describe: Gracefully handle profiles that lack provenance info.

Fixes <https://bugs.gnu.org/48313>.

Previously, 'current-channels' would return the empty list when
'current-profile' is true but the profile lacks provenance info.

* guix/describe.scm (current-channels)[build-time-metadata]: New
procedure.  Call it when 'manifest-entry-channel' returns #f for all of
ENTRIES.
* guix/scripts/describe.scm (guix-describe): When PROFILE is true, pass
third argument to 'display-profile-info'.

2 files changed, 21 insertions, 5 deletions

diff --git a/guix/describe.scm b/guix/describe.scm
index 0683ad8a278..711b7b42903 100644
--- a/guix/describe.scm
+++ b/guix/describe.scm
@@ -122,15 +122,24 @@ lives in, or the empty list if this is not applicable."
   (mlambda ()
     "Return the list of channels currently available, including the 'guix'
 channel.  Return the empty list if this information is missing."
+    (define (build-time-metadata)
+      (match (channel-metadata)
+        (#f '())
+        (sexp (or (and=> (sexp->channel sexp 'guix) list) '()))))
+
     (match (current-profile-entries)
       (()
        ;; As a fallback, if we're not running from a profile, use 'guix'
        ;; channel metadata from (guix config).
-       (match (channel-metadata)
-         (#f '())
-         (sexp (or (and=> (sexp->channel sexp 'guix) list) '()))))
+       (build-time-metadata))
       (entries
-       (filter-map manifest-entry-channel entries)))))
+       (match (filter-map manifest-entry-channel entries)
+         (()
+          ;; This profile lacks provenance metadata, so fall back to
+          ;; build-time metadata as returned by 'channel-metadata'.
+          (build-time-metadata))
+         (lst
+          lst))))))
 
 (define (package-path-entries)
   "Return two values: the list of package path entries to be added to the
diff --git a/guix/scripts/describe.scm b/guix/scripts/describe.scm
index b5f6249176a..a3e3338f7e1 100644
--- a/guix/scripts/describe.scm
+++ b/guix/scripts/describe.scm
@@ -301,4 +301,11 @@ text.  The hyperlink links to a web view of COMMIT, when available."
            (channels
             (display-profile-info #f format channels))))
         (profile
-         (display-profile-info (canonicalize-profile profile) format))))))
+         ;; For the current profile, resort to 'current-channels', which has a
+         ;; fallback to metadata from (guix config) in case PROFILE lacks it.
+         (let ((channels (if (and (current-profile)
+                                  (string=? profile (current-profile)))
+                             (current-channels)
+                             (profile-channels profile))))
+           (display-profile-info (canonicalize-profile profile)
+                                 format channels)))))))