[async] Gracefully handle exceptions in async_hooks.

When --async-stack-traces is on and there's an exception from within an
async_hook "after" handler, we will be faced with a settled promise. In
that case we cannot do anything, since the promise will not have any
reactions on it anymore, but we should also not crash of course.

Bug: chromium:896700, v8:7522
Change-Id: I6e3d212d0433da40740489ff7421c5a98cf9bff3
Reviewed-on: https://chromium-review.googlesource.com/c/1290550Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56801}

src/isolate.cc

@@ -614,9 +614,10 @@ bool IsBuiltinFunction(Isolate* isolate, HeapObject* object,
 
 void CaptureAsyncStackTrace(Isolate* isolate, Handle<JSPromise> promise,
                             FrameArrayBuilder* builder) {
-  CHECK_EQ(Promise::kPending, promise->status());
-
   while (!builder->full()) {
+    // Check that the {promise} is not settled.
+    if (promise->status() != Promise::kPending) return;
+
     // Check that we have exactly one PromiseReaction on the {promise}.
     if (!promise->reactions()->IsPromiseReaction()) return;
     Handle<PromiseReaction> reaction(

test/mjsunit/regress/regress-crbug-896700.js

+// Copyright 2018 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --async-stack-traces --expose-async-hooks
+
+async_hooks.createHook({
+  after() { throw new Error(); }
+}).enable();
+Promise.resolve().then();