I just observed a case of $SUBJECT: I typed control-D to end a shell
session, and bash replied with "Exit", but the xterm window didn't
close as it normally does. What's more, clicking on the red
window-close button did nothing, which seemed even more surprising.
I looked closer, and found:
1. According to ps, that xterm instance was chewing 100% CPU. Its
child process was still there, too, but listed as "(bash)", which
I think means it was a zombie. So this suggests a failure to respond
to SIGCHLD properly.
2. I tried to attach to the process with gdb to get a stack trace.
gdb wouldn't let me (something about an OS/kernel failure). On
reflection maybe I should have tried with Activity Monitor, but that
didn't occur to me in the heat of the moment.
3. I then tried "kill -ABRT", hoping that would produce a debuggable
core dump. It stopped the xterm alright, but I can't find a core.
Anyway, this is the first time I've seen anything like this, so it
isn't terribly reproducible. Just mentioning it in case anyone has
seen the like, or has a better idea about how to gather evidence
if it happens again.
regards, tom lane