bug#48561: "Daemon not running" exception when avahi-daemon is not running

[Mathieu Othacehe]

Hello Maxime,

>   warning: avahi daemon is not running, cannot auto-discover substitute 
> servers!

This should be fixed with the attached patch.

Thanks,

Mathieu

>From a50bbf99f65d26bbdb0d16112a49335bf913b822 Mon Sep 17 00:00:00 2001
From: Mathieu Othacehe <othacehe@gnu.org>
Date: Fri, 21 May 2021 15:21:15 +0200
Subject: [PATCH] scripts: discover: Warn when the daemon is unreachable.

* guix/scripts/discover (guix-discover): Print a warning message when the
daemon is unreachable.
---
 guix/scripts/discover.scm | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/guix/scripts/discover.scm b/guix/scripts/discover.scm
index be1eaa6e95..e42f7662d0 100644
--- a/guix/scripts/discover.scm
+++ b/guix/scripts/discover.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2020 Mathieu Othacehe <othacehe@gnu.org>
+;;; Copyright © 2020, 2021 Mathieu Othacehe <othacehe@gnu.org>
 ;;; Copyright © 2021 Simon Tournier <zimon.toutoune@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -26,6 +26,7 @@
   #:use-module (guix build syscalls)
   #:use-module (guix build utils)
   #:use-module (guix scripts publish)
+  #:use-module (avahi)
   #:use-module (ice-9 rdelim)
   #:use-module (srfi srfi-37)
   #:export (read-substitute-urls
@@ -138,5 +139,13 @@ to synchronize with the writer."
       (parameterize ((%publish-file publish-file))
         (mkdir-p (dirname publish-file))
         (false-if-exception (delete-file publish-file))
-        (avahi-browse-service-thread service-proc
-                                     #:types %services)))))
+        (catch 'avahi-error
+          (lambda ()
+            (avahi-browse-service-thread service-proc
+                                         #:types %services))
+          (lambda (key err function . _)
+            (cond
+             ((eq? err error/no-daemon)
+              (warning (G_ "Avahi daemon is not running, \
+cannot auto-discover substitutes servers.~%"))))
+            (exit 1)))))))
-- 
2.31.1