Subject: Re: [PATCH] t9300-fast-import: don't hang if background fast-import exits too early

From: SZEDER Gábor <[email-masked]>
Date: Sun, 1 Dec 2019 10:31:48 +0100

On Sat, Nov 30, 2019 at 01:16:30PM -0800, Junio C Hamano wrote:
> SZEDER Gábor <[email-masked]> writes:
> 
> > +	(
> > +		git fast-import $options <&8 >&9 &
> > +		echo $! >V.fi.pid
> > +		wait $!
> > +		echo >&2 "background fast-import terminated too early with exit code $?"
> > +		# Un-block the read loop in the main shell process.
> > +		echo >&9 UNEXPECTED
> > +	) &
> > +	echo $! >V.sh.pid
> >  	# We don't mind if fast-import has already died by the time the test
> >  	# ends.
> > -	test_when_finished "
> > +	test_when_finished '
> >  		exec 8>&-; exec 9>&-;
> > -		kill $(cat V.pid) && wait $(cat V.pid)
> > -		true"
> > +		kill $(cat V.sh.pid) && wait $(cat V.sh.pid)
> > +		kill $(cat V.fi.pid) && wait $(cat V.sh.pid)
> > +		true'
> 
> The original interpolates the PID of the fast-import when
> "when-finished" program is registered, so it is OK if somebody else
> removed V.pid file; the new one interpolates when "when-finished"
> program is run, reading from V.??.pid, so somebody needs to make
> sure these pid files will stay around.  I do not think it is an
> issue as I suspect we've left it to the global clean-up procedure
> that removes the trash directory to remove the pid file.

In the original the same shell process starts 'git fast-import',
writes its pidfile, and registers the test_when_finished commands, so
we can be sure that the pid file is already present when the shell
runs the $(cat V.pid) command substitutions.

With this patch that's not the case anymore, because the background
subshell starts 'git fast-import' and writes the pidfile, but the main
shell process registers the test_when_finished commands.  IOW these
two shell processes are racing, and it's possible that the
test_when_finished command is executed before the background subshell
can write the pidfile.  So double quotes around the block of
test_when_finished commands are not good.

> By the way, does the second "kill && wait" wait for the right
> process?

Ouch, it clearly doesn't.  Copy-paste error I suppose.
Thanks for spotting it.

> 
> >  background_import_still_running () {
> > -	if ! kill -0 "$(cat V.pid)"
> > +	if ! kill -0 "$(cat V.fi.pid)"
> >  	then
> >  		echo >&2 "background fast-import terminated too early"
> >  		false
> >  	fi
> >  }