According to https://gitlab.gnome.org/GNOME/glib/-/merge_requests/3101, glib
now searches for the session bus socket in runstatedir. The dbus service must
thus have its socket in /run/dbus.

For interoperability with the dbus standard, /run/dbus is also symlinked to
/var/run/dbus.

* gnu/services/dbus.scm (dbus-activation): Symlink /run/dbus to /var/run/dbus.
(%dbus-accounts): Run dbus in /run/dbus.
(dbus-root-service-type): Save the pid file in /run/dbus.
---

> Perhaps, but it's not okay to fail if it's a regular directory.  We should
> move those!

I’m not sure I understand.  What comes to my mind is:

1. Try to make the symlink. If it fails with EEXIST:
2. Try to read /var/run/dbus as a symlink. If it points to /run/dbus already,
   stop. Otherwise:
3. Move everything in /var/run/dbus to /run/dbus.
4. Delete the now-empty /var/run/dbus.
5. Symlink /run/dbus to /var/run/dbus.

Is it what you meant?

Best regards,

Vivien

 gnu/services/dbus.scm | 38 +++++++++++++++++++++++++++++++++++---
 1 file changed, 35 insertions(+), 3 deletions(-)

diff --git a/gnu/services/dbus.scm b/gnu/services/dbus.scm
index 5a0c634393..44bf0c910b 100644
--- a/gnu/services/dbus.scm
+++ b/gnu/services/dbus.scm
@@ -163,7 +163,7 @@ (define %dbus-accounts
          (group "messagebus")
          (system? #t)
          (comment "D-Bus system bus user")
-         (home-directory "/var/run/dbus")
+         (home-directory "/run/dbus")
          (shell (file-append shadow "/sbin/nologin")))))
 
 (define dbus-setuid-programs
@@ -186,7 +186,39 @@ (define (dbus-activation config)
         (let ((user (getpwnam "messagebus")))
           ;; This directory contains the daemon's socket so it must be
           ;; world-readable.
-          (mkdir-p/perms "/var/run/dbus" user #o755))
+          (mkdir-p/perms "/run/dbus" user #o755))
+
+        (catch 'system-error
+          (lambda ()
+            (symlink "/run/dbus" "/var/run/dbus"))
+          (lambda args
+            (let ((errno (system-error-errno args)))
+              (cond
+               ((= errno EEXIST)
+                (let ((existing-name
+                       (false-if-exception
+                        (readlink "/var/run/dbus"))))
+                  (unless (equal? existing-name "/run/dbus")
+                    ;; Move the content of /var/run/dbus to /run/dbus, and
+                    ;; retry.
+                    (let ((dir (opendir "/var/run/dbus")))
+                      (let move-to-/run/dbus ()
+                        (let ((next (readdir dir)))
+                          (unless (or (equal? next ".")
+                                      (equal? next "..")
+                                      (eof-object? next))
+                            (rename-file (string-append "/var/run/dbus/" next)
+                                         (string-append "/run/dbus/" next)))
+                          (unless (eof-object? next)
+                            (move-to-/run/dbus))))
+                      (closedir dir)
+                      (rmdir "/var/run/dbus")
+                      (symlink "/run/dbus" "/var/run/dbus")))))
+               (else
+                (format (current-error-port)
+                        "Failed to symlink /run/dbus to /var/run/dbus: ~s~%"
+                        (strerror errno))
+                (error "cannot create /var/run/dbus"))))))
 
         (unless (file-exists? "/etc/machine-id")
           (format #t "creating /etc/machine-id...~%")
@@ -210,7 +242,7 @@ (define dbus-shepherd-service
                              '(#:environment-variables '("DBUS_VERBOSE=1")
                                #:log-file "/var/log/dbus-daemon.log")
                              '())
-                      #:pid-file "/var/run/dbus/pid"))
+                      #:pid-file "/run/dbus/pid"))
             (stop #~(make-kill-destructor)))))))
 
 (define dbus-root-service-type

base-commit: b18b2d13488f2a92331ccad2dc8cbb54ee15582f
-- 
2.41.0