Optionally aborting on uncaught exception

[Dave Pacheco]

Last year I worked with members of the V8 team to add low-overhead debug

information to V8 so that we could debug Node.js programs postmortem.[1]  This

information has proven quite valuable, and we've configured our Node programs to

invoke abort(3C) (via Node's process.abort()) whenever we get the

"uncaughtException" event.  We've nailed several nasty production issues based

on the resulting core files without having to reproduce the problem.  See [2]

for a particularly nasty example.

That said, there's one problem with this approach: by the time we get to the

uncaughtException handler, all of the stack state associated with the exception

is gone.  If we blow an assertion or trip over an undefined variable inside a

function, it's often valuable to know what the arguments to that function were,

but that's all gone by the time the event is emitted.  My proposal is to add a

command line flag that causes V8 to call OS::Abort when an uncaught exception is

emitted.  The diffs are pretty small:

https://github.com/davepacheco/v8/commit/d917219c4c43511ecc05b0a466fa52058d9cfe53

but the result is that we get a core file with the current JavaScript stack

intact, including arguments.  This particular change helped us nail two

instances of RangeError (see [3] for my write-up of one of them).

If the team is okay with this change, I'd like to submit a code review and have

it integrated into V8.

Thanks,

Dave

[1] https://chromiumcodereview.appspot.com/8803024

[2] https://github.com/joyent/node/issues/4785

[3] http://dtrace.org/blogs/dap/2012/05/14/debugging-rangeerror-from-a-core-dump/

[Yang Guo]

Hi Dave,

Not sure if this really needs adding a command line flag. Wouldn't it be sufficient to set a break point in DoThrow when you need it? Or is it not possible to attach gdb? In that case, please follow https://code.google.com/p/v8/wiki/Contributing so that we can land the patch for you. The patch by itself looks alright to me, but you might want to consider adding OS::DumpBacktrace() as well.

Yang

[Jakob Kummerow]

Yang, Dave has already uploaded the patch at https://codereview.chromium.org/13071005/. It would be great if you had time to take a look, I'm not sure when I'll get around to it myself.

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