bug#26976: On Hydra, offload crashes while trying to build linux-libre source

[ludo@gnu.org (Ludovic Courtès)]

Hello,

ludo@gnu.org (Ludovic Courtès) skribis:

> AIUI, that means there’s one output compression buffer per session, and
> it’s not thread-safe (in Guile 2.2 finalizers are called from a separate
> thread.)
>
> I think the fix, in Guile-SSH, is to associate each libssh object
> (session, channel, etc.) with a mutex, and to protect all uses of the
> libssh object by that mutex.

I’ve pushed a workaround that seems to work (‘guix copy’ commands that
previously segfaulted on hydra.gnu.org no longer do):

  https://git.savannah.gnu.org/cgit/guix.git/commit/?id=8e469b67f95cfe5b95405b503b8ee315fdf8ce66

The idea is simply to explicitly close all the channel ports.  That way,
when their finalizer gets called, it doesn’t invoke libssh code.

(guix build offload) and (guix ssh) already explicitly close all the
channel ports they open, so I found that the culprit is ‘node-eval’ in
Guile-SSH.  The patch changes ‘node-eval’ to explicitly close the RREPL
channel upon completion.  (BTW, performance-wise, it may be best to
avoid opening a new channel every time ‘node-eval’ is called.)

That may be good enough for Guix, but of course that’s a workaround and
not a proper fix.

I’ll do some more testing and then maybe try switching hydra.gnu.org to
Guile 2.2 again.

Thanks,
Ludo’.