[bug#73992] [Shepherd PATCH] shepherd: Add support for rebooting using kexec

[Jakob Kirsch via Guix-patches via <guix-patches@gnu.org>]

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This is my improved patch for using kexec with shepherd as I sent the last one to the wrong address (someone should update the description of the shepherd project to link to the correct patch submission list).

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From 762fbee53fb890a7cf5e26abcc2dcfad03b1bab4 Mon Sep 17 00:00:00 2001
From: Jakob Kirsch <jakob.kirsch@web.de>
Date: Thu, 24 Oct 2024 19:45:31 +0200
Subject: [PATCH v3] shepherd: Add support for rebooting using kexec

---
 AUTHORS                             |  1 +
 configure.ac                        |  4 ++++
 doc/shepherd.texi                   |  9 +++++++++
 modules/shepherd/scripts/reboot.scm | 12 ++++++++++--
 modules/shepherd/service.scm        |  9 +++++++++
 modules/shepherd/system.scm.in      | 10 ++++++++++
 tests/status-sexp.sh                |  2 +-
 7 files changed, 44 insertions(+), 3 deletions(-)

diff --git a/AUTHORS b/AUTHORS
index 19132a7..6642bdc 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -10,3 +10,4 @@ when it was known as GNU dmd.  Others have since contributed:
              Alex Sassmannshausen <alex.sassmannshausen@gmail.com>
             David Thompson <dthompson2@worcester.edu>
              Andy Wingo <wingo@pobox.com>
+            Jakob Kirsch <jakob.kirsch@web.de>
diff --git a/configure.ac b/configure.ac
index bcfef13..db1dc95 100644
--- a/configure.ac
+++ b/configure.ac
@@ -124,6 +124,7 @@ case "$host_os" in
     AC_COMPUTE_INT([RB_DISABLE_CAD], [RB_DISABLE_CAD], [#include <sys/reboot.h>])
     AC_COMPUTE_INT([RB_POWER_OFF], [RB_POWER_OFF], [#include <sys/reboot.h>])
     AC_COMPUTE_INT([RB_SW_SUSPEND], [RB_SW_SUSPEND], [#include <sys/reboot.h>])
+    AC_COMPUTE_INT([RB_KEXEC], [RB_KEXEC], [#include <sys/reboot.h>])
     ;;
   gnu*)
     # On GNU/Hurd, the Mach-derived reboot.h uses different names, and
@@ -131,6 +132,7 @@ case "$host_os" in
     AC_COMPUTE_INT([RB_HALT_SYSTEM], [RB_HALT], [#include <sys/reboot.h>])
     AC_COMPUTE_INT([RB_POWER_OFF], [RB_HALT], [#include <sys/reboot.h>])
     RB_DISABLE_CAD="#f"
+    RB_KEXEC="#f"
     ;;
   *)
     # What is this?  GNU/kFreeBSD?
@@ -138,6 +140,7 @@ case "$host_os" in
     AC_COMPUTE_INT([RB_HALT_SYSTEM], [RB_HALT_SYSTEM], [#include <sys/reboot.h>])
     AC_COMPUTE_INT([RB_POWER_OFF], [RB_HALT_SYSTEM], [#include <sys/reboot.h>])
     RB_DISABLE_CAD="#f"
+    RB_KEXEC="#f"
     ;;
 esac

@@ -145,6 +148,7 @@ AC_SUBST([RB_DISABLE_CAD])
 AC_SUBST([RB_AUTOBOOT])
 AC_SUBST([RB_HALT_SYSTEM])
 AC_SUBST([RB_POWER_OFF])
+AC_SUBST([RB_KEXEC])
 AC_MSG_RESULT([done])

 AC_MSG_CHECKING([<sys/prctl.h> constants])
diff --git a/doc/shepherd.texi b/doc/shepherd.texi
index 0e627b3..921bf40 100644
--- a/doc/shepherd.texi
+++ b/doc/shepherd.texi
@@ -14,6 +14,7 @@ Copyright @copyright{} @value{NEW-YEARS} Ludovic Courtès@*
 Copyright @copyright{} 2020 Brice Waegeneire@*
 Copyright @copyright{} 2020 Oleg Pykhalov@*
 Copyright @copyright{} 2020, 2023 Jan (janneke) Nieuwenhuizen@*
+Copyright @copyright{} 2024 Jakob Kirsch@*

 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
@@ -600,6 +601,14 @@ It is equivalent to running @command{herd stop shepherd}.  The
 Send commands to the socket special file @var{file}. If this option is
 not specified, @file{@var{localstatedir}/run/shepherd/socket} is taken.

+@item -k
+@itemx --do-kexec
+Reboot the system using kexec. The kernel that was previously loaded
+using @command{kexec -l @var{file}} is executed instead of rebooting
+into the BIOS in order to keep the down time to a minimum or to chainload
+another kernel. This feature is only available on Linux-based systems and will
+throw an exception on GNU/Hurd. This is also equivalent to running @command{herd kexec shepherd}.
+
 @end table

 @c @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
diff --git a/modules/shepherd/scripts/reboot.scm b/modules/shepherd/scripts/reboot.scm
index 6be5414..4c2448e 100644
--- a/modules/shepherd/scripts/reboot.scm
+++ b/modules/shepherd/scripts/reboot.scm
@@ -30,6 +30,8 @@
 (define (main . args)
   (initialize-cli)

+  (define action 'stop)
+
   (parameterize ((program-name "reboot"))
     (let ((socket-file %system-socket-file)
           (command-args '()))
@@ -44,13 +46,19 @@
                       #:argument-name "FILE"
                       #:description "send commands to FILE"
                       #:action (lambda (file)
-                                 (set! socket-file file))))
+                                 (set! socket-file file)))
+                    (option
+                      #:long-name "do-kexec" #:short-name #\k
+                      #:takes-argument? #f
+                      #:description "reboot using kexec"
+                      #:action (lambda () (set! action 'kexec))
+                    ))

       (set! command-args (reverse command-args))
       (with-system-error-handling
        (let ((sock (open-connection socket-file)))
          ;; Send the command without further ado.
-         (write-command (shepherd-command 'stop 'root) sock)
+         (write-command (shepherd-command action 'root) sock)

          ;; Receive output if we're not already dead.
          (match (read sock)
diff --git a/modules/shepherd/service.scm b/modules/shepherd/service.scm
index 5942b0a..6a5e112 100644
--- a/modules/shepherd/service.scm
+++ b/modules/shepherd/service.scm
@@ -2707,6 +2707,15 @@ Clients such as 'herd' can read it and format it in a human-readable way."
           (lambda (key)
             (local-output (l10n "Shutting down..."))
             (power-off)))))
+
+     (kexec
+       "Reboot the system and run kexec."
+       (lambda (running)
+        (catch 'quit
+          (cut stop root-service)
+          (lambda (key)
+            (local-output (l10n "Rebooting with kexec..."))
+            (reboot-kexec)))))
      ;; Evaluate arbitrary code.
      (load
       "Load the Scheme code from FILE into shepherd.  This is potentially
diff --git a/modules/shepherd/system.scm.in b/modules/shepherd/system.scm.in
index 4c83175..1168c15 100644
--- a/modules/shepherd/system.scm.in
+++ b/modules/shepherd/system.scm.in
@@ -26,6 +26,7 @@
   #:use-module (srfi srfi-26)
   #:export (disable-reboot-on-ctrl-alt-del
             reboot
+            reboot-kexec
             halt
             power-off
             max-file-descriptors
@@ -51,6 +52,7 @@
 (define RB_HALT_SYSTEM @RB_HALT_SYSTEM@)
 (define RB_POWER_OFF @RB_POWER_OFF@)
 (define RB_DISABLE_CAD @RB_DISABLE_CAD@)          ; integer | #f
+(define RB_KEXEC @RB_KEXEC@)                      ; integer | #f

 (define (syscall->procedure return-type name argument-types)
   "Return a procedure that wraps the C function NAME using the dynamic FFI,
@@ -95,6 +97,14 @@ ctrlaltdel(8) and see kernel/reboot.c in Linux."
   "Perform a hard reset of the system now.  Return #f on failure."
   (%libc-reboot RB_AUTOBOOT))

+(define (reboot-kexec)
+  "Execute kernel loaded with 'kexec -l' now. Kexec is a feature of the
+Linux kernel that allows you to instantly reboot into a new kernel while skipping
+the BIOS phase.  Return #f on failure or throw an exception on non-Linux systems."
+  (if RB_KEXEC
+    (%libc-reboot RB_KEXEC)
+    (throw 'system-error 'kexec  "~A" (list (strerror ENOSYS)) (list ENOSYS))))
+
 (define (halt)
   "Halt the system.  Return #f on failure."
   (%libc-reboot RB_HALT_SYSTEM))
diff --git a/tests/status-sexp.sh b/tests/status-sexp.sh
index 14b1e72..3fa0283 100644
--- a/tests/status-sexp.sh
+++ b/tests/status-sexp.sh
@@ -91,7 +91,7 @@ root_service_sexp="
       (transient? #f)
       (respawn-limit (5 . 7))
       (respawn-delay 0.1)
-      (actions (help status halt power-off load eval unload reload daemonize restart))
+      (actions (help status halt power-off kexec load eval unload reload daemonize restart))
       (exit-statuses ())
       (recent-messages ())
       (log-files (\"$PWD/$log\"))
--
2.46.0