[error] Clear external_caught_exception in Error formatting

Clearing the pending exception is not enough - if we want to swallow an
exception while currently on top of an external handler (e.g. TryCatch),
we also need to clear external_caught_exception.

BUG=chromium:719380

Review-Url: https://codereview.chromium.org/2870423002
Cr-Commit-Position: refs/heads/master@{#45247}

src/messages.cc

@@ -890,13 +890,14 @@ MaybeHandle<Object> AppendErrorString(Isolate* isolate, Handle<Object> error,
     Handle<Object> pending_exception =
         handle(isolate->pending_exception(), isolate);
     isolate->clear_pending_exception();
+    isolate->set_external_caught_exception(false);
 
     err_str = ErrorUtils::ToString(isolate, pending_exception);
     if (err_str.is_null()) {
       // Formatting the thrown exception threw again, give up.
       DCHECK(isolate->has_pending_exception());
       isolate->clear_pending_exception();
-
+      isolate->set_external_caught_exception(false);
       builder->AppendCString("<error>");
     } else {
       // Formatted thrown exception successfully, append it.
@@ -991,6 +992,7 @@ MaybeHandle<Object> ErrorUtils::FormatStackTrace(Isolate* isolate,
       Handle<Object> pending_exception =
           handle(isolate->pending_exception(), isolate);
       isolate->clear_pending_exception();
+      isolate->set_external_caught_exception(false);
 
       maybe_frame_string = ErrorUtils::ToString(isolate, pending_exception);
       if (maybe_frame_string.is_null()) {
@@ -1222,6 +1224,7 @@ Handle<String> FormatMessage(Isolate* isolate, int template_index,
            .ToHandle(&msg)) {
     DCHECK(isolate->has_pending_exception());
     isolate->clear_pending_exception();
+    isolate->set_external_caught_exception(false);
     return isolate->factory()->NewStringFromAsciiChecked("<error>");
   }
 

test/mjsunit/regress/regress-719380.js

+// 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.
+
+TypeError.prototype.__defineGetter__("name", () => { throw 42; });
+console.log({ toString: () => { throw new TypeError() }});
+try { new WebAssembly.Table({}); } catch (e) {}