Subject: Re: bug#29826: nondeterministic Broken pipe

From: [email-masked] (Ludovic Courtès)
Date: Tue, 02 Jan 2018 23:17:08 +0100

Hello,

Mark H Weaver <[email-masked]> skribis:

> [email-masked] (Ludovic Courtès) writes:

[...]

>> Not sure!  We specifically ignore EPIPE in cases where it matters, such
>> as for the output of ‘guix package --search’, ‘guix package -A’, etc.
>> In other cases, it’s probably an error, so it’s worth reporting.
>>
>> WDYT?
>
> I see from the comment in (guix ui) where SIGPIPE is ignored, the
> rationale:
>
>   ;; Ignore SIGPIPE.  If the daemon closes the connection, we prefer to be
>   ;; notified via an EPIPE later.
>   (sigaction SIGPIPE SIG_IGN)
>
> Instead of unconditionally ignoring SIGPIPE here in (initialize-guix),
> it might be better to ignore SIGPIPE only if we open a connection to the
> daemon with the intent of mutating the store, and perhaps in some other
> cases where we're mutating information on disk (e.g. switching
> generations).  In those cases, we have a job to do that should ideally
> be completed regardless of whether anyone is still listening to our
> STDOUT.
>
> However, in many other cases, we don't mutate anything on disk, and our
> *only* job is printing information to the user, e.g. when showing
> version/usage information, the list of available packages, the list of
> generations, etc.  In those cases, I think it would be better to let
> SIGPIPE kill us, because there is no reason to keep the 'guix' process
> alive if its output is going nowhere.  These are also the cases where
> it's most useful to pipe 'guix' output into other commands.
>
> So, I think we should consider removing (sigaction SIGPIPE SIG_IGN) from
> (initialize-guix), and instead putting it in various other selected
> places.
>
> What do you think?

Why not.  An option would be to move (sigaction SIGPIPE SIG_IGN) to
‘open-connection’, though that’s not following “library design best
practices.”

If we do that, can we really remove the ‘leave-on-EPIPE’ uses that we
have in (guix scripts package) for instance?  At first sight they are in
‘process-query’, which corresponds to operations that don’t rely on the
store, so that should be safe.

There are a few other uses of ‘leave-on-EPIPE’ that happen while the
store is opened (in ‘guix size’, ‘guix challenge’).  We’d have to keep
these.

Thoughts?

Ludo’.