(currentDepth >= mStackDepthAfterPush);
+  return currentDepth == mStackDepthAfterPush;
+}
+
 AutoJSContext::AutoJSContext(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_IN_IMPL)
   : mCx(nullptr)
 {

js/xpconnect/src/nsCxPusher.h

@@ -31,10 +31,15 @@ public:
 
   nsIScriptContext* GetScriptContext() { return mScx; }
 
+  // Returns true if this AutoCxPusher performed the push that is currently at
+  // the top of the cx stack.
+  bool IsStackTop();
+
 private:
   mozilla::Maybe<JSAutoRequest> mAutoRequest;
   mozilla::Maybe<JSAutoCompartment> mAutoCompartment;
   nsCOMPtr<nsIScriptContext> mScx;
+  uint32_t mStackDepthAfterPush;
 #ifdef DEBUG
   JSContext* mPushedContext;
   unsigned mCompartmentDepthOnEntry;