Bug 1292699 - Add a small helper to deal with cases when native side needs to ensure that slow script dialog shows up, r=bholley

--HG--
extra : rebase_source : ab0fc332e5b54921d7a05e0c008c65b29947ee27

dom/base/ScriptSettings.cpp

@@ -812,4 +812,22 @@ AutoSafeJSContext::AutoSafeJSContext(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_IN_IMP
              "returned null, and inited correctly otherwise!");
 }
 
+AutoSlowOperation::AutoSlowOperation(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM_IN_IMPL)
+  : AutoJSAPI()
+{
+  MOZ_ASSERT(NS_IsMainThread());
+
+  MOZ_GUARD_OBJECT_NOTIFIER_INIT;
+
+  Init();
+}
+
+void
+AutoSlowOperation::CheckForInterrupt()
+{
+  // JS_CheckForInterrupt expects us to be in a compartment.
+  JSAutoCompartment ac(cx(), xpc::UnprivilegedJunkScope());
+  JS_CheckForInterrupt(cx());
+}
+
 } // namespace mozilla

dom/base/ScriptSettings.h

@@ -443,6 +443,23 @@ private:
   MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
 };
 
+/**
+ * Use AutoSlowOperation when native side calls many JS callbacks in a row
+ * and slow script dialog should be activated if too much time is spent going
+ * through those callbacks.
+ * AutoSlowOperation puts a JSAutoRequest on the stack so that we don't continue
+ * to reset the watchdog and CheckForInterrupt can be then used to check whether
+ * JS execution should be interrupted.
+ */
+class MOZ_RAII AutoSlowOperation : public dom::AutoJSAPI
+{
+public:
+  explicit AutoSlowOperation(MOZ_GUARD_OBJECT_NOTIFIER_ONLY_PARAM);
+  void CheckForInterrupt();
+private:
+  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
+};
+
 } // namespace mozilla
 
 #endif // mozilla_dom_ScriptSettings_h

dom/base/nsDOMMutationObserver.cpp

@@ -885,10 +885,7 @@ nsDOMMutationObserver::HandleMutationsInternal()
     return;
   }
 
-  // We need the AutoSafeJSContext to ensure the slow script dialog is
-  // triggered. AutoSafeJSContext does that by pushing JSAutoRequest to stack.
-  // This needs to be outside the while loop.
-  AutoSafeJSContext cx;
+  AutoSlowOperation aso;
 
   nsTArray<RefPtr<nsDOMMutationObserver> >* suppressedObservers = nullptr;
 
@@ -910,7 +907,7 @@ nsDOMMutationObserver::HandleMutationsInternal()
       }
     }
     delete observers;
-    JS_CheckForInterrupt(cx);
+    aso.CheckForInterrupt();
   }
 
   if (suppressedObservers) {

dom/promise/Promise.cpp

@@ -992,7 +992,7 @@ Promise::PerformMicroTaskCheckpoint()
     return false;
   }
 
-  AutoSafeJSContext cx;
+  AutoSlowOperation aso;
 
   do {
     nsCOMPtr<nsIRunnable> runnable = microtaskQueue.front().forget();
@@ -1004,7 +1004,7 @@ Promise::PerformMicroTaskCheckpoint()
     if (NS_WARN_IF(NS_FAILED(rv))) {
       return false;
     }
-    JS_CheckForInterrupt(cx);
+    aso.CheckForInterrupt();
     runtime->AfterProcessMicrotask();
   } while (!microtaskQueue.empty());