Re: git fails with a broken pipe when one quits the pager

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Feb 1, 2021, 12:10 UTC
Message-ID: <87im7cng42.fsf@evledraar.gmail.com>
In-Reply-To: <20210201103429.GT623063@zira.vinc17.org>

On Mon, Feb 01 2021, Vincent Lefevre wrote:

Show 17 quoted lines

> On 2021-01-31 21:49:49 +0100, ï¿½var Arnfjï¿½rï¿½ Bjarmason wrote:
>> On Sun, Jan 31 2021, Vincent Lefevre wrote:
>> > FYI, I already have the exit status already in my prompt (the above
>> > commands were just for the example). Still, the git behavior is
>> > disturbing.
>> >
>> > Moreover, this doesn't solve the issue when doing something like
>> >
>> >   git log && some_other_command
>> 
>> What issue? That we're returning an exit code per getting a SIGHUP here
>> is a feature. Consider:
>> 
>>     git -c core.pager=/bin/false log && echo showed you the output
>
> If the pager exists with a non-zero exit status, it is normal to
> return a non-zero exit status. This was not the bug I reported.

Is it normal? Isn't this subject to the same race noted in https://lore.kernel.org/git/20191115040909.GA21654@sigill.intra.peff.net/

I.e. we start the /bin/false process, then start spewing output to it, so maybe we'll get a SIGPIPE first because it's not being consumed, or maybe /bin/false (or whatever else exits with non-zero) will exit first.

Unrelated to that there's at least a bug in wait_for_pager_signal() in how we log the pager's exit. We just rely on "ret" in finish_command_in_signal() before calling trace2_child_exit(), but should probably log -1 there or something and defer.

Show 6 quoted lines

>> > No! I want to be warned about real SIGPIPEs.
>> 
>> Not being able to write "git log" output is a real SIGPIPE.
>
> Which is not the case here, because the full output has never been
> requested by the user.

They requested it by running "git log", which e.g. for git.git is ~1 million lines. Then presumably paged down just a few pages and issued "q" in their pager. At which point we'll fail on the write() in git-log.

The pager's exit status is usually/always 0 in those cases (e.g. https://pubs.opengroup.org/onlinepubs/9699919799/utilities/more.html). So we've got the SIGPIPE to indicate the output wasn't fully consumed.

Show 20 quoted lines

>> Is it because in your mind it's got something to do with the "|" shell
>> piping construct? The SIGPIPE is sent by the kernel, so it's no less
>> expected in cases like:
>> 
>>     git log && echo foo
>> 
>> Than:
>> 
>>     git log | cat
>
> See the difference (without the patch) between
>
> $ git log && echo foo; echo $?
> 141
>
> and
>
> $ git log | head; echo $?
> [...]
> 0

Presumably that first command is one where you exited your pager before the output wasn't fully consumed, see above.

Show 19 quoted lines

> [...]
>> Maybe we have users who'd like to work around zsh's "setopt
>> PRINT_EXIT_VALUE" mode (would you want this patch if you could make zsh
>> ignore 141?).
>
> zsh is working as expected, and as I've already said, I ***WANT***
> SIGPIPE to be reported by the shell, as it may indicate a real failure
> in a script. BTW, I even have a script using git that relies on that:
>
> { git rev-list --author "$@[-1]" HEAD &&
>   git rev-list --grep   "$@[-1]" HEAD } | \
>   git "${@[1,-2]:-lv}" --no-walk --stdin
>
> return $((pipestatus[2] ? pipestatus[2] : pipestatus[1]))
>
> Here it is important not to lose any information. No pager is
> involved, the full output is needed. If for some reason, the
> LHS of the pipe fails due to a SIGPIPE but the right hand side
> succeeds, the error will be reported.

Sorry, I really don't see how this is different. I think this goes back to my "'|' shell piping construct[...]" question in the E-Mail you're replying to.

in both the "git log &&" case and potentially here you'll get a program writing to a pipe getting a SIGPIPE, which is then reflected in the exit code.

> The fact is that with a pager, the SIGPIPE with a pager is normal.
> Thus with a pager, git is reporting a spurious SIGPIPE, and this
> is disturbing.

I don't get what you're trying to say here, sorry.

Maybe this helps. So first, I don't know if your report came out of reading the recent "set -o pipefail" traffic on-list. As you can see in [1] I'm not some zealot for PIPEFAIL always being returned no matter what.

The difference between that though and what you're proposing is there you have the shell getting an exit code and opting to ignore it, as opposed to the program itself sweeping it under the rug.

I don't think either that just because you run a pager you're obligated to ferry down a SIGPIPE if you get it. E.g. mysql and postgresql both have interactive shells where you can open pagers. I didn't bother to check, but you can imagine doing a "show tables" or whatever and only viewing the first page, then quitting in the pager.

If that's part of a long interactive SQL session it would make no sense for the eventual exit code of mysql(1) or psql(1) to reflect that.

But with git we're (mostly) executing one-shot commands, e.g. with "git log" you give it some params, and it spews all the output at you, maybe with the help of a pager.

So then if we fail on the write() I don't see how it doesn't make sense to return the appropriate exit code for that failure downstream.

1. https://lore.kernel.org/git/20210116153554.12604-12-avarab@gmail.com/