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From: Jeff King <peff@peff.net>
To: Brandon Williams <bmwill@google.com>
Cc: Michael Haggerty <mhagger@alum.mit.edu>,
	Lars Schneider <larsxschneider@gmail.com>,
	Martin Ågren <martin.agren@gmail.com>,
	Git Users <git@vger.kernel.org>
Subject: Re: [PATCH] config: use a static lock_file struct
Date: Thu, 31 Aug 2017 00:09:07 -0400
Message-ID: <20170831040907.a4qpydb3w2aghatm@sigill.intra.peff.net> (raw)
In-Reply-To: <20170830210602.GC50018@google.com>

On Wed, Aug 30, 2017 at 02:06:02PM -0700, Brandon Williams wrote:

> > We could extend that protection by having sigchain_push_common() set
> > sa_mask to cover all of the related signals. On Linux and BSD the
> > current code using signal() also implies SA_RESTART. We could add that
> > to our flags, though I suspect in practice it doesn't matter. Whenever
> > we establish a handler like this our intent is to never return from it.
> > 
> > That just protects us from calling the _same_ handler from itself. But
> > that's probably enough in practice, and means handlers don't have to
> > worry about "critical sections". The other alternative is sigprocmask()
> > to block signals entirely during a section. I'm not sure if there are
> > portability questions there (it looks like we have a mingw wrapper
> > there, but it's a complete noop).
> 
> Yeah there's a lot about signals that I'm not very clear on.  I do know
> that Eric helped me out on the fork-exec series I worked on earlier in
> the year and I believe it was to turn on/off signals during process
> launches in 45afb1ca9 (run-command: block signals between fork and
> execve, 2017-04-19).  Though that bit of code is strictly for unix so I
> wouldn't know how that would work on windows machines.  Portability does
> seem to always be a challenging problem.

Based on the sketch I wrote above, I figured it would be pretty easy to
convert sigchain to sigaction. But after taking a look at
compat/mingw.c, I don't think Windows would be on board. sigaction()
there is is a stub implementation that _only_ handles SIGALRM and
nothing else.

So I think the best we could do is put big #ifdefs around it to use
sigaction on other platforms, and fall back to signal() on Windows.
That's do-able, but my enthusiasm is waning as the complexity increases.
Getting two SIGINTs in a row seems plausible, but we already handle that
well. Getting SIGINT and then a _different_ signal while we're in the
handler seems less likely in practice. The only combination I can think
that would be common is TERM+KILL, but of course we're not catching KILL
in the first place.

So this seems like adding complexity for very little benefit.

-Peff

  reply index

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-08-27  7:37 [PATCH] pkt-line: re-'static'-ify buffer in packet_write_fmt_1() Martin Ågren
2017-08-27 15:41 ` Jeff King
2017-08-27 18:25   ` Jeff King
2017-08-27 18:21 ` Lars Schneider
2017-08-27 19:09   ` Martin Ågren
2017-08-27 19:15     ` Jeff King
2017-08-27 20:04     ` Lars Schneider
2017-08-27 23:23       ` Jeff King
2017-08-28  4:11         ` Martin Ågren
2017-08-28 17:52           ` Stefan Beller
2017-08-28 17:58             ` Jeff King
2017-09-05 10:03               ` Junio C Hamano
2017-08-29 17:51           ` Lars Schneider
2017-08-29 18:53             ` Jeff King
2017-08-29 18:58               ` [PATCH] config: use a static lock_file struct Jeff King
2017-08-29 19:09                 ` Brandon Williams
2017-08-29 19:10                   ` Brandon Williams
2017-08-29 19:12                   ` Jeff King
2017-08-30  3:25                     ` Michael Haggerty
2017-08-30  4:31                       ` Jeff King
2017-08-30  4:55                         ` Michael Haggerty
2017-08-30  4:55                         ` Jeff King
2017-08-30  5:55                           ` Jeff King
2017-08-30  7:07                             ` Michael Haggerty
2017-09-02  8:44                               ` Jeff King
2017-09-02 13:50                                 ` Jeff King
2017-08-30  6:55                           ` Michael Haggerty
2017-08-30 19:53                             ` Jeff King
2017-08-30 19:57                               ` Brandon Williams
2017-08-30 20:11                                 ` Jeff King
2017-08-30 21:06                                   ` Brandon Williams
2017-08-31  4:09                                     ` Jeff King [this message]
2017-09-06  3:59                 ` Junio C Hamano
2017-09-06 12:41                   ` Jeff King
2017-08-29 19:22               ` [PATCH] pkt-line: re-'static'-ify buffer in packet_write_fmt_1() Martin Ågren
2017-08-29 21:48                 ` Jeff King
2017-08-30  5:31                   ` Jeff King
2017-09-05 10:03                     ` Junio C Hamano
2017-10-10  4:06                       ` [PATCH 0/2] Do not call cmd_*() as a subroutine Junio C Hamano
2017-10-10  4:06                         ` [PATCH 1/2] describe: do not use " Junio C Hamano
2017-10-10 13:43                           ` SZEDER Gábor
2017-10-11  6:00                             ` Junio C Hamano
2017-10-10  4:06                         ` [PATCH 2/2] merge-ours: " Junio C Hamano

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