diff mbox series

[bug#36555,1/2] guix system: Add 'reconfigure' module.

Message ID 87ef30i9fl.fsf@sdf.lonestar.org
State Accepted
Headers show
Series Refactor out common behavior for system reconfiguration. | expand

Commit Message

Jakob L. Kreuze July 8, 2019, 7:59 p.m. UTC 
* guix/scripts/system/reconfigure.scm: New file.
* Makefile.am (MODULES): Add it.
* guix/scripts/system.scm (bootloader-installer-script): Export variable.
* gnu/machine/ssh.scm (switch-to-system, upgrade-shepherd-services)
(install-bootloader): Delete variable.
* gnu/machine/ssh.scm (deploy-managed-host): Rewrite procedure.
---
 Makefile.am                         |   1 +
 gnu/machine/ssh.scm                 | 232 +++++++---------------------
 guix/scripts/system.scm             |   1 +
 guix/scripts/system/reconfigure.scm | 158 +++++++++++++++++++
 4 files changed, 219 insertions(+), 173 deletions(-)
 create mode 100644 guix/scripts/system/reconfigure.scm

Comments

Ludovic Courtès July 13, 2019, 10:23 a.m. UTC | #1 
Hello!

zerodaysfordays@sdf.lonestar.org (Jakob L. Kreuze) skribis:

> * guix/scripts/system/reconfigure.scm: New file.
> * Makefile.am (MODULES): Add it.
> * guix/scripts/system.scm (bootloader-installer-script): Export variable.
> * gnu/machine/ssh.scm (switch-to-system, upgrade-shepherd-services)
> (install-bootloader): Delete variable.
> * gnu/machine/ssh.scm (deploy-managed-host): Rewrite procedure.

[...]

> +  (define (run-switch-to-system machine)
> +    "Monadic procedure serializing the items in MACHINE necessary to build a
> +G-Expression with 'switch-to-system'."
> +    (mlet %store-monad ((script (switch-system-program (machine-system machine))))
> +        (machine-remote-eval machine #~(primitive-load #$script))))
> +
> +  (define (run-upgrade-shepherd-services machine)
> +    "Monadic procedure serializing the items in MACHINE necessary to build a
> +G-Expression with 'upgrade-shepherd-services'."
> +    (mlet* %store-monad ((target-services target-services)
> +                         (script (upgrade-services-program target-services)))
> +      (machine-remote-eval machine #~(primitive-load #$script))))

These would look nicer if ‘switch-system-program’ and
‘upgrade-services-program’ returns a <program-file> because you could
just write:

  (machine-remote-eval #~(primitive-load #$(switch-system-program …))
                       machine)

(I realize the order of arguments is reversed; to stick to what ‘eval’
does, I’d tend to put the ‘machine’ argument second—but that’s a
separate issue.  :-))

> +(define (switch-system-program os)
> +  "Return as a monadic value a derivation to build a scheme file that, upon
> +being evaluated, will create a new generation for SYSTEM-DERIVATION and
> +execute ACTIVATION-SCRIPT."
> +  (gexp->script
> +   "switch-to-system.scm"
> +   (with-extensions (list guile-gcrypt)
> +     (with-imported-modules (source-module-closure '((guix config)
> +                                                     (guix profiles)
> +                                                     (guix utils)))
> +       #~(begin
> +           (use-modules (guix config)
> +                        (guix profiles)
> +                        (guix utils))
> +
> +           (define %system-profile
> +             (string-append %state-directory "/profiles/system"))
> +
> +           (let* ((number (1+ (generation-number %system-profile)))
> +                  (generation (generation-file-name %system-profile number)))
> +             (switch-symlinks generation #$os)
> +             (switch-symlinks %system-profile generation)
> +             (setenv "GUIX_NEW_SYSTEM" #$os)
> +             (with-output-to-string
> +               (lambda ()
> +                 (primitive-load
> +                  #$(operating-system-activation-script os))))))))))

Can we remove ‘with-output-to-string’?  I’d rather see what’s going on.
:-)

If that’s too verbose, we can use ‘invoke/quiet’.

> +;; XXX: Currently, this does NOT attempt to restart running services. See
> +;; <https://issues.guix.info/issue/33508> for details.
> +(define (upgrade-services-program target-services)
> +  "Return as a monadic value a derivation to build a scheme file that, upon
> +being evaluated, will use TARGET-SERVICES, a list
> +of (shepherd-service-canonical-name, shepherd-service-file) pairs to determine
> +which services are obsolete and need to be unloaded, as well as which services
> +are new and need to be started."
> +  (gexp->script
> +   "upgrade-shepherd-services.scm"
> +   (with-imported-modules '((gnu services herd))
> +    #~(begin
> +        (use-modules (gnu services herd)
> +                     (srfi srfi-1))
> +
> +        (define running
> +          (filter live-service-running (current-services)))
> +
> +        (define (essential? service)
> +          ;; Return #t if SERVICE is essential and should not be unloaded
> +          ;; under any circumstance.
> +          (memq (first (live-service-provision service))
> +                '(root shepherd)))
> +
> +        (define (obsolete? service)
> +          ;; Return #t if SERVICE can be safely unloaded.
> +          (and (not (essential? service))
> +               (every (lambda (requirements)
> +                        (not (memq (first (live-service-provision service))
> +                                   requirements)))
> +                      (map live-service-requirement running))))
> +
> +        (define to-unload
> +          (filter obsolete?
> +                  (remove (lambda (service)
> +                            (memq (first (live-service-provision service))
> +                                  (map first '#$target-services)))
> +                          running)))
> +
> +        (define to-start
> +          (remove (lambda (service-pair)
> +                    (memq (first service-pair)
> +                          (map (compose first live-service-provision)
> +                               running)))
> +                  '#$target-services))
> +
> +        ;; Unload obsolete services.
> +        (for-each (lambda (service)
> +                    (false-if-exception
> +                     (unload-service service)))
> +                  to-unload)
> +
> +        ;; Load the service files for any new services and start them.
> +        (load-services/safe (map second to-start))
> +        (for-each start-service (map first to-start))))))

It seems that this sort-of inlines parts of ‘shepherd-service-upgrade’
but without traversing the service dependency graph to determine the
compilete set of obsolete services, no?  I feel that we should be
reusing ‘shepherd-service-upgrade’ or similar bits.  (I realize this is
already in ‘master’ for ‘guix deploy’, but since this is going to be
shared with ‘guix system’, we’d rather be extra cautious.)

Also, I think we should remove ‘false-if-exception’ around
‘unload-service’.

> +(define (install-bootloader-program installer-script bootcfg bootcfg-file target)
> +  "Return as a monadic value a derivation to build a scheme file that, upon
> +being evaluated, will install BOOTCFG to BOOTCFG-FILE, a target path, on
> +TARGET, a mount point, and subsequently run INSTALLER-SCRIPT."
> +  (gexp->script
> +   "install-bootloader.scm"
> +   (with-extensions (list guile-gcrypt)
> +     (with-imported-modules (source-module-closure '((gnu build install)
> +                                                     (guix store)
> +                                                     (guix utils)))
> +       #~(begin
> +           (use-modules (gnu build install)
> +                        (guix store)
> +                        (guix utils))
> +           (let* ((gc-root (string-append "/" %gc-roots-directory "/bootcfg"))
> +                  (temp-gc-root (string-append gc-root ".new")))
> +
> +             (switch-symlinks temp-gc-root gc-root)
> +
> +             (let ((installer-result
> +                    (false-if-exception
> +                     (begin
> +                       (install-boot-config #$bootcfg #$bootcfg-file #$target)
> +                       (with-output-to-string
> +                         (lambda ()
> +                           (primitive-load #$installer-script)))))))
> +               (unless installer-result
> +                 (delete-file temp-gc-root)
> +                 (error "failed to install bootloader"))
> +               (rename-file temp-gc-root gc-root)
> +               installer-result)))))))

I’d rather not swallow stdout and not use ‘error’.  Or at least, code
that runs ‘install-bootloader-program’ should be able to produce a
meaningful (and i18n’d) error message.  So the caller could do something
like:

  (define result
    (machine-eval #~(…
                     (guard (c ((message-condition? c)
                                (cons 'error (condition-message c))))
                       (invoke/quiet #$(install-bootloader-program …))
                       '(success)))
                  machine))

  (match result
    (('error message)
     (leave (G_ "failed to install bootloader:~%~a~%") message))
    (('success)
     #t))

Does that make sense?

That’s quite some boilerplate to the challenge will be to factorize it.

Ultimately, the code in (guix scripts system reconfigure) should be
parameterized by an evaluation procedure that would be either
‘machine-eval’ or some hypothetical ‘local-eval’ procedure to evaluate
things locally.

Thanks,
Ludo’.
Jakob L. Kreuze July 13, 2019, 5:44 p.m. UTC | #2 
Hi, Ludovic!

Ludovic Courtès <ludo@gnu.org> writes:

> These would look nicer if ‘switch-system-program’ and
> ‘upgrade-services-program’ returns a <program-file> because you could
> just write:
>
>   (machine-remote-eval #~(primitive-load #$(switch-system-program …))
>                        machine)
>
> (I realize the order of arguments is reversed; to stick to what ‘eval’
> does, I’d tend to put the ‘machine’ argument second—but that’s a
> separate issue.  :-))

I'm using 'gexp->script', so they should be returning a 'program-file'.
I've just neglected the conveniences I'm afforded with ungexp, it seems.
#~(primitive-load #$(switch-system-program …)) is, indeed, quite a bit
cleaner :)

> Can we remove ‘with-output-to-string’? I’d rather see what’s going on.
> :-)
>
> If that’s too verbose, we can use ‘invoke/quiet’.

I'm not too concerned with verbosity; rather, in the case for 'guix
deploy', the script's output mixes with the REPL output and that causes
'remote-eval' to fail with a match error. I think it would be better to
continue using 'with-output-to-string', but to preseve its return value
so we can show it to the user from 'guix deploy' or 'guix system
reconfigure'. Users of 'guix deploy' would also be able to see the
script's output this way.

> It seems that this sort-of inlines parts of ‘shepherd-service-upgrade’
> but without traversing the service dependency graph to determine the
> compilete set of obsolete services, no? I feel that we should be
> reusing ‘shepherd-service-upgrade’ or similar bits. (I realize this is
> already in ‘master’ for ‘guix deploy’, but since this is going to be
> shared with ‘guix system’, we’d rather be extra cautious.)

Does 'live-service-requirement' not encompass the full service
dependency graph? Regardless, I'll look into reusing
'shepherd-service-upgrade' as it's well-testsed.

> Also, I think we should remove ‘false-if-exception’ around
> ‘unload-service’.

Agreed. When you have time to look at it, I've raised a few questions
about this in v2 of this series.

> I’d rather not swallow stdout and not use ‘error’. Or at least, code
> that runs ‘install-bootloader-program’ should be able to produce a
> meaningful (and i18n’d) error message. So the caller could do
> something like:
>
>   (define result
>     (machine-eval #~(…
>                      (guard (c ((message-condition? c)
>                                 (cons 'error (condition-message c))))
>                        (invoke/quiet #$(install-bootloader-program …))
>                        '(success)))
>                   machine))
>
>   (match result
>     (('error message)
>      (leave (G_ "failed to install bootloader:~%~a~%") message))
>     (('success)
>      #t))
>
> Does that make sense?

Yes, and thank you for providing that snippet :)

> That’s quite some boilerplate to the challenge will be to factorize
> it.
>
> Ultimately, the code in (guix scripts system reconfigure) should be
> parameterized by an evaluation procedure that would be either
> ‘machine-eval’ or some hypothetical ‘local-eval’ procedure to evaluate
> things locally.

Noted. That should be a relatively small change, so I'll see about
tackling that in my next revision for this series.

Regards,
Jakob
diff mbox series

Patch

diff --git a/Makefile.am b/Makefile.am
index dd7720e87..58a96d348 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -245,6 +245,7 @@  MODULES =					\
   guix/scripts/describe.scm			\
   guix/scripts/system.scm			\
   guix/scripts/system/search.scm		\
+  guix/scripts/system/reconfigure.scm		\
   guix/scripts/lint.scm				\
   guix/scripts/challenge.scm			\
   guix/scripts/import/crate.scm			\
diff --git a/gnu/machine/ssh.scm b/gnu/machine/ssh.scm
index a7d1a967a..95198bb2a 100644
--- a/gnu/machine/ssh.scm
+++ b/gnu/machine/ssh.scm
@@ -30,10 +30,13 @@ 
   #:use-module (guix monads)
   #:use-module (guix records)
   #:use-module (guix remote)
+  #:use-module (guix scripts system)
+  #:use-module (guix scripts system reconfigure)
   #:use-module (guix ssh)
   #:use-module (guix store)
   #:use-module (ice-9 match)
   #:use-module (srfi srfi-19)
+  #:use-module (srfi srfi-26)
   #:use-module (srfi srfi-35)
   #:export (managed-host-environment-type
 
@@ -105,118 +108,6 @@  an environment type of 'managed-host."
 ;;; System deployment.
 ;;;
 
-(define (switch-to-system machine)
-  "Monadic procedure creating a new generation on MACHINE and execute the
-activation script for the new system configuration."
-  (define (remote-exp drv script)
-    (with-extensions (list guile-gcrypt)
-      (with-imported-modules (source-module-closure '((guix config)
-                                                      (guix profiles)
-                                                      (guix utils)))
-        #~(begin
-            (use-modules (guix config)
-                         (guix profiles)
-                         (guix utils))
-
-            (define %system-profile
-              (string-append %state-directory "/profiles/system"))
-
-            (let* ((system #$drv)
-                   (number (1+ (generation-number %system-profile)))
-                   (generation (generation-file-name %system-profile number)))
-              (switch-symlinks generation system)
-              (switch-symlinks %system-profile generation)
-              ;; The implementation of 'guix system reconfigure' saves the
-              ;; load path and environment here. This is unnecessary here
-              ;; because each invocation of 'remote-eval' runs in a distinct
-              ;; Guile REPL.
-              (setenv "GUIX_NEW_SYSTEM" system)
-              ;; The activation script may write to stdout, which confuses
-              ;; 'remote-eval' when it attempts to read a result from the
-              ;; remote REPL. We work around this by forcing the output to a
-              ;; string.
-              (with-output-to-string
-                (lambda ()
-                  (primitive-load #$script))))))))
-
-  (let* ((os (machine-system machine))
-         (script (operating-system-activation-script os)))
-    (mlet* %store-monad ((drv (operating-system-derivation os)))
-      (machine-remote-eval machine (remote-exp drv script)))))
-
-;; XXX: Currently, this does NOT attempt to restart running services. This is
-;; also the case with 'guix system reconfigure'.
-;;
-;; See <https://issues.guix.info/issue/33508>.
-(define (upgrade-shepherd-services machine)
-  "Monadic procedure unloading and starting services on the remote as needed
-to realize the MACHINE's system configuration."
-  (define target-services
-    ;; Monadic expression evaluating to a list of (name output-path) pairs for
-    ;; all of MACHINE's services.
-    (mapm %store-monad
-          (lambda (service)
-            (mlet %store-monad ((file ((compose lower-object
-                                                shepherd-service-file)
-                                       service)))
-              (return (list (shepherd-service-canonical-name service)
-                            (derivation->output-path file)))))
-          (service-value
-           (fold-services (operating-system-services (machine-system machine))
-                          #:target-type shepherd-root-service-type))))
-
-  (define (remote-exp target-services)
-    (with-imported-modules '((gnu services herd))
-      #~(begin
-          (use-modules (gnu services herd)
-                       (srfi srfi-1))
-
-          (define running
-            (filter live-service-running (current-services)))
-
-          (define (essential? service)
-            ;; Return #t if SERVICE is essential and should not be unloaded
-            ;; under any circumstance.
-            (memq (first (live-service-provision service))
-                  '(root shepherd)))
-
-          (define (obsolete? service)
-            ;; Return #t if SERVICE can be safely unloaded.
-            (and (not (essential? service))
-                 (every (lambda (requirements)
-                          (not (memq (first (live-service-provision service))
-                                     requirements)))
-                        (map live-service-requirement running))))
-
-          (define to-unload
-            (filter obsolete?
-                    (remove (lambda (service)
-                              (memq (first (live-service-provision service))
-                                    (map first '#$target-services)))
-                            running)))
-
-          (define to-start
-            (remove (lambda (service-pair)
-                      (memq (first service-pair)
-                            (map (compose first live-service-provision)
-                                 running)))
-                    '#$target-services))
-
-          ;; Unload obsolete services.
-          (for-each (lambda (service)
-                      (false-if-exception
-                       (unload-service service)))
-                    to-unload)
-
-          ;; Load the service files for any new services and start them.
-          (load-services/safe (map second to-start))
-          (for-each start-service (map first to-start))
-
-          #t)))
-
-  (mlet %store-monad ((target-services target-services))
-    (machine-remote-eval machine (remote-exp target-services))))
-
 (define (machine-boot-parameters machine)
   "Monadic procedure returning a list of 'boot-parameters' for the generations
 of MACHINE's system profile, ordered from most recent to oldest."
@@ -275,71 +166,66 @@  of MACHINE's system profile, ordered from most recent to oldest."
                            (boot-parameters-kernel-arguments params))))))))
           generations))))
 
-(define (install-bootloader machine)
-  "Create a bootloader entry for the new system generation on MACHINE, and
-configure the bootloader to boot that generation by default."
-  (define bootloader-installer-script
-    (@@ (guix scripts system) bootloader-installer-script))
-
-  (define (remote-exp installer bootcfg bootcfg-file)
-    (with-extensions (list guile-gcrypt)
-      (with-imported-modules (source-module-closure '((gnu build install)
-                                                      (guix store)
-                                                      (guix utils)))
-        #~(begin
-            (use-modules (gnu build install)
-                         (guix store)
-                         (guix utils))
-            (let* ((gc-root (string-append "/" %gc-roots-directory "/bootcfg"))
-                   (temp-gc-root (string-append gc-root ".new")))
-
-              (switch-symlinks temp-gc-root gc-root)
-
-              (unless (false-if-exception
-                       (begin
-                         ;; The implementation of 'guix system reconfigure'
-                         ;; saves the load path here. This is unnecessary here
-                         ;; because each invocation of 'remote-eval' runs in a
-                         ;; distinct Guile REPL.
-                         (install-boot-config #$bootcfg #$bootcfg-file "/")
-                         ;; The installation script may write to stdout, which
-                         ;; confuses 'remote-eval' when it attempts to read a
-                         ;; result from the remote REPL. We work around this
-                         ;; by forcing the output to a string.
-                         (with-output-to-string
-                           (lambda ()
-                             (primitive-load #$installer)))))
-                (delete-file temp-gc-root)
-                (error "failed to install bootloader"))
-
-              (rename-file temp-gc-root gc-root)
-              #t)))))
-
-  (mlet* %store-monad ((boot-parameters (machine-boot-parameters machine)))
-    (let* ((os (machine-system machine))
-           (bootloader ((compose bootloader-configuration-bootloader
-                                 operating-system-bootloader)
-                        os))
-           (bootloader-target (bootloader-configuration-target
-                               (operating-system-bootloader os)))
-           (installer (bootloader-installer-script
-                       (bootloader-installer bootloader)
-                       (bootloader-package bootloader)
-                       bootloader-target
-                       "/"))
-           (menu-entries (map boot-parameters->menu-entry boot-parameters))
-           (bootcfg (operating-system-bootcfg os menu-entries))
-           (bootcfg-file (bootloader-configuration-file bootloader)))
-      (machine-remote-eval machine (remote-exp installer bootcfg bootcfg-file)))))
-
 (define (deploy-managed-host machine)
   "Internal implementation of 'deploy-machine' for MACHINE instances with an
 environment type of 'managed-host."
+  (define target-services
+    ;; Monadic expression evaluating to a list of
+    ;; (shepherd-service-canonical-name, shepherd-service-file) pairs for the
+    ;; services in MACHINE's operating system configuration.
+    (mapm %store-monad
+          (lambda (service)
+            (mlet %store-monad ((file ((compose lower-object
+                                                shepherd-service-file)
+                                       service)))
+              (return (list (shepherd-service-canonical-name service)
+                            (derivation->output-path file)))))
+          (service-value
+           (fold-services (operating-system-services (machine-system machine))
+                          #:target-type shepherd-root-service-type))))
+
+  (define (run-switch-to-system machine)
+    "Monadic procedure serializing the items in MACHINE necessary to build a
+G-Expression with 'switch-to-system'."
+    (mlet %store-monad ((script (switch-system-program (machine-system machine))))
+        (machine-remote-eval machine #~(primitive-load #$script))))
+
+  (define (run-upgrade-shepherd-services machine)
+    "Monadic procedure serializing the items in MACHINE necessary to build a
+G-Expression with 'upgrade-shepherd-services'."
+    (mlet* %store-monad ((target-services target-services)
+                         (script (upgrade-services-program target-services)))
+      (machine-remote-eval machine #~(primitive-load #$script))))
+
+  (define (run-install-bootloader machine)
+    "Monadic procedure serializing the items in MACHINE necessary to build a
+G-Expression with 'install-bootloader'."
+    (mlet %store-monad ((boot-parameters (machine-boot-parameters machine)))
+      (let* ((os (machine-system machine))
+             (bootloader ((compose bootloader-configuration-bootloader
+                                   operating-system-bootloader)
+                          os))
+             (target (bootloader-configuration-target
+                      (operating-system-bootloader os)))
+             (installer (bootloader-installer-script
+                         (bootloader-installer bootloader)
+                         (bootloader-package bootloader)
+                         target
+                         "/"))
+             (menu-entries (map boot-parameters->menu-entry boot-parameters))
+             (bootcfg (operating-system-bootcfg os menu-entries))
+             (bootcfg-file (bootloader-configuration-file bootloader)))
+        (mlet %store-monad ((script (install-bootloader-program installer
+                                                                bootcfg
+                                                                bootcfg-file
+                                                                "/")))
+          (machine-remote-eval machine #~(primitive-load #$script))))))
+
   (maybe-raise-unsupported-configuration-error machine)
-  (mbegin %store-monad
-    (switch-to-system machine)
-    (upgrade-shepherd-services machine)
-    (install-bootloader machine)))
+  (mapm %store-monad (cut <> machine)
+        (list run-switch-to-system
+              run-upgrade-shepherd-services
+              run-install-bootloader)))
 
 
 ;;;
diff --git a/guix/scripts/system.scm b/guix/scripts/system.scm
index 60c1ca5c9..21858ee7d 100644
--- a/guix/scripts/system.scm
+++ b/guix/scripts/system.scm
@@ -70,6 +70,7 @@ 
   #:use-module (ice-9 match)
   #:use-module (rnrs bytevectors)
   #:export (guix-system
+            bootloader-installer-script
             read-operating-system))
 
 
diff --git a/guix/scripts/system/reconfigure.scm b/guix/scripts/system/reconfigure.scm
new file mode 100644
index 000000000..e14ea4f2f
--- /dev/null
+++ b/guix/scripts/system/reconfigure.scm
@@ -0,0 +1,158 @@ 
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2014, 2015, 2016, 2017, 2018, 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2016 Alex Kost <alezost@gmail.com>
+;;; Copyright © 2016, 2017, 2018 Chris Marusich <cmmarusich@gmail.com>
+;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
+;;; Copyright © 2018 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2019 Christopher Baines <mail@cbaines.net>
+;;; Copyright © 2019 Jakob L. Kreuze <zerodaysfordays@sdf.lonestar.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (guix scripts system reconfigure)
+  #:autoload   (gnu packages gnupg) (guile-gcrypt)
+  #:use-module (gnu system)
+  #:use-module (guix gexp)
+  #:use-module (guix modules)
+  #:export (switch-system-program
+            upgrade-services-program
+            install-bootloader-program))
+
+;;; Commentary:
+;;;
+;;; This module implements the "effectful" parts of system
+;;; reconfiguration. Although building a system derivation is a pure
+;;; operation, a number of impure operations must be carried out for the
+;;; system configuration to be realized -- chiefly, creation of generation
+;;; symlinks and invocation of activation scripts.
+;;;
+;;; Code:
+
+(define (switch-system-program os)
+  "Return as a monadic value a derivation to build a scheme file that, upon
+being evaluated, will create a new generation for SYSTEM-DERIVATION and
+execute ACTIVATION-SCRIPT."
+  (gexp->script
+   "switch-to-system.scm"
+   (with-extensions (list guile-gcrypt)
+     (with-imported-modules (source-module-closure '((guix config)
+                                                     (guix profiles)
+                                                     (guix utils)))
+       #~(begin
+           (use-modules (guix config)
+                        (guix profiles)
+                        (guix utils))
+
+           (define %system-profile
+             (string-append %state-directory "/profiles/system"))
+
+           (let* ((number (1+ (generation-number %system-profile)))
+                  (generation (generation-file-name %system-profile number)))
+             (switch-symlinks generation #$os)
+             (switch-symlinks %system-profile generation)
+             (setenv "GUIX_NEW_SYSTEM" #$os)
+             (with-output-to-string
+               (lambda ()
+                 (primitive-load
+                  #$(operating-system-activation-script os))))))))))
+
+;; XXX: Currently, this does NOT attempt to restart running services. See
+;; <https://issues.guix.info/issue/33508> for details.
+(define (upgrade-services-program target-services)
+  "Return as a monadic value a derivation to build a scheme file that, upon
+being evaluated, will use TARGET-SERVICES, a list
+of (shepherd-service-canonical-name, shepherd-service-file) pairs to determine
+which services are obsolete and need to be unloaded, as well as which services
+are new and need to be started."
+  (gexp->script
+   "upgrade-shepherd-services.scm"
+   (with-imported-modules '((gnu services herd))
+    #~(begin
+        (use-modules (gnu services herd)
+                     (srfi srfi-1))
+
+        (define running
+          (filter live-service-running (current-services)))
+
+        (define (essential? service)
+          ;; Return #t if SERVICE is essential and should not be unloaded
+          ;; under any circumstance.
+          (memq (first (live-service-provision service))
+                '(root shepherd)))
+
+        (define (obsolete? service)
+          ;; Return #t if SERVICE can be safely unloaded.
+          (and (not (essential? service))
+               (every (lambda (requirements)
+                        (not (memq (first (live-service-provision service))
+                                   requirements)))
+                      (map live-service-requirement running))))
+
+        (define to-unload
+          (filter obsolete?
+                  (remove (lambda (service)
+                            (memq (first (live-service-provision service))
+                                  (map first '#$target-services)))
+                          running)))
+
+        (define to-start
+          (remove (lambda (service-pair)
+                    (memq (first service-pair)
+                          (map (compose first live-service-provision)
+                               running)))
+                  '#$target-services))
+
+        ;; Unload obsolete services.
+        (for-each (lambda (service)
+                    (false-if-exception
+                     (unload-service service)))
+                  to-unload)
+
+        ;; Load the service files for any new services and start them.
+        (load-services/safe (map second to-start))
+        (for-each start-service (map first to-start))))))
+
+(define (install-bootloader-program installer-script bootcfg bootcfg-file target)
+  "Return as a monadic value a derivation to build a scheme file that, upon
+being evaluated, will install BOOTCFG to BOOTCFG-FILE, a target path, on
+TARGET, a mount point, and subsequently run INSTALLER-SCRIPT."
+  (gexp->script
+   "install-bootloader.scm"
+   (with-extensions (list guile-gcrypt)
+     (with-imported-modules (source-module-closure '((gnu build install)
+                                                     (guix store)
+                                                     (guix utils)))
+       #~(begin
+           (use-modules (gnu build install)
+                        (guix store)
+                        (guix utils))
+           (let* ((gc-root (string-append "/" %gc-roots-directory "/bootcfg"))
+                  (temp-gc-root (string-append gc-root ".new")))
+
+             (switch-symlinks temp-gc-root gc-root)
+
+             (let ((installer-result
+                    (false-if-exception
+                     (begin
+                       (install-boot-config #$bootcfg #$bootcfg-file #$target)
+                       (with-output-to-string
+                         (lambda ()
+                           (primitive-load #$installer-script)))))))
+               (unless installer-result
+                 (delete-file temp-gc-root)
+                 (error "failed to install bootloader"))
+               (rename-file temp-gc-root gc-root)
+               installer-result)))))))