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From: Jeff King <peff@peff.net>
To: Junio C Hamano <gitster@pobox.com>
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: Mon, 15 May 2017 23:53:58 -0400	[thread overview]
Message-ID: <20170516035357.2wutcd2tyax4tjyn@sigill.intra.peff.net> (raw)
In-Reply-To: <xmqqr2zp31vg.fsf@gitster.mtv.corp.google.com>

On Tue, May 16, 2017 at 12:40:51PM +0900, Junio C Hamano wrote:

> 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.

If it's right. :) It's all theory at this point.

My /bin/sh isn't bash, but I should be able to build with
SHELL_PATH=/bin/bash to reproduce. But I can't:

   $ bash
   $ foo() { echo >&2 "in foo"; }
   $ export -f foo
   $ bash -c foo
   in foo

   $ strace -f 2>&1 git.compile rebase -x 'foo;' HEAD^ | grep foo
   Executing: foo;
   [pid  1788] execve("/bin/bash", ["/bin/bash", "-c", "foo;", "foo;"], [/* 60 vars */] <unfinished ...>
   foo;: foo: command not found

So I'm not sure why the direct "bash -c" can find it, but somehow the
variable doesn't make it through to the "bash -c" at the lower level.
Replacing "foo;" with "env" shows the environment, but BASH_FUNC_foo
isn't set in it. I'm not sure where it's getting eaten, though.

> The "semicolon" trick is way too obscure, but perhaps can be
> documented as an escape hatch?

Yeah, I agree this should be documented if it can't be fixed. I wasn't
sure if we were giving up just yet.

Either way, though, it wouldn't hurt to mention optimizing out "maybe
shell" optimization, because it can occasionally produce user-visible
effects. Where would be a good place? In git(1), I guess?

It's almost something that could go in gitcli(7), but it's not really
about the CLI in particular. In most cases the shell-exec'd commands are
from config, but not always (as this case shows). So git-config(1)
probably isn't the right place.

AFAIK, we don't talk about this behavior at all in the existing
documentation. And I really don't know where we'd put it that somebody
would find it without having to read the documentation exhaustively.

-Peff

  reply	other threads:[~2017-05-16  3:54 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
2017-05-16  3:53     ` Jeff King [this message]
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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