guix gc: Add '--delete-generations'.

* guix/scripts/gc.scm (show-help, %options): Add
'--delete-generations'.  Change '--delete' shorthand to '-D'.
(delete-old-generations): New procedure.
(guix-gc)[delete-generations]: New procedure.
Call it when ACTION is 'collect-garbage' and OPTS contains
'delete-generations.
* doc/guix.texi (Invoking guix gc): Document it.

doc/guix.texi

@@ -3438,8 +3438,22 @@ as @code{500MiB}, as described above.
 When @var{free} or more is already available in @file{/gnu/store}, do
 nothing and exit immediately.
 
+@item --delete-generations[=@var{duration}]
+@itemx -d [@var{duration}]
+Before starting the garbage collection process, delete all the generations
+older than @var{duration}, for all the user profiles; when run as root, this
+applies to all the profiles @emph{of all the users}.
+
+For example, this command deletes all the generations of all your profiles
+that are older than 2 months (except generations that are current), and then
+proceeds to free space until at least 10 GiB are available:
+
+@example
+guix gc -d 2m -F 10G
+@end example
+
 @item --delete
-@itemx -d
+@itemx -D
 Attempt to delete all the store files and directories specified as
 arguments.  This fails if some of the files are not in the store, or if
 they are still live.

guix/scripts/gc.scm

@@ -22,6 +22,8 @@
   #:use-module (guix store)
   #:use-module (guix store roots)
   #:autoload   (guix build syscalls) (free-disk-space)
+  #:autoload   (guix profiles) (generation-profile)
+  #:autoload   (guix scripts package) (delete-generations)
   #:use-module (ice-9 match)
   #:use-module (ice-9 regex)
   #:use-module (srfi srfi-1)
@@ -48,7 +50,10 @@ Invoke the garbage collector.\n"))
   (display (G_ "
   -F, --free-space=FREE  attempt to reach FREE available space in the store"))
   (display (G_ "
-  -d, --delete           attempt to delete PATHS"))
+  -d, --delete-generations[=PATTERN]
+                         delete profile generations matching PATTERN"))
+  (display (G_ "
+  -D, --delete           attempt to delete PATHS"))
   (display (G_ "
       --list-roots       list the user's garbage collector roots"))
   (display (G_ "
@@ -98,6 +103,16 @@ Invoke the garbage collector.\n"))
             lst)
           '()))))
 
+(define (delete-old-generations store profile pattern)
+  "Remove the generations of PROFILE that match PATTERN, a duration pattern.
+Do nothing if none matches."
+  (let* ((current (generation-number profile))
+         (numbers (matching-generations pattern profile
+                                        #:duration-relation >)))
+
+    ;; Make sure we don't inadvertently remove the current generation.
+    (delete-generations store profile (delv current numbers))))
+
 (define %options
   ;; Specification of the command-line options.
   (list (option '(#\h "help") #f #f
@@ -123,10 +138,25 @@ Invoke the garbage collector.\n"))
         (option '(#\F "free-space") #t #f
                 (lambda (opt name arg result)
                   (alist-cons 'free-space (size->number arg) result)))
-        (option '(#\d "delete") #f #f
+        (option '(#\D "delete") #f #f            ;used to be '-d' (lower case)
                 (lambda (opt name arg result)
                   (alist-cons 'action 'delete
                               (alist-delete 'action result))))
+        (option '(#\d "delete-generations") #f #t
+                (lambda (opt name arg result)
+                  (if (and arg (store-path? arg))
+                      (begin
+                        (warning (G_ "'-d' as an alias for '--delete' \
+is deprecated; use '-D'~%"))
+                        `((action . delete)
+                          (argument . ,arg)
+                          (alist-delete 'action result)))
+                      (begin
+                        (when (and arg (not (string->duration arg)))
+                          (leave (G_ "~s does not denote a duration~%")
+                                 arg))
+                        (alist-cons 'delete-generations (or arg "")
+                                    result)))))
         (option '("optimize") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'action 'optimize
@@ -212,6 +242,14 @@ Invoke the garbage collector.\n"))
             (info (G_ "freeing ~h MiBs~%") (/ to-free 1024. 1024.))
             (collect-garbage store to-free)))))
 
+  (define (delete-generations store pattern)
+    ;; Delete the generations matching PATTERN of all the user's profiles.
+    (let ((profiles (delete-duplicates
+                     (filter-map generation-profile (gc-roots)))))
+      (for-each (lambda (profile)
+                  (delete-old-generations store profile pattern))
+                profiles)))
+
   (define (list-roots)
     ;; List all the user-owned GC roots.
     (let ((roots (filter (if (zero? (getuid)) (const #t) user-owned?)
@@ -245,6 +283,10 @@ Invoke the garbage collector.\n"))
          (assert-no-extra-arguments)
          (let ((min-freed  (assoc-ref opts 'min-freed))
                (free-space (assoc-ref opts 'free-space)))
+           (match (assoc-ref opts 'delete-generations)
+             (#f #t)
+             ((? string? pattern)
+              (delete-generations store pattern)))
            (cond
             (free-space
              (ensure-free-space store free-space))