bug#34033: Offloading sometimes hangs

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hello Ludovic,

Ludovic Courtès <ludo@gnu.org> writes:

> Hello,
>
> Ludovic Courtès <ludo@gnu.org> skribis:
>
>> A simple thing would be to somehow get libssh to pass POLLIN | POLLRDHUP
>> instead of just POLLIN.
>
> Reported here:
>
>   https://www.libssh.org/archive/libssh/2019-01/0000000.html
>
> A fix has been proposed by upstream and should be committed shortly.
>
>> Additionally, we could change Guile-SSH so that we can specify a timeout
>> when reading from a channel.
>
> Turns out we can set a per-session timeout, which we already do (see
> #:timeout in ‘open-ssh-session’ in (guix scripts offload)) but
> ‘ssh_channel_read’ would ignore it and instead pass an infinite timeout
> to poll(2):
>
>   https://www.libssh.org/archive/libssh/2019-01/0000001.html
>
> This issue happens to be fixed in libssh 0.8.x, so I upgraded our libssh
> package in commit a8b0556ea1e439c89dc1ba33c8864e8b9b811f08.
>
> (That still doesn’t tell us why our ‘guix offload’ processes would
> occasionally be stuck but at least it ensures the build farm keeps
> making progress even when that happens.)
>
> Ludo’.

Seems the patch in the response at the URL you linked is awaiting some
feedback/review.  Is this the reason 'guix substitute' hangs for so long
when the substitute server is down? (like 1 minute or so).

Maxim