Allow microtasks to throw exceptions and handle them gracefully

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index f7605d8d21b346e85a78c557aca0b74d78cb624c..5bd2b69815b71f2e6d961e98d61e749fde2b1c32 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -810,6 +810,15 @@ Object* Isolate::TerminateExecution() {
 }
 
 
+bool Isolate::IsExecutionTerminating() {
+  if (!IsInitialized()) return false;
+  if (has_scheduled_exception()) {
+    return scheduled_exception() == heap()->termination_exception();
+  }
+  return false;
+}
+
+
 void Isolate::CancelTerminateExecution() {
   if (try_catch_handler()) {
     try_catch_handler()->has_terminated_ = false;
@@ -2281,9 +2290,15 @@ void Isolate::RunMicrotasks() {
     for (int i = 0; i < num_tasks; i++) {
       HandleScope scope(this);
       Handle<JSFunction> microtask(JSFunction::cast(queue->get(i)), this);
-      // TODO(adamk): This should ignore/clear exceptions instead of Checking.
-      Execution::Call(this, microtask, factory()->undefined_value(),
-                      0, NULL).Check();
+      MaybeHandle<Object> result = Execution::Call(
+          this, microtask, factory()->undefined_value(), 0, NULL);
+      if (result.is_null()) {
+        // Microtasks should not throw unless the embedder has asked us to
+        // TerminateExecution.
+        ASSERT(IsExecutionTerminating());
+        handle_scope_implementer()->DecrementCallDepth();

[jochen (gone - plz use gerrit), 2014/05/22 07:45:38]

why not just break?

[adamk, 2014/05/22 08:02:16]

We're inside a while and a for loop. I would have added a goto but I didn't see
any others in v8...

+        return;
+      }
     }
   }