guix gc: Correctly handle '--delete-generations' with no arguments.

Previously, 'guix gc --delete-generations' would crash: the "" pattern
would be passed to 'matching-generations', which would return #f instead
of returning a list.

Reported by Raghav Gururajan <rvgn@disroot.org>
in <https://bugs.gnu.org/36466>.

* guix/ui.scm (matching-generations): Raise an error when passed an
invalid pattern.
* guix/scripts/gc.scm (delete-old-generations): Check if PATTERN is
true.
(%options): Leave ARG as-is for 'delete-generations'.
(guix-gc): Use 'assq' instead of 'assoc-ref' for 'delete-generations'.
* guix/scripts/package.scm (delete-matching-generations):
Replace (string-null? pattern) with (not pattern).  Remove 'else'
clause.
(%options): Leave ARG as-is for 'delete-generations'.
* guix/scripts/pull.scm (%options): Leave ARG as-is for
'list-generations'.
(process-query): Replace (string-null? pattern) with (not pattern).
* guix/scripts/system.scm (list-generations): Likewise, and remove
'else' clause.
(process-command): Use #f instead of "" when no pattern is given.

1 files changed, 5 insertions, 1 deletions

diff --git a/guix/ui.scm b/guix/ui.scm
index 7d6ab9a2a7..76f6fc8eed 100644
--- a/guix/ui.scm
+++ b/guix/ui.scm
@@ -1484,7 +1484,11 @@ DURATION-RELATION with the current time."
         ((string->duration str)
          =>
          filter-by-duration)
-        (else #f)))
+        (else
+         (raise
+          (condition (&message
+                      (message (format #f (G_ "invalid syntax: ~a~%")
+                                       str))))))))
 
 (define (display-generation profile number)
   "Display a one-line summary of generation NUMBER of PROFILE."