v8 shell crashes with Fatal error in v8::ToLocalChecked Empty MaybeLocal.

[Jane Chen]

[jchen@jchen-z620 x64.release]$ ./shell
V8 version 4.7.0 (candidate) [sample shell]
> read();
(shell):1: Bad parameters
read();
^

#
# Fatal error in v8::ToLocalChecked
# Empty MaybeLocal.
#

Illegal instruction (core dumped)

This is with a build made with either:

make -j8 x64.debug -werror=no GYPFLAGS="-Dv8_use_external_startup_data=0 -Dcomponent=shared_library"

or:

make -j8 x64.release -werror=no GYPFLAGS="-Dv8_use_external_startup_data=0 -Dcomponent=shared_library"

I see this with my embedding application as well as long as I throw an exception from a callback.  Am I missing something or should I use a more stable branch?  If so, which branch of v8 works with the above?

Thanks in advance.

[Jakob Kummerow]

The whole point of a MaybeLocal is that it may (or may not) contain a value. Calling .ToLocalChecked() on it is unsafe, unless you happen to know that it can't be empty. shell.cc:416 is doing it wrong. d8.cc:883 is doing it right.

A callback throwing an exception is the canonical example of a function that possibly returns no value, in which case its MaybeLocal return value is empty. The reason we've introduced MaybeLocals is so embedders can be sure they've covered all the places where they have to check for exceptions being thrown, since they can't accidentally convert from MaybeLocal to Local without explicitly checking for emptiness.

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[Jane Chen]

Thanks.  Any tip in debugging this kind of failures?  I don't seem to get a stack even though I'm using debug build both for v8 and my embedding app.

[Jane Chen]

Actually, forget that.  I do get stack for shell.  Not sure why I'm not getting it in my app.  Looking...