Fix crash from turning on DevTools in the middle of catch prediction

src/isolate.cc

 // Copyright 2012 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 "src/isolate.h"
 
 #include <stdlib.h>
 
 #include <fstream>  // NOLINT(readability/streams)
 #include <sstream>
@@ skipping 1733 matching lines @@
           // await, the function which has this exception event has not yet
           // returned, so the generated Promise has not yet been marked
           // by AsyncFunctionAwaitCaught with promiseHandledHintSymbol.
           Handle<Symbol> key = factory()->promise_handled_hint_symbol();
           JSObject::SetProperty(Handle<JSObject>::cast(retval), key,
                                 factory()->true_value(), STRICT)
               .Assert();
         }
         return retval;
       case HandlerTable::PROMISE:
-        return promise_on_stack->promise();
+        return promise_on_stack
+                   ? Handle<Object>::cast(promise_on_stack->promise())
+                   : undefined;
       case HandlerTable::ASYNC_AWAIT: {
         // If in the initial portion of async/await, continue the loop to pop up
         // successive async/await stack frames until an asynchronous one with
         // dependents is found, or a non-async stack frame is encountered, in
         // order to handle the synchronous async/await catch prediction case:
         // assume that async function calls are awaited.
-        retval = promise_on_stack->promise();
+        if (promise_on_stack) retval = promise_on_stack->promise();

[gsathya, 2016/09/24 01:14:09]

Can you not return early if you hit an undefined? 

[Dan Ehrenberg, 2016/09/24 03:08:20]

You're right, that's a cleaner way to do it. Fixed.

         if (PromiseHasUserDefinedRejectHandler(retval)) {
           return retval;
         }
-        promise_on_stack = promise_on_stack->prev();
+        if (promise_on_stack) promise_on_stack = promise_on_stack->prev();
         continue;

[gsathya, 2016/09/24 01:14:09]

Do you need this continue?

[Dan Ehrenberg, 2016/09/24 03:08:20]

In this switch statement, each case ends with either return or continue. I think
the pattern makes the code easier to read because it means that the reader
doesn't have to look to the end of the loop to figure out what's going to
happen.

       }
     }
   }
   return retval;
 }
 
 
 void Isolate::SetCaptureStackTraceForUncaughtExceptions(
       bool capture,
       int frame_limit,
@@ skipping 1504 matching lines @@
   // Then check whether this scope intercepts.
   if ((flag & intercept_mask_)) {
     intercepted_flags_ |= flag;
     return true;
   }
   return false;
 }
 
 }  // namespace internal
 }  // namespace v8