From: Junio C Hamano <gitster@pobox.com>
To: Jeff King <peff@peff.net>
Cc: Eric Rannaud <eric.rannaud@gmail.com>,
git@vger.kernel.org,
Johannes Schindelin <johannes.schindelin@gmx.de>,
Jeremy Serror <jeremy.serror@gmail.com>
Subject: Re: git rebase regression: cannot pass a shell expression directly to --exec
Date: Tue, 16 May 2017 12:40:51 +0900 [thread overview]
Message-ID: <xmqqr2zp31vg.fsf@gitster.mtv.corp.google.com> (raw)
In-Reply-To: 20170516032503.bzkxmtqpmppxgi75@sigill.intra.peff.net
Jeff King <peff@peff.net> writes:
> Interesting. The "exec" string is still run with a shell. E.g.:
> ...
> I wonder if this is falling afoul of the optimization in run-command's
> prepare_shell_cmd() to skip shell invocations for "simple" commands.
> ...
> So I suspect if you added an extraneous semi-colon, your case would work
> again (and that would confirm for us that this is indeed the problem).
Wow, that's a brilliant analysis.
> The optimization in run-command is very old, but the switch to
> rebase--helper presumably moved us from doing that exec in the actual
> shell script to doing it via the C code.
>
> Which means your exported-function technique has been broken for _most_
> of Git all along, but it just now affected this particular spot.
>
> I'm not sure how to feel about it. In the face of exported functions, we
> can never do the shell-skipping optimization, because we don't know how
> the shell is going to interpret even a simple-looking command. And it is
> kind of a neat trick. But I also hate to add extra useless shell
> invocations for the predominantly common case that people aren't using
> this trick (or aren't even using a shell that supports function
> exports).
I was about to write this off as "an unfortunate regression, a
fallout that will likely left unfixed, due to lack of a good
practical workaround."
The point of rewriting things in C and using run_command() interface
was to avoid shell overhead. We are doing an exec already, but
adding a shell invocation in the middle will double the number of
exec (and probably add an extra fork as well), which probably is
measurable on systems with slow fork/exec.
The "semicolon" trick is way too obscure, but perhaps can be
documented as an escape hatch?
next prev parent reply other threads:[~2017-05-16 3:40 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-15 18:08 git rebase regression: cannot pass a shell expression directly to --exec Eric Rannaud
2017-05-16 3:25 ` Jeff King
2017-05-16 3:37 ` Jeff King
2017-05-16 16:41 ` Jonathan Nieder
2017-05-16 16:47 ` Jeff King
2017-05-16 17:11 ` Eric Rannaud
2017-05-16 17:15 ` Brandon Williams
2017-05-16 17:23 ` Jeff King
2017-05-16 17:30 ` Brandon Williams
2017-05-16 19:14 ` Linus Torvalds
2017-05-16 19:35 ` [TANGENT] run-command: use vfork instead of fork Eric Wong
2017-05-16 20:47 ` Linus Torvalds
2017-05-16 21:11 ` Brandon Williams
2017-05-16 20:12 ` git rebase regression: cannot pass a shell expression directly to --exec Johannes Schindelin
2017-05-16 20:27 ` Linus Torvalds
2017-05-16 17:37 ` Jonathan Nieder
2017-05-16 3:40 ` Junio C Hamano [this message]
2017-05-16 3:53 ` Jeff King
2017-05-16 4:08 ` Jeff King
2017-05-16 16:45 ` Eric Rannaud
2017-05-16 10:46 ` Johannes Schindelin
2017-05-16 10:23 ` Johannes Schindelin
2017-05-16 16:18 ` Jeff King
2017-05-16 16:59 ` Eric Rannaud
2017-05-16 17:14 ` Kevin Daudt
2017-05-16 17:29 ` Eric Rannaud
2017-05-16 17:41 ` Jeff King
2017-05-16 17:21 ` Eric Rannaud
2017-05-16 17:37 ` Jeff King
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