Clear pending message during Isolate::CancelScheduledExceptionFromTryCatch.

Without doing this, a JSMessageObject can be kept alive by the isolate, which
in turn keeps the context alive, until the message is cleared.

BUG=v8:5941

Review-Url: https://codereview.chromium.org/2675203005
Cr-Commit-Position: refs/heads/master@{#43043}

src/isolate.cc

@@ -1460,6 +1460,9 @@ void Isolate::CancelScheduledExceptionFromTryCatch(v8::TryCatch* handler) {
     DCHECK(scheduled_exception() != heap()->termination_exception());
     clear_scheduled_exception();
   }
+  if (thread_local_top_.pending_message_obj_ == handler->message_obj_) {
+    clear_pending_message();
+  }
 }
 
 

test/unittests/BUILD.gn

@@ -10,6 +10,7 @@ v8_executable("unittests") {
   sources = [
     "../../testing/gmock-support.h",
     "../../testing/gtest-support.h",
+    "api/exception-unittest.cc",
     "api/isolate-unittest.cc",
     "base/atomic-utils-unittest.cc",
     "base/bits-unittest.cc",

test/unittests/api/exception-unittest.cc

+// Copyright 2017 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.
+
+#include "include/v8.h"
+#include "src/flags.h"
+#include "test/unittests/test-utils.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace v8 {
+namespace {
+
+using APIExceptionTest = TestWithIsolate;
+
+class ScopedExposeGc {
+ public:
+  ScopedExposeGc() : was_exposed_(i::FLAG_expose_gc) {
+    i::FLAG_expose_gc = true;
+  }
+  ~ScopedExposeGc() { i::FLAG_expose_gc = was_exposed_; }
+
+ private:
+  const bool was_exposed_;
+};
+
+TEST_F(APIExceptionTest, ExceptionMessageDoesNotKeepContextAlive) {
+  ScopedExposeGc expose_gc;
+  Persistent<Context> weak_context;
+  {
+    HandleScope handle_scope(isolate());
+    Local<Context> context = Context::New(isolate());
+    weak_context.Reset(isolate(), context);
+    weak_context.SetWeak();
+
+    Context::Scope context_scope(context);
+    TryCatch try_catch(isolate());
+    isolate()->ThrowException(Undefined(isolate()));
+  }
+  isolate()->RequestGarbageCollectionForTesting(
+      Isolate::kFullGarbageCollection);
+  EXPECT_TRUE(weak_context.IsEmpty());
+}
+
+}  // namespace
+}  // namespace v8

test/unittests/unittests.gyp

@@ -8,6 +8,7 @@
   'variables': {
     'v8_code': 1,
     'unittests_sources': [  ### gcmole(all) ###
+      'api/exception-unittest.cc',
       'api/isolate-unittest.cc',
       'base/atomic-utils-unittest.cc',
       'base/bits-unittest.cc',