guix package: '--delete-generations' deletes generations older than specified.

* guix/scripts/package.scm (matching-generations): Add
  'duration-relation' keyword parameter.
  (guix-package)[process-action](delete-generations): Pass
  #:duration-relation >.
* tests/guix-package.sh: Add test.
* doc/guix.texi (Invoking guix package): Clarify the meaning of
  durations for '--list-durations' and '--delete-durations'.

1 files changed, 12 insertions, 6 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 2e6bdc595e..29928c5af4 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -711,18 +711,24 @@ second one.
 
 @item @emph{Durations}.  You can also get the last @emph{N}@tie{}days, weeks,
 or months by passing an integer along with the first letter of the
-duration, e.g., @code{--list-generations=20d}.
+duration.  For example, @code{--list-generations=20d} lists generations
+that are up to 20 days old.
 @end itemize
 
 @item --delete-generations[=@var{pattern}]
 @itemx -d [@var{pattern}]
-Delete all generations except the current one.  Note that the zeroth
-generation is never deleted.
+When @var{pattern} is omitted, delete all generations except the current
+one.
 
 This command accepts the same patterns as @option{--list-generations}.
-When @var{pattern} is specified, delete the matching generations.  If
-the current generation matches, it is deleted atomically, i.e., by
-switching to the previous available generation.
+When @var{pattern} is specified, delete the matching generations.  When
+@var{pattern} specifies a duration, generations @emph{older} than the
+specified duration match.  For instance, @code{--delete-generations=1m}
+deletes generations that are more than one month old.
+
+If the current generation matches, it is deleted atomically---i.e., by
+switching to the previous available generation.  Note that the zeroth
+generation is never deleted.
 
 @end table