Re: [PATCH] rebase -i: ignore signals when forking subprocesses

From: Phillip Wood <phillip.wood123@gmail.com>
Date: Sep 14, 2023, 13:50 UTC
Message-ID: <f7f29dc3-c5cb-4522-95db-1bf13bd1e416@gmail.com>
In-Reply-To: <20230914095601.GE2254894@coredump.intra.peff.net>

On 14/09/2023 10:56, Jeff King wrote:

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> On Fri, Sep 08, 2023 at 10:59:06AM +0100, Phillip Wood wrote:
> 
>> Do we want a whole new session? As I understand it to launch a foreground
>> job shells put the child in its own process group and then call tcsetpgrp()
>> to change the foreground process group of the controlling terminal to that
>> of the child. I agree that would be a better way of doing things on unix.
> 
> One thing I am not clear on is the convention on who does the process
> group and controlling terminal setup. Should Git do it to say "I am
> handing off control of the terminal to the editor that I am spawning"?
> Or should the editor say "I am an editor which has a user interface that
> takes over the terminal; I will control it while I am running".

As I understand it the editor has a controlling terminal (assuming there is a controlling terminal associated with the editor's session id), not the other way round. If the editor is launched in the background then it will receive SIGTTIN when it tries to read from it's controlling terminal which stops the process unless the process installs a signal handler.

> The latter makes much more sense to me, as Git cannot know how the
> editor plans to behave. But as I understand it, this kind of job control
> stuff is implemented by the calling shell, which does the tcsetpgrp()
> call.

Yes, my understanding is that the shell puts all the processes in a pipeline in the same process group and calls tcsetpgrp() if it wants that job to be run in the foreground.

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> So I dunno. It sounds to me like the "right" thing here is making Git
> more shell-like in handing control to a program (like the editor) that
> we expect to be in the foreground of the terminal. As opposed to the
> "ignore SIGINT temporarily" thing which feels more like band-aid. But
> I'm wary of getting into a rabbit hole of portability headaches and
> weird corners of Unix terminal-handling conventions.

I'm wary of that too, it has the potential to end up adding a lot of fiddly code checking if git is in the foreground or background and propagating SIGTSTP and friends up to the shell. In any case we'd need the "ignore SIGINT" thing for windows anyway. Ignoring SIGINT and SIGQUIT in the parent is what system(3) does. As long as git exits promptly when the interrupted child is killed I think that is reasonable. Would you be happier if we re-raised the signal once we have cleaned up any state that needs to be written before exiting?

Best Wishes

Phillip