publish: Add '--nar-path'.

* guix/scripts/publish.scm (show-help, %options): Add '--nar-path'.
(%default-options): Add 'nar-path'.
(guix-publish): Honor it.

doc/guix.texi

@@ -6523,6 +6523,14 @@ This allows the user's Guix to keep substitute information in cache for
 guarantee that the store items it provides will indeed remain available
 for as long as @var{ttl}.
 
+@item --nar-path=@var{path}
+Use @var{path} as the prefix for the URLs of ``nar'' files
+(@pxref{Invoking guix archive, normalized archives}).
+
+By default, nars are served at a URL such as
+@code{/nar/gzip/@dots{}-coreutils-8.25}.  This option allows you to
+change the @code{/nar} part to @var{path}.
+
 @item --public-key=@var{file}
 @itemx --private-key=@var{file}
 Use the specific @var{file}s as the public/private key pair used to sign

guix/scripts/publish.scm

@@ -71,6 +71,8 @@ (define (show-help)
                          compress archives at LEVEL"))
   (display (_ "
       --ttl=TTL          announce narinfos can be cached for TTL seconds"))
+  (display (_ "
+      --nar-path=PATH    use PATH as the prefix for nar URLs"))
   (display (_ "
       --public-key=FILE  use FILE as the public key for signatures"))
   (display (_ "
@@ -152,6 +154,9 @@ (define %options
                       (leave (_ "~a: invalid duration~%") arg))
                     (alist-cons 'narinfo-ttl (time-second duration)
                                 result))))
+        (option '("nar-path") #t #f
+                (lambda (opt name arg result)
+                  (alist-cons 'nar-path arg result)))
         (option '("public-key") #t #f
                 (lambda (opt name arg result)
                   (alist-cons 'public-key-file arg result)))
@@ -167,6 +172,9 @@ (define %options
 (define %default-options
   `((port . 8080)
 
+    ;; By default, serve nars under "/nar".
+    (nar-path . "nar")
+
     (public-key-file . ,%public-key-file)
     (private-key-file . ,%private-key-file)
 
@@ -589,6 +597,7 @@ (define (guix-publish . args)
                                            (sockaddr:addr addr)
                                            port)))
            (socket  (open-server-socket address))
+           (nar-path  (assoc-ref opts 'nar-path))
            (repl-port (assoc-ref opts 'repl))
 
            ;; Read the key right away so that (1) we fail early on if we can't
@@ -615,5 +624,6 @@ (define (guix-publish . args)
           (repl:spawn-server (repl:make-tcp-server-socket #:port repl-port)))
         (with-store store
           (run-publish-server socket store
+                              #:nar-path nar-path
                               #:compression compression
                               #:narinfo-ttl ttl))))))