Bug 989528 - Introduce a mechanism to explicitly enter a null compartment. r=luke

js/src/jsapi.cpp

@@ -984,6 +984,19 @@ JSAutoCompartment::~JSAutoCompartment()
     cx_->leaveCompartment(oldCompartment_);
 }
 
+JSAutoNullCompartment::JSAutoNullCompartment(JSContext *cx)
+  : cx_(cx),
+    oldCompartment_(cx->compartment())
+{
+    AssertHeapIsIdleOrIterating(cx_);
+    cx_->enterNullCompartment();
+}
+
+JSAutoNullCompartment::~JSAutoNullCompartment()
+{
+    cx_->leaveCompartment(oldCompartment_);
+}
+
 JS_PUBLIC_API(void)
 JS_SetCompartmentPrivate(JSCompartment *compartment, void *data)
 {

js/src/jsapi.h

@@ -1677,6 +1677,15 @@ class JS_PUBLIC_API(JSAutoCompartment)
     ~JSAutoCompartment();
 };
 
+class JS_PUBLIC_API(JSAutoNullCompartment)
+{
+    JSContext *cx_;
+    JSCompartment *oldCompartment_;
+  public:
+    JSAutoNullCompartment(JSContext *cx);
+    ~JSAutoNullCompartment();
+};
+
 /* NB: This API is infallible; a nullptr return value does not indicate error. */
 extern JS_PUBLIC_API(JSCompartment *)
 JS_EnterCompartment(JSContext *cx, JSObject *target);

js/src/jscntxt.h

@@ -350,6 +350,7 @@ class ExclusiveContext : public ThreadSafeContext
 #endif
 
     inline void enterCompartment(JSCompartment *c);
+    inline void enterNullCompartment();
     inline void leaveCompartment(JSCompartment *oldCompartment);
 
     void setWorkerThread(WorkerThread *workerThread);

js/src/jscntxtinlines.h

@@ -407,6 +407,13 @@ js::ExclusiveContext::enterCompartment(JSCompartment *c)
     setCompartment(c);
 }
 
+inline void
+js::ExclusiveContext::enterNullCompartment()
+{
+    enterCompartmentDepth_++;
+    setCompartment(nullptr);
+}
+
 inline void
 js::ExclusiveContext::leaveCompartment(JSCompartment *oldCompartment)
 {
@@ -417,7 +424,8 @@ js::ExclusiveContext::leaveCompartment(JSCompartment *oldCompartment)
     // compartment.
     JSCompartment *startingCompartment = compartment_;
     setCompartment(oldCompartment);
-    startingCompartment->leave();
+    if (startingCompartment)
+        startingCompartment->leave();
 }
 
 inline void