channels: Record 'guix' channel metadata in (guix config).

Partially fixes <https://bugs.gnu.org/45896>.

* guix/config.scm.in (%channel-metadata): New variable.
* guix/describe.scm (channel-metadata): Use it.
(current-channels): New procedure.
(current-profile-entries): Clarify docstring.
* guix/self.scm (compiled-guix): Add #:channel-metadata and pass it to
'make-config.scm'.
(make-config.scm): Add #:channel-metadata and define '%channel-metadata'
in the generated file.
(guix-derivation): Add #:channel-metadata and pass it to 'compiled-guix'.
* guix/channels.scm (build-from-source): Replace 'name', 'source', and
'commit' parameters with 'instance'.  Pass #:channel-metadata to BUILD.
(build-channel-instance): Adjust accordingly.
* build-aux/build-self.scm (build-program): Add #:channel-metadata
and pass it to 'guix-derivation'.
(build): Add #:channel-metadata and pass it to 'build-program'.
* guix/scripts/describe.scm (display-profile-info): Add optional
'channels' parameter.  Pass it to 'display-profile-content'.
(display-profile-content): Add optional 'channels' parameter and honor
it.  Iterate on CHANNELS rather than on the manifest entries of
PROFILE.
(guix-describe): When PROFILE is #f, call 'current-channels' and pass it
to 'display-profile-info', unless it returns the empty list.

1 files changed, 26 insertions, 2 deletions

diff --git a/guix/describe.scm b/guix/describe.scm
index ac89fc0d7c..6a31c707f0 100644
--- a/guix/describe.scm
+++ b/guix/describe.scm
@@ -23,12 +23,13 @@
   #:use-module ((guix utils) #:select (location-file))
   #:use-module ((guix store) #:select (%store-prefix store-path?))
   #:use-module ((guix config) #:select (%state-directory))
-  #:autoload   (guix channels) (sexp->channel)
+  #:autoload   (guix channels) (sexp->channel manifest-entry-channel)
   #:use-module (srfi srfi-1)
   #:use-module (ice-9 match)
   #:export (current-profile
             current-profile-date
             current-profile-entries
+            current-channels
             package-path-entries
 
             package-provenance
@@ -87,10 +88,19 @@ as a number of seconds since the Epoch, or #f if it could not be determined."
                            (string-append (dirname file) "/" target)))))
              (const #f)))))))
 
+(define (channel-metadata)
+  "Return the 'guix' channel metadata sexp from (guix config) if available;
+otherwise return #f."
+  ;; Older 'build-self.scm' would create a (guix config) file without the
+  ;; '%channel-metadata' variable.  Thus, properly deal with a lack of
+  ;; information.
+  (let ((module (resolve-interface '(guix config))))
+    (and=> (module-variable module '%channel-metadata) variable-ref)))
+
 (define current-profile-entries
   (mlambda ()
     "Return the list of entries in the 'guix pull' profile the calling process
-lives in, or #f if this is not applicable."
+lives in, or the empty list if this is not applicable."
     (match (current-profile)
       (#f '())
       (profile
@@ -105,6 +115,20 @@ lives in, or #f if this is not applicable."
               (string=? (manifest-entry-name entry) "guix"))
             (current-profile-entries))))
 
+(define current-channels
+  (mlambda ()
+    "Return the list of channels currently available, including the 'guix'
+channel.  Return the empty list if this information is missing."
+    (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) '()))))
+      (entries
+       (filter-map manifest-entry-channel entries)))))
+
 (define (package-path-entries)
   "Return two values: the list of package path entries to be added to the
 package search path, and the list to be added to %LOAD-COMPILED-PATH.  These