Re: git rebase regression: cannot pass a shell expression directly to --exec

From: Junio C Hamano <gitster@pobox.com>
Date: May 16, 2017, 03:40 UTC
Message-ID: <xmqqr2zp31vg.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <20170516032503.bzkxmtqpmppxgi75@sigill.intra.peff.net>

Jeff King <peff@peff.net> writes:

Show 7 quoted lines

> 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.

Show 14 quoted lines

> 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?