Re: Changes to job handling cause hangs in wait

From: Herbert Xu <herbert@gondor.apana.org.au>
To: Harald van Dijk <harald@gigawatt.nl>
Cc: Andrej Shadura <andrew.shadura@collabora.co.uk>,
	dash@vger.kernel.org, Michael Biebl <biebl@debian.org>,
	974705@bugs.debian.org
Subject: Re: Changes to job handling cause hangs in wait
Date: Tue, 1 Dec 2020 21:34:48 +1100	[thread overview]
Message-ID: <20201201103448.GA28234@gondor.apana.org.au> (raw)
In-Reply-To: <0e6bb3c5-96d9-18e1-e6bb-87cb34f85fab@gigawatt.nl>

On Tue, Dec 01, 2020 at 10:14:04AM +0000, Harald van Dijk wrote:
> 
> POSIX says:
> 
>   "If  the  wait utility is invoked with no operands, it shall wait until
> all process IDs known to the invoking shell have terminated and exit with a
> zero exit status."
> 
> I would say that child processes that were created before dash was started
> do not have process IDs known to dash.

Well I disagree and the fact that the original ksh does the same
thing is an important fact.

> > bash -c set -e; export USER=`id -nu`; . /etc/profile >/dev/null 2>&1 || true;  . ~/.profile >/dev/null 2>&1 || true; buildtree="/tmp/autopkgtest-lxc.is4n6xxr/downtmp/build.f2G/real-tree"; mkdir -p -m 1777 -- "/tmp/autopkgtest-lxc.is4n6xxr/downtmp/timedated-artifacts"; export AUTOPKGTEST_ARTIFACTS="/tmp/autopkgtest-lxc.is4n6xxr/downtmp/timedated-artifacts"; export ADT_ARTIFACTS="$AUTOPKGTEST_ARTIFACTS"; mkdir -p -m 755 "/tmp/autopkgtest-lxc.is4n6xxr/downtmp/autopkgtest_tmp"; export AUTOPKGTEST_TMP="/tmp/autopkgtest-lxc.is4n6xxr/downtmp/autopkgtest_tmp"; export ADTTMP="$AUTOPKGTEST_TMP"; export DEBIAN_FRONTEND=noninteractive; export LANG=C.UTF-8; export DEB_BUILD_OPTIONS=parallel=2; unset LANGUAGE LC_CTYPE LC_NUMERIC LC_TIME LC_COLLATE   LC_MONETARY LC_MESSAGES LC_PAPER LC_NAME LC_ADDRESS   LC_TELEPHONE LC_MEASUREMENT LC_IDENTIFICATION LC_ALL;rm -f /tmp/autopkgtest_script_pid; set -C; echo $$ > /tmp/autopkgtest_script_pid; set +C; trap "rm -f /tmp/autopkgtest_script_pid" EXIT INT Q
 UIT PIPE; cd "$buildtree"; export AUTOPKGTEST_NORMAL_USER=; export ADT_NORMAL_USER=; chmod +x /tmp/autopkgtest-lxc.is4n6xxr/downtmp/build.f2G/real-tree/debian/tests/timedated; touch /tmp/autopkgtest-lxc.is4n6xxr/downtmp/timedated-stdout /tmp/autopkgtest-lxc.is4n6xxr/downtmp/timedated-stderr; /tmp/autopkgtest-lxc.is4n6xxr/downtmp/build.f2G/real-tree/debian/tests/timedated 2> >(tee -a /tmp/autopkgtest-lxc.is4n6xxr/downtmp/timedated-stderr >&2) > >(tee -a /tmp/autopkgtest-lxc.is4n6xxr/downtmp/timedated-stdout);
> > 
> > For some reason this is causing the final two tee's to be created
> > as children of debian/tests/timedated rather than the bash shell.
> 
> This is because of the same optimisation that dash also has, where it tries
> to avoid creating a subshell for the last command in a list when it can just
> exec() without a fork() instead. A minimal example without an explicit exec
> is
> 
>   bash -c 'dash -c ": & wait" <(sleep 1d)'

I'm not sure about that because bash itself is still hanging around,
if it were really the -c optimisation then bash should not appear in
the ps output at all.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt