machine: Implement 'roll-back-machine'.

* gnu/machine.scm (roll-back-machine, &deploy-error, deploy-error?)
(deploy-error-should-roll-back)
(deploy-error-captured-args): New variable.
* gnu/machine/ssh.scm (roll-back-managed-host): New variable.
* guix/scripts/deploy.scm (guix-deploy): Roll-back systems when a
deployment fails.

gnu/machine.scm

@@ -24,6 +24,7 @@
   #:use-module (guix records)
   #:use-module (guix store)
   #:use-module ((guix utils) #:select (source-properties->location))
+  #:use-module (srfi srfi-35)
   #:export (environment-type
             environment-type?
             environment-type-name
@@ -40,7 +41,13 @@
             machine-display-name
 
             deploy-machine
-            machine-remote-eval))
+            roll-back-machine
+            machine-remote-eval
+
+            &deploy-error
+            deploy-error?
+            deploy-error-should-roll-back
+            deploy-error-captured-args))
 
 ;;; Commentary:
 ;;;
@@ -66,6 +73,7 @@
   ;; of the form '(machine-remote-eval machine exp)'.
   (machine-remote-eval environment-type-machine-remote-eval) ; procedure
   (deploy-machine      environment-type-deploy-machine)      ; procedure
+  (roll-back-machine   environment-type-roll-back-machine)   ; procedure
 
   ;; Metadata.
   (name        environment-type-name)       ; symbol
@@ -105,3 +113,20 @@ are built and deployed to MACHINE beforehand."
 MACHINE, activating it on MACHINE and switching MACHINE to the new generation."
   (let ((environment (machine-environment machine)))
     ((environment-type-deploy-machine environment) machine)))
+
+(define (roll-back-machine machine)
+  "Monadic procedure rolling back to the previous system generation on
+MACHINE. Return the number of the generation that was current before switching
+and the new generation number."
+  (let ((environment (machine-environment machine)))
+    ((environment-type-roll-back-machine environment) machine)))
+
+
+;;;
+;;; Error types.
+;;;
+
+(define-condition-type &deploy-error &error
+  deploy-error?
+  (should-roll-back deploy-error-should-roll-back)
+  (captured-args deploy-error-captured-args))

gnu/machine/ssh.scm

@@ -17,6 +17,7 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (gnu machine ssh)
+  #:use-module (gnu bootloader)
   #:use-module (gnu machine)
   #:autoload   (gnu packages gnupg) (guile-gcrypt)
   #:use-module (gnu system)
@@ -34,6 +35,7 @@
   #:use-module (guix store)
   #:use-module (guix utils)
   #:use-module (ice-9 match)
+  #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-19)
   #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-34)
@@ -341,6 +343,18 @@ of MACHINE's system profile, ordered from most recent to oldest."
                            (boot-parameters-kernel-arguments params))))))))
           generations))))
 
+(define-syntax-rule (with-roll-back should-roll-back? mbody ...)
+  "Catch exceptions that arise when binding MBODY, a monadic expression in
+%STORE-MONAD, and collect their arguments in a &deploy-error condition, with
+the 'should-roll-back' field set to SHOULD-ROLL-BACK?"
+  (catch #t
+    (lambda ()
+      mbody ...)
+    (lambda args
+      (raise (condition (&deploy-error
+                         (should-roll-back should-roll-back?)
+                         (captured-args args)))))))
+
 (define (deploy-managed-host machine)
   "Internal implementation of 'deploy-machine' for MACHINE instances with an
 environment type of 'managed-host."
@@ -353,9 +367,60 @@ environment type of 'managed-host."
            (bootloader-configuration (operating-system-bootloader os))
            (bootcfg (operating-system-bootcfg os menu-entries)))
       (mbegin %store-monad
-        (switch-to-system eval os)
+        (with-roll-back #f
+          (switch-to-system eval os))
+        (with-roll-back #t
+          (mbegin %store-monad
             (upgrade-shepherd-services eval os)
-        (install-bootloader eval bootloader-configuration bootcfg)))))
+            (install-bootloader eval bootloader-configuration bootcfg)))))))
+
+
+;;;
+;;; Roll-back.
+;;;
+
+(define (roll-back-managed-host machine)
+  "Internal implementation of 'roll-back-machine' for MACHINE instances with
+an environment type of 'managed-host."
+  (define remote-exp
+    (with-extensions (list guile-gcrypt)
+      (with-imported-modules (source-module-closure '((guix config)
+                                                      (guix profiles)))
+        #~(begin
+            (use-modules (guix config)
+                         (guix profiles))
+
+            (define %system-profile
+              (string-append %state-directory "/profiles/system"))
+
+            (define target-generation
+              (relative-generation %system-profile -1))
+
+            (if target-generation
+                (switch-to-generation %system-profile target-generation)
+                'error)))))
+
+  (define roll-back-failure
+    (condition (&message (message (G_ "could not roll-back machine")))))
+
+  (mlet* %store-monad ((boot-parameters (machine-boot-parameters machine))
+                       (_ -> (if (< (length boot-parameters) 2)
+                                 (raise roll-back-failure)))
+                       (entries -> (map boot-parameters->menu-entry
+                                        (list (second boot-parameters))))
+                       (old-entries -> (map boot-parameters->menu-entry
+                                            (drop boot-parameters 2)))
+                       (bootloader -> (operating-system-bootloader
+                                       (machine-operating-system machine)))
+                       (bootcfg (lower-object
+                                 ((bootloader-configuration-file-generator
+                                   (bootloader-configuration-bootloader
+                                    bootloader))
+                                  bootloader entries
+                                  #:old-entries old-entries)))
+                       (remote-result (machine-remote-eval machine remote-exp)))
+    (when (eqv? 'error remote-result)
+      (raise roll-back-failure))))
 
 
 ;;;
@@ -366,6 +431,7 @@ environment type of 'managed-host."
   (environment-type
    (machine-remote-eval managed-host-remote-eval)
    (deploy-machine      deploy-managed-host)
+   (roll-back-machine   roll-back-managed-host)
    (name                'managed-host-environment-type)
    (description         "Provisioning for machines that are accessible over SSH
 and have a known host-name. This entails little more than maintaining an SSH

guix/scripts/deploy.scm

@@ -28,6 +28,8 @@
   #:use-module (guix grafts)
   #:use-module (ice-9 format)
   #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-34)
+  #:use-module (srfi srfi-35)
   #:use-module (srfi srfi-37)
   #:export (guix-deploy))
 
@@ -88,7 +90,18 @@ Perform the deployment specified by FILE.\n"))
     (with-store store
       (set-build-options-from-command-line store opts)
       (for-each (lambda (machine)
-                  (info (G_ "deploying to ~a...") (machine-display-name machine))
+                  (info (G_ "deploying to ~a...~%")
+                        (machine-display-name machine))
                   (parameterize ((%graft? (assq-ref opts 'graft?)))
-                    (run-with-store store (deploy-machine machine))))
+                    (guard (c ((message-condition? c)
+                               (report-error (G_ "failed to deploy ~a: '~a'~%")
+                                             (machine-display-name machine)
+                                             (condition-message c)))
+                              ((deploy-error? c)
+                               (when (deploy-error-should-roll-back c)
+                                 (info (G_ "rolling back ~a...~%")
+                                       (machine-display-name machine))
+                                 (run-with-store store (roll-back-machine machine)))
+                               (apply throw (deploy-error-captured-args c))))
+                      (run-with-store store (deploy-machine machine)))))
                 machines))))