[Dovecot] assertion failed and debugging dovecot

I'm getting this error on a particular message in my Maildir:

Oct 11 21:59:48 aurora dovecot: IMAP(doug): file ostream-file.c: line 646 (_send_istream): assertion failed: (instream->v_offset <= in_size)

To me, more import than the question "why is this happening" is "how can I debug this?" Attaching gdb to the running imap process doesn't give me access to the proper function names to set up breakpoints and get a stack trace.

Can someone with the experience give a quick how-to on debugging functions deep in dovecot?

Thanks,

Doug