bug#42740: Segfault in libssh during ‘guix copy’

[Ludovic Courtès]

Hello!

Maxim Cournoyer <maxim.cournoyer@gmail.com> skribis:

> Ludovic Courtès <ludo@gnu.org> writes:

[...]

>> I’ve pushed this change on our side to explicitly close channels and
>> sessions:
>>
>>   
>> https://git.savannah.gnu.org/cgit/guix.git/commit/?id=61fe9ced7da7eefceb931af0cb7363b721f5bdd6
>>
>> This workaround is similar to that of 2017:
>>
>>   
>> https://git.savannah.gnu.org/cgit/guix.git/commit/?id=8e469b67f95cfe5b95405b503b8ee315fdf8ce66
>>
>> It’s really just a workaround so I think we should fix the core issue in
>> Guile-SSH (or libssh) so it doesn’t pop up again next month—it’s hard to
>> ensure code that opens a channel explicitly closes it.
>
> Do you think the issue lies in guile-ssh or in libssh itself?  Sorry for
> not having caught these problems earlier; it seemed to work reliably
> when I last tested it.

No worries; these issues are non-deterministic and we can spend months
without noticing.

Perhaps libssh is documented as not being thread-safe, in which case
it’s up to its users to do the right thing.  In that case, the problem
would be in Guile-SSH, since in Guile 2.2/3.0 finalizers can be called
from a separate thread.

It’s also probably easier to fix in Guile-SSH (technically & socially).

>> Anyway, I would welcome tests using ‘guix copy’, ‘guix deploy’, and
>> offloading.  (For offloading, make sure to run the daemon from your
>> build tree.)
>
> While attempting to use offload on the core-updates branch, I
> encountered stalls and file errors, but with your patch it seems to work
> reliable (it's been offloading builds for the last 15 minutes or so
> without interruption).
>
> So your workaround fixes seem to work as intended.

Good, thanks for testing.  We should update the ‘guix’ package again
soonish.

Ludo’.