Hello Guix!

Lately we’ve had a lot of nar URLs return 404, mostly for server-side
issues (setup issue on hydra.gnu.org that broke ‘guix publish’ cache
management, and a ‘guix publish’ cache eviction policy that’s too
aggressive.)

The attached tool allows you to query the status a substitute server
(and, as a side effect, triggers a nar regeneration if the server uses
‘guix publish --cache’).  So it goes like this:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix weather --substitute-urls=https://hydra.gnu.org
computing 5,864 package derivations for x86_64-linux...
looking for 6,121 store items on https://hydra.gnu.org...
updating list of substitutes from 'https://hydra.gnu.org'... 100.0%
https://hydra.gnu.org
  81.2% substitutes available (4,970 out of 6,121)
  17,852.6 MiB of nars (compressed)
  46,415.5 MiB on disk (uncompressed)
  0.050 seconds per request (306.0 seconds in total)
  20.0 requests per second
--8<---------------cut here---------------end--------------->8---

Here it’s telling us that hydra.gnu.org has 81% of the substitutes for
x86_64 of the current public packages (those shown by “guix package
-A”).

We can add multiple -s flags, thought that quickly takes ages,
especially on that box.

Thoughts?

Ludo’.