billiard.einfo.Frame: add f_back attribute

[terceiro]

Standard Python frame objects have a f_back attribute, which is used
in places such as the Django AdminEmailHandler. The lack of this
attribute in the billiard Frame causes a crash when celery is used in
Django apps, configured to use Django logging, and AdminEmailHandler is
in use. See for example Django bug #27543:

https://code.djangoproject.com/ticket/27543

In general, this also makes the billiard Frame object more compatible
with any code that handles frames, and could be using f_back.

[thedrow]

Thanks for the contribution.

[ianhoffman]

Hey @thedrow and @terceiro -- just wanted to check in real quick and ask what's up with this bugfix. Anything still needed to do to get it into the development branch of celery? I'd be interested in helping if possible.

[thedrow]

Only lack of time is preventing me from releasing.
I'll try to get to it.

[ianhoffman]

Thanks! I'll keep an eye out. In the meantime have fixed by (in case this is useful to anyone):

class MyEmailHandler(AdminEmailHandler):
    def emit(self, record):
        if record.exc_info:
            from billiard import einfo
            exc_typ, exc_val, tb = record.exc_info
            if isintance(tb, einfo.Traceback):
                record.exc_info = exc_type, exc_val, exc_val.__traceback__
        super().emit(record)

[thedrow]

Released.

[ianhoffman]

Thanks @thedrow! Getting this into production soon.