ssh: Add Kerberos-support to ssh:// daemon URLs

* guix/ssh.scm (open-ssh-session): Fall back to GSSAPI if public key
authentication does not work

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -6811,8 +6811,9 @@ instruct it to listen for TCP connections (@pxref{Invoking guix-daemon,
 @item ssh
 @cindex SSH access to build daemons
 These URIs allow you to connect to a remote daemon over
-SSH@footnote{This feature requires Guile-SSH (@pxref{Requirements}).}.
-A typical URL might look like this:
+SSH. This feature requires Guile-SSH (@pxref{Requirements}) and a working
+@code{guile} binary in @code{PATH} on the destination machine. It supports
+public key and GSSAPI authentication. A typical URL might look like this:
 @example
 ssh://charlie@@guix.example.org:22

guix/ssh.scm

@@ -157,11 +157,16 @@ (define* (open-ssh-session host #:key user port identity
           (session-set! session 'timeout timeout)
           session)
          (x
-          (disconnect! session)
-          (raise (condition
-                  (&message
-                   (message (format #f (G_ "SSH authentication failed for '~a': ~a~%")
-                                    host (get-error session)))))))))
+          (match (userauth-gssapi! session)
+            ('success
+             (session-set! session 'timeout timeout)
+             session)
+            (x
+             (disconnect! session)
+             (raise (condition
+                     (&message
+                      (message (format #f (G_ "SSH authentication failed for '~a': ~a~%")
+                                       host (get-error session)))))))))))
       (x
        ;; Connection failed or timeout expired.
        (raise (condition