guix system: 'describe' shows the running system, not the current one.

* guix/profiles.scm (generation-number): Add optional 'base-profile'
parameter and use it.
* guix/scripts/system.scm (process-command): Add "/run/current-system"
as first argument to 'generation-number'.
* doc/guix.texi (Invoking guix system): Clarify that 'guix system
describe' shows the running system.

3 files changed, 24 insertions, 7 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index b10f0d085b..ceec0d0cf5 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -35811,9 +35811,21 @@ bootloader boot menu:
 @table @code
 
 @item describe
-Describe the current system generation: its file name, the kernel and
+Describe the running system generation: its file name, the kernel and
 bootloader used, etc., as well as provenance information when available.
 
+@quotation Note
+The @emph{running} system generation---referred to by
+@file{/run/current-system}---is not necessarily the @emph{current}
+system generation---referred to by @file{/var/guix/profiles/system}: it
+differs when, for instance, you chose from the bootloader menu to boot
+an older generation.
+
+It can also differ from the @emph{booted} system generation---referred
+to by @file{/run/booted-system}---for instance because you reconfigured
+the system in the meantime.
+@end quotation
+
 @item list-generations
 List a summary of each generation of the operating system available on
 disk, in a human-readable way.  This is similar to the
diff --git a/guix/profiles.scm b/guix/profiles.scm
index 1d354ecb78..96be421727 100644
--- a/guix/profiles.scm
+++ b/guix/profiles.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013-2022 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2013 Nikita Karetnikov <nikita@karetnikov.org>
 ;;; Copyright © 2014, 2016 Alex Kost <alezost@gmail.com>
 ;;; Copyright © 2015 Mark H Weaver <mhw@netris.org>
@@ -2037,9 +2037,14 @@ paths."
   (make-regexp (string-append "^" (regexp-quote (basename profile))
                               "-([0-9]+)")))
 
-(define (generation-number profile)
-  "Return PROFILE's number or 0.  An absolute file name must be used."
-  (or (and=> (false-if-exception (regexp-exec (profile-regexp profile)
+(define* (generation-number profile
+                            #:optional (base-profile profile))
+  "Return PROFILE's number or 0.  An absolute file name must be used.
+
+Optionally, if BASE-PROFILE is provided, use it instead of PROFILE to
+construct the regexp matching generations.  This is useful in special cases
+like: (generation-number \"/run/current-system\" %system-profile)."
+  (or (and=> (false-if-exception (regexp-exec (profile-regexp base-profile)
                                               (basename (readlink profile))))
              (compose string->number (cut match:substring <> 1)))
       0))
diff --git a/guix/scripts/system.scm b/guix/scripts/system.scm
index 414e931c8a..430815902d 100644
--- a/guix/scripts/system.scm
+++ b/guix/scripts/system.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014-2022 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2016 Alex Kost <alezost@gmail.com>
 ;;; Copyright © 2016, 2017, 2018 Chris Marusich <cmmarusich@gmail.com>
 ;;; Copyright © 2017, 2019 Mathieu Othacehe <m.othacehe@gmail.com>
@@ -1328,7 +1328,7 @@ argument list and OPTS is the option alist."
                       (x (leave (G_ "wrong number of arguments~%"))))))
        (list-generations pattern)))
     ((describe)
-     (match (generation-number %system-profile)
+     (match (generation-number "/run/current-system" %system-profile)
        (0
         (leave (G_ "no system generation, nothing to describe~%")))
        (generation