Re: Is t7006-pager.sh racy?

[Jeff King <peff@peff.net>]

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.

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 --

-Peff