Hi, thanks for the bug report, and sorry for the breakage.

Lars-Dominik Braun <lars@6xq.net> skriver:

> Hi,
>
> after the update to libvirt 8.6.0 in
> 3a76c2bfd94557c9776aa11240fec14580aec1b0 networks don’t start any more:
>
>     > LANG=C virsh net-start default
>     error: Failed to start network default
>     error: Unable to find 'dnsmasq' binary in $PATH: No such file or directory
>
> I tried to patch dnsmasq’s path like follows, but then the testcase
> networkxml2conftest fails and cannot find dnsmasq either.

I don't understand why that test is failing with your patch.  It has a
hard coded "/usr/sbin/dnsmasq", but patching that does not make a
difference.  It seems the lookup via virFindFileInPath, which ultimately
calls out to GLib's g_find_program_in_path, fails; but ostensibly using
an absolute file name should not make a difference[0].

  [0]: https://docs.gtk.org/glib/func.find_program_in_path.html

Anyway, I was able to reproduce the failure in the system test:

--8<---------------cut here---------------start------------->8---
diff --git a/gnu/tests/virtualization.scm b/gnu/tests/virtualization.scm
index 4bd56e5d9d..557f30db4f 100644
--- a/gnu/tests/virtualization.scm
+++ b/gnu/tests/virtualization.scm
@@ -106,6 +106,26 @@ (define marionette
                          "-c" "qemu:///system" "connect"))
              marionette))
 
+          (test-eq "create default network"
+            0
+            (marionette-eval
+             '(begin
+                (chdir "/tmp")
+                (system* #$(file-append libvirt "/bin/virsh")
+                         "-c" "qemu:///system" "net-define"
+                         #$(file-append libvirt
+                                        "/etc/libvirt/qemu/networks/default.xml")))
+             marionette))
+
+          (test-eq "start default network"
+            0
+            (marionette-eval
+             '(begin
+                (chdir "/tmp")
+                (system* #$(file-append libvirt "/bin/virsh")
+                         "-c" "qemu:///system" "net-start" "default"))
+             marionette))
+
           (test-end))))
 
   (gexp->derivation "libvirt-test" test))
--8<---------------cut here---------------end--------------->8---

And can confirm your patch fixes that, which is arguably more important.
So I went ahead and committed your patch (and disabled the failing
test), and the system test to avoid future regressions in this area.

Fixed in:

  acbf2f9def gnu: libvirt: Use absolute dnsmasq.
  3e0abde17b tests: libvirt: Ensure the default network can be started.