[bug#64737] [PATCH 2/2] scripts: time-machine: Error when attempting to visit too old commits.

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

* doc/guix.texi (Invoking guix time-machine): Document limitation.
* guix/scripts/time-machine.scm (%oldest-possible-commit): New variable.
(guix-time-machine): Raise an error when the channel commit is too old.

Suggested-by: Simon Tournier <zimon.toutoune@gmail.com>
---

 doc/guix.texi                 |  6 ++++++
 guix/scripts/time-machine.scm | 23 ++++++++++++++++++++++-
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index 1d8ebcd72f..30fef813c0 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -5056,6 +5056,12 @@ Invoking guix time-machine
 large number of packages; the result is cached though and subsequent
 commands targeting the same commit are almost instantaneous.
 
+Due to @command{guix time-machine} relying on the ``inferiors''
+mechanism (@pxref{Inferiors}), the oldest commit it can travel to is
+commit @samp{2ca299caf} (``Add (guix inferior) and (guix scripts
+repl).''), dated July 10@sup{th}, 2018.  An error is returned when
+attempting to navigate to older commits.
+
 @quotation Note
 The history of Guix is immutable and @command{guix time-machine}
 provides the exact same software as they are in a specific Guix
diff --git a/guix/scripts/time-machine.scm b/guix/scripts/time-machine.scm
index d7c71ef705..36a40a1538 100644
--- a/guix/scripts/time-machine.scm
+++ b/guix/scripts/time-machine.scm
@@ -2,6 +2,7 @@
 ;;; Copyright © 2019 Konrad Hinsen <konrad.hinsen@fastmail.net>
 ;;; Copyright © 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2021 Simon Tournier <zimon.toutoune@gmail.com>
+;;; Copyright © 2023 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -19,13 +20,15 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (guix scripts time-machine)
+  #:use-module (guix channels)
+  #:use-module (guix diagnostics)
   #:use-module (guix ui)
   #:use-module (guix scripts)
   #:use-module (guix inferior)
   #:use-module (guix store)
   #:use-module (guix status)
   #:use-module ((guix git)
-                #:select (with-git-error-handling))
+                #:select (update-cached-checkout with-git-error-handling))
   #:use-module ((guix utils)
                 #:select (%current-system))
   #:use-module ((guix scripts pull)
@@ -38,9 +41,16 @@ (define-module (guix scripts time-machine)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-11)
   #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-37)
+  #:use-module (srfi srfi-71)
   #:export (guix-time-machine))
 
+;;; The commit introducing the 'inferiors' mechanism; it is the oldest commit
+;;; that can be travelled to.
+(define %oldest-possible-commit
+  "2ca299caf64489f4e1e665ec1158fb0309b0b565")
+
 \f
 ;;;
 ;;; Command-line options.
@@ -139,9 +149,20 @@ (define-command (guix-time-machine . args)
     (with-git-error-handling
      (let* ((opts         (parse-args args))
             (channels     (channel-list opts))
+            (guix-channel (find guix-channel? channels))
             (command-line (assoc-ref opts 'exec))
+            (ref          (assoc-ref opts 'ref))
+            (checkout commit relation (update-cached-checkout
+                                       (channel-url guix-channel)
+                                       #:ref (or ref '())
+                                       #:starting-commit
+                                       %oldest-possible-commit))
             (substitutes?  (assoc-ref opts 'substitutes?))
             (authenticate? (assoc-ref opts 'authenticate-channels?)))
+       (unless (memq relation '(ancestor self))
+         (raise (formatted-message
+                 (G_ "cannot travel past commit `~a' from July 10th, 2018")
+                 (string-take %oldest-possible-commit 12))))
        (when command-line
          (let* ((directory
                  (with-store store
-- 
2.41.0