tests: ssh: Abstract session connection and authentication.

* gnu/tests/ssh.scm (run-ssh-test): Introduce make-session-for-test,
call-with-connected-session and call-with-connected-session/auth.
(run-ssh-test)["connect"]: Rename to "shell command".  Abstract its session
connection and authentication work into the above three functions.

gnu/tests/ssh.scm

 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2016 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2017 Clément Lassieur <clement@lassieur.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -101,6 +102,47 @@ (define (wait-for-file file)
                          (error "file didn't show up" ,file))))
                marionette))
 
+            (define (make-session-for-test)
+              "Make a session with predefined parameters for a test."
+              (make-session #:user "root"
+                            #:port 2222
+                            #:host "localhost"
+                            #:log-verbosity 'protocol))
+
+            (define (call-with-connected-session proc)
+              "Call the one-argument procedure PROC with a freshly created and
+connected SSH session object, return the result of the procedure call.  The
+session is disconnected when the PROC is finished."
+              (let ((session (make-session-for-test)))
+                (dynamic-wind
+                  (lambda ()
+                    (let ((result (connect! session)))
+                      (unless (equal? result 'ok)
+                        (error "Could not connect to a server"
+                               session result))))
+                  (lambda () (proc session))
+                  (lambda () (disconnect! session)))))
+
+            (define (call-with-connected-session/auth proc)
+              "Make an authenticated session.  We should be able to connect as
+root with an empty password."
+              (call-with-connected-session
+               (lambda (session)
+                 ;; Try the simple authentication methods.  Dropbear requires
+                 ;; 'none' when there are no passwords, whereas OpenSSH accepts
+                 ;; 'password' with an empty password.
+                 (let loop ((methods (list (cut userauth-password! <> "")
+                                           (cut userauth-none! <>))))
+                   (match methods
+                     (()
+                      (error "all the authentication methods failed"))
+                     ((auth rest ...)
+                      (match (pk 'auth (auth session))
+                        ('success
+                         (proc session))
+                        ('denied
+                         (loop rest)))))))))
+
             (mkdir #$output)
             (chdir #$output)
 
@@ -131,37 +173,19 @@ (define (wait-for-file file)
                          (current-services))))
                marionette))
 
-            ;; Connect to the guest over SSH.  We should be able to connect as
-            ;; "root" with an empty password.  Make sure we can run a shell
+            ;; Connect to the guest over SSH.  Make sure we can run a shell
             ;; command there.
-            (test-equal "connect"
+            (test-equal "shell command"
               'hello
-              (let* ((session (make-session #:user "root"
-                                            #:port 2222 #:host "localhost"
-                                            #:log-verbosity 'protocol)))
-                (match (connect! session)
-                  ('ok
-                   ;; Try the simple authentication methods.  Dropbear
-                   ;; requires 'none' when there are no passwords, whereas
-                   ;; OpenSSH accepts 'password' with an empty password.
-                   (let loop ((methods (list (cut userauth-password! <> "")
-                                             (cut userauth-none! <>))))
-                     (match methods
-                       (()
-                        (error "all the authentication methods failed"))
-                       ((auth rest ...)
-                        (match (pk 'auth (auth session))
-                          ('success
-                           ;; FIXME: 'get-server-public-key' segfaults.
-                           ;; (get-server-public-key session)
-                           (let ((channel (make-channel session)))
-                             (channel-open-session channel)
-                             (channel-request-exec channel
-                                                   "echo hello > /root/witness")
-                             (and (zero? (channel-get-exit-status channel))
-                                  (wait-for-file "/root/witness"))))
-                          ('denied
-                           (loop rest))))))))))
+              (call-with-connected-session/auth
+               (lambda (session)
+                 ;; FIXME: 'get-server-public-key' segfaults.
+                 ;; (get-server-public-key session)
+                 (let ((channel (make-channel session)))
+                   (channel-open-session channel)
+                   (channel-request-exec channel "echo hello > /root/witness")
+                   (and (zero? (channel-get-exit-status channel))
+                        (wait-for-file "/root/witness"))))))
 
             (test-end)
             (exit (= (test-runner-fail-count (test-runner-current)) 0)))))