bug#45045: substitute fails if discovery is enabled

[Maxime Devos]

Hi,

Discovery of substitute servers on the local network doesn't seem to
work properly, see output of
`sudo guix system reconfigure config.scm --save-provenance`
at end of e-mail. Disabling substitutes (e.g. `guix build hello --no-
substitutes`) seems to work-around the issue. 

Some network details: there are no substitute servers on the local
network (except the local computer).

Greetings,
Maxime

--- Appendices

Guix version:
guix (GNU Guix) c2352335ca3ccbfee5b1581a832e38bc9a15c4f9

My service defintion:

 (services
  (append
   (list (service mate-desktop-service-type)
         (service cups-service-type 
                  (cups-configuration
                   (web-interface? #t)
                   (extensions
                    (list cups-filters escpr hplip-minimal brlaser))))
         (service zram-device-service-type
                  (zram-device-configuration))
         (service sane-service-type)
         (set-xorg-configuration
          (xorg-configuration
           (keyboard-layout keyboard-layout)))
         (bluetooth-service)
         (service tlp-service-type)
         (service guix-publish-service-type
                  (guix-publish-configuration
                   (port 8080)
                   (host "0.0.0.0")
                   (compression '(("lzip" 7))))))
   (modify-services %desktop-services
                    (network-manager-service-type c => (network-
manager-configuration
                                                        (vpn-plugins
(list network-manager-openconnect))))

                    (guix-service-type c => (guix-configuration
                                             (inherit c)
                                             (discover? #t))))))

Output of `sudo guix system reconfigure config.scm --save-provenance`:

substitute: In ice-9/boot-9.scm:
substitute:   1736:10 16 (with-exception-handler _ _ #:unwind? _ # _)
substitute: In unknown file:
substitute:           15 (apply-smob/0 #<thunk 7fa9732d3580>)
substitute: In ice-9/boot-9.scm:
substitute:     718:2 14 (call-with-prompt _ _ #<procedure default-
prompt-handle…>)
substitute: In ice-9/eval.scm:
substitute:     619:8 13 (_ #(#(#<directory (guile-user)
7fa972ef9f00>)))
substitute: In guix/ui.scm:
substitute:    2101:2 12 (run-guix-command substitute "--query")
substitute: In ice-9/boot-9.scm:
substitute:   1731:15 11 (with-exception-handler #<procedure
7fa970ea2cf0 at ic…> …)
substitute:   3297:17 10 (resolve-interface (guix scripts substitute)
#:select _ …)
substitute: In ice-9/threads.scm:
substitute:     390:8  9 (_ _)
substitute: In ice-9/boot-9.scm:
substitute:   3223:13  8 (_)
substitute: In ice-9/threads.scm:
substitute:     390:8  7 (_ _)
substitute: In ice-9/boot-9.scm:
substitute:   3507:20  6 (_)
substitute:    2806:4  5 (save-module-excursion _)
substitute:   3527:26  4 (_)
substitute: In unknown file:
substitute:            3 (primitive-load-path "guix/scripts/substitute"
#<proced…>)
substitute: In guix/scripts/substitute.scm:
substitute:   1107:33  2 (_)
substitute: In guix/scripts/discover.scm:
substitute:     98:26  1 (read-substitute-urls #:file _)
substitute: In unknown file:
substitute:            0 (open-file "/var/guix/discover/publish" "r0")
substitute: 
substitute: ERROR: In procedure open-file:
substitute: In procedure open-file: No such file or directory:
"/var/guix/discover/publish"
guix environment: error: `/gnu/store/qvsmk5hcx5fchl1whd0jzvrfvzbkky5d-
guix-1.2.0-3.35a32fe/bin/guix substitute' died unexpectedly

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