From: "SZEDER Gábor" <szeder.dev@gmail.com>
To: Jeff King <peff@peff.net>
Cc: "Junio C Hamano" <gitster@pobox.com>,
git@vger.kernel.org, "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
Subject: Re: Is t7006-pager.sh racy?
Date: Thu, 28 Oct 2021 21:55:27 +0200 [thread overview]
Message-ID: <20211028195527.GA2574@szeder.dev> (raw)
In-Reply-To: <YXbsPrU6nRSboQ7r@coredump.intra.peff.net>
On Mon, Oct 25, 2021 at 01:41:18PM -0400, Jeff King wrote:
> On Sun, Oct 24, 2021 at 07:03:49PM +0200, SZEDER Gábor wrote:
>
> > On Sat, Oct 23, 2021 at 05:04:42PM -0700, Junio C Hamano wrote:
> > > It seems under --stress it is fairly easy to break the said test,
> > > especially the one near the end
> >
> > I couldn't reproduce a failure with --stress, but after a cursory look
> > into those tests I doubt that either that test or any of the
> > preceeding SIGPIPE tests added in c24b7f6736 (pager: test for exit
> > code with and without SIGPIPE, 2021-02-02) actually check what they
> > are supposed to.
>
> Yeah, I am puzzled that they are using test_terminal in the first place
> (as opposed to just "git -p"). And you are right that a raw git-log is
> unlikely to be slow enough to get SIGPIPE in most cases.
>
> My usual test for an intentional SIGPIPE is "yes". So something like:
>
> git -p \
> -c core.pager='exit 0' \
> -c alias.yes='!yes' \
> yes
>
> will reliably trigger SIGPIPE from yes, which git.c will then translate
> into an exit code of 141.
Oh, that's clever. Alas it's not applicable to our tests, because
'yes' is not portable; 8648732e29 (t/test-lib.sh: provide a shell
implementation of the 'yes' utility, 2009-08-28).
> If you really want to see SIGPIPE from a builtin (which arguably is the
> more interesting case here, though I think it behaves the same with
> respect to the pager), it's a bit trickier. One way to do it is with a
> command that doesn't generate output until after it gets EOF on stdin.
>
> So something like "git log --stdin" works, but you have to contort
> yourself a bit to make it race-free:
>
> -- >8 --
> # The I/O setup here is:
> #
> # fifo:log-in stdout
> # shell -----------> git-log ------> pager
> # ^ /
> # \-------------------------------/
> # fifo:pager-closed
> #
> # The pager closes its stdin, which will give git-log SIGPIPE. But the
> # tricky part is that after doing so, it signals via fifo to the shell,
> # which then writes to git-log's stdin, triggering it to actually
> # generate output (and get SIGPIPE).
> #
> # You can verify that it's race-free by inserting a "sleep 3" at the
> # front of the pager command (before the exec) and seeing that the
> # other processes wait (and we still get SIGPIPE).
>
> mkfifo pager-closed
> mkfifo log-in
> git config core.pager 'exec 0<&-; echo ready >pager-closed; exit 0'
> (git -p log --stdin <log-in; echo $? >exit-code) &
>
> # we have to open a descriptor rather than just "echo HEAD >log-in", because
> # that will give git-log an immediate EOF on its input when echo closes it, and
> # we must wait until the signal from pager-closed. Likewise we cannot wait
> # for that signal before the echo, because the subshell is blocking on opening
> # log-in until somebody is hooked up to the write end of the pipe.
> exec 9>log-in
> read ok <pager-closed
> echo HEAD >&9
> exec 9>&-
>
> # now we can wait for the subshell to finish and retrieve any output
> # it produced
> wait
> cat exit-code
> -- >8 --
Ugh. I think this would work reliably, but... ugh :)
I wonder whether we could do this as a new pair of 'test-tool'
helpers, one to run the pager through the usual pager-invoking
machinery and to generate a lot of output, the other to be used as the
early-exiting pager, with a pipe between the two to ensure that the
SIGPIPE does happen. Well, essentially the same that you outlined
above but in C instead of shell, which I somehow find less "ugh".
next prev parent reply other threads:[~2021-10-28 19:55 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-24 0:04 Is t7006-pager.sh racy? Junio C Hamano
2021-10-24 17:03 ` SZEDER Gábor
2021-10-25 17:41 ` Jeff King
2021-10-28 19:55 ` SZEDER Gábor [this message]
2021-10-28 22:10 ` Jeff King
2021-11-21 18:40 ` Jeff King
2021-11-21 22:05 ` Jeff King
2021-11-21 22:54 ` [PATCH] t7006: clean up SIGPIPE handling in trace2 tests Jeff King
2021-11-21 23:10 ` Jeff King
2021-11-22 2:17 ` Junio C Hamano
2021-11-22 4:51 ` Jeff King
2021-11-22 4:54 ` Jeff King
2021-11-22 5:49 ` Junio C Hamano
2021-11-22 6:05 ` Junio C Hamano
2021-11-22 19:11 ` Jeff King
2021-11-22 21:28 ` [PATCH] run-command: unify signal and regular logic for wait_or_whine() Jeff King
2021-12-01 14:03 ` Is t7006-pager.sh racy? Ævar Arnfjörð Bjarmason
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
List information: http://vger.kernel.org/majordomo-info.html
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20211028195527.GA2574@szeder.dev \
--to=szeder.dev@gmail.com \
--cc=avarab@gmail.com \
--cc=git@vger.kernel.org \
--cc=gitster@pobox.com \
--cc=peff@peff.net \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://80x24.org/mirrors/git.git
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).