pack: Improve naming of the packs store file names.

Instead of just naming them by their pack type, add information from the
package(s) they contain to make it easier to differentiate them.

* guix/scripts/pack.scm (define-with-source): New macro.
(manifest->friendly-name): Extract procedure from ...
(docker-image): ... here, now defined via the above macro.  Adjust REPOSITORY
argument value accordingly.
(guix-pack): Derive NAME using MANIFEST->FRIENDLY-NAME.

guix/scripts/pack.scm

@@ -172,6 +172,28 @@ dependencies are registered."
   (computed-file "store-database" build
                  #:options `(#:references-graphs ,(zip labels items))))
 
+(define-syntax-rule (define-with-source (variable args ...) body body* ...)
+  "Bind VARIABLE to a procedure accepting ARGS defined as BODY, also setting
+its source property."
+  (begin
+    (define (variable args ...)
+      body body* ...)
+    (eval-when (load eval)
+      (set-procedure-property! variable 'source
+                               '(define (variable args ...) body body* ...)))))
+
+(define-with-source (manifest->friendly-name manifest)
+  "Return a friendly name computed from the entries in MANIFEST, a
+<manifest> object."
+  (let loop ((names (map manifest-entry-name
+                         (manifest-entries manifest))))
+    (define str (string-join names "-"))
+    (if (< (string-length str) 40)
+        str
+        (match names
+          ((_) str)
+          ((names ... _) (loop names))))))
+
 
 ;;;
 ;;; Tarball format.
@@ -540,7 +562,7 @@ the image."
          (file-append (store-database (list profile))
                       "/db/db.sqlite")))
 
-  (define defmod 'define-module)                  ;trick Geiser
+  (define defmod 'define-module)        ;trick Geiser
 
   (define build
     ;; Guile-JSON and Guile-Gcrypt are required by (guix docker).
@@ -558,6 +580,8 @@ the image."
                          (srfi srfi-1) (srfi srfi-19)
                          (ice-9 match))
 
+            #$(procedure-source manifest->friendly-name)
+
             (define environment
               (map (match-lambda
                      ((spec . value)
@@ -581,19 +605,6 @@ the image."
               `((directory "/tmp" ,(getuid) ,(getgid) #o1777)
                 ,@(append-map symlink->directives '#$symlinks)))
 
-            (define tag
-              ;; Compute a meaningful "repository" name, which will show up in
-              ;; the output of "docker images".
-              (let ((manifest (profile-manifest #$profile)))
-                (let loop ((names (map manifest-entry-name
-                                       (manifest-entries manifest))))
-                  (define str (string-join names "-"))
-                  (if (< (string-length str) 40)
-                      str
-                      (match names
-                        ((_) str)
-                        ((names ... _) (loop names))))))) ;drop one entry
-
             (setenv "PATH" #+(file-append archiver "/bin"))
 
             (build-docker-image #$output
@@ -601,7 +612,8 @@ the image."
                                      (call-with-input-file "profile"
                                        read-reference-graph))
                                 #$profile
-                                #:repository tag
+                                #:repository (manifest->friendly-name
+                                              (profile-manifest #$profile))
                                 #:database #+database
                                 #:system (or #$target %host-type)
                                 #:environment environment
@@ -1209,8 +1221,6 @@ Create a bundle of PACKAGE.\n"))
                                        manifest)
                                       manifest)))
                    (pack-format (assoc-ref opts 'format))
-                   (name        (string-append (symbol->string pack-format)
-                                               "-pack"))
                    (target      (assoc-ref opts 'target))
                    (bootstrap?  (assoc-ref opts 'bootstrap?))
                    (compressor  (if bootstrap?
@@ -1244,7 +1254,10 @@ Create a bundle of PACKAGE.\n"))
                                     (hooks (if bootstrap?
                                                '()
                                                %default-profile-hooks))
-                                    (locales? (not bootstrap?)))))
+                                    (locales? (not bootstrap?))))
+                   (name (string-append (manifest->friendly-name manifest)
+                                        "-" (symbol->string pack-format)
+                                        "-pack")))
               (define (lookup-package package)
                 (manifest-lookup manifest (manifest-pattern (name package))))