bug#49781: Successful build on ci.guix not available as substitute

["Ludovic Courtès" <ludovic.courtes@inria.fr>]

Hi!

I stumbled upon the case of a build that was successful on ci.guix but
that does not have a corresponding substitute:

  https://ci.guix.gnu.org/build/806609/details

and:

--8<---------------cut here---------------start------------->8---
$ wget -O- https://ci.guix.gnu.org/slm0x0j05vyl3vqbjhgc0nch6jca9fp3.narinfo
--2021-07-30 10:53:15--  https://ci.guix.gnu.org/slm0x0j05vyl3vqbjhgc0nch6jca9fp3.narinfo
Ni solvigas ci.guix.gnu.org (ci.guix.gnu.org)... 141.80.181.40
Konektado al ci.guix.gnu.org (ci.guix.gnu.org)|141.80.181.40|:443... konektita.
HTTP peto sendita, ni atendas respondon... 404 Not Found
2021-07-30 10:53:15 ERARO 404: Not Found.
--8<---------------cut here---------------end--------------->8---

The store item vanished from berlin, though the .drv is still there:

--8<---------------cut here---------------start------------->8---
ludo@berlin ~$ ls /gnu/store/slm0x0j05vyl3vqbjhgc0nch6jca9fp3-xnnpack-0.0-1.bbe8824
ls: cannot access '/gnu/store/slm0x0j05vyl3vqbjhgc0nch6jca9fp3-xnnpack-0.0-1.bbe8824': No such file or directory
ludo@berlin ~$ guix gc --derivers /gnu/store/slm0x0j05vyl3vqbjhgc0nch6jca9fp3-xnnpack-0.0-1.bbe8824
/gnu/store/hrn9hj7v1img3il6afh3r036aczvwnyd-xnnpack-0.0-1.bbe8824.drv
ludo@berlin ~$ ls $(guix gc --derivers /gnu/store/slm0x0j05vyl3vqbjhgc0nch6jca9fp3-xnnpack-0.0-1.bbe8824)
/gnu/store/hrn9hj7v1img3il6afh3r036aczvwnyd-xnnpack-0.0-1.bbe8824.drv
--8<---------------cut here---------------end--------------->8---

It’s the first time I see this.  My understanding is that Cuirass makes
a GET request on ‘guix publish’ upon build completion.  Could there be a
race condition or something that can explain this?

Thanks,
Ludo’.