GNU bug report logs - #63190
[Shepherd] Nested calls lead to a hang

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From: Ludovic Courtès <ludo <at> gnu.org>
To: Brian Cully <bjc <at> spork.org>
Cc: 63190 <at> debbugs.gnu.org, Bruno Victal <mirai <at> makinata.eu>
Subject: bug#63190: [Shepherd] Nested calls lead to a hang
Date: Sat, 13 May 2023 11:45:51 +0200

Brian Cully <bjc <at> spork.org> skribis:

> Ludovic Courtès <ludo <at> gnu.org> writes:
>
>> (Whether that leads to a deadlock depends; at first sight, I’d say
>> there’s no reason for this to deadlock in general, but you can of
>> course
>> end up with a logic bug like A starts B, which spawns a client to
>> start
>> A, which doesn’t start because it’s waiting for B.)
>
> It's been a while since I looked at this, but my rough recollection is
> the deadlock occurs because shepherd can only process one request over
> its socket at a time.

That’s not the case in 0.9: it can process several requests
concurrently.  However, as I wrote in a followup message, the client
socket created by (gnu services herd) lacks SOCK_NONBLOCK, which can
thus block the process on reads and writes.

Ludo’.

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