publish: Support systemd-style socket activation.

* guix/scripts/publish.scm (systemd-socket): New procedure.
(guix-publish): Add 'style' variable.  Adjust startup message depending
on whether STYLE is 'systemd.
* doc/guix.texi (Invoking guix publish): Mention socket activation.

doc/guix.texi

@@ -14042,6 +14042,11 @@ spawn an HTTP server on port 8080:
 guix publish
 @end example
 
+@cindex socket activation, for @command{guix publish}
+@command{guix publish} can also be started following the systemd
+``socket activation'' protocol (@pxref{Service De- and Constructors,
+@code{make-systemd-constructor},, shepherd, The GNU Shepherd Manual}).
+
 Once a publishing server has been authorized, the daemon may download
 substitutes from it.  @xref{Getting Substitutes from Other Servers}.
 

guix/scripts/publish.scm

@@ -1235,6 +1235,23 @@ headers."
     (bind sock address)
     sock))
 
+(define (systemd-socket)
+  "If this program is being spawned through systemd-style \"socket
+activation\", whereby the listening socket is passed as file descriptor 3,
+return the corresponding socket.  Otherwise return #f."
+  (and (equal? (and=> (getenv "LISTEN_PID") string->number)
+               (getpid))
+       (match (getenv "LISTEN_FDS")
+         ((= string->number 1)
+          (let ((sock (fdopen 3 "r+0")))
+            (configure-socket sock)
+            sock))
+         ((= string->number (? integer? n))
+          (leave (G_ "~a: unexpected number of startup file descriptors")
+                 n))
+         (_
+          #f))))
+
 (define (gather-user-privileges user)
   "Switch to the identity of USER, a user name."
   (catch 'misc-error
@@ -1280,7 +1297,12 @@ headers."
                       (make-socket-address (sockaddr:fam addr)
                                            (sockaddr:addr addr)
                                            port)))
-           (socket  (open-server-socket address))
+           (socket style (match (systemd-socket)
+                           (#f
+                            (values (open-server-socket address)
+                                    'normal))
+                           (socket
+                            (values socket 'systemd))))
            (nar-path  (assoc-ref opts 'nar-path))
            (repl-port (assoc-ref opts 'repl))
            (cache     (assoc-ref opts 'cache))
@@ -1305,10 +1327,12 @@ consider using the '--user' option!~%")))
                      (cache-bypass-threshold
                       (or (assoc-ref opts 'cache-bypass-threshold)
                           (cache-bypass-threshold))))
-        (info (G_ "publishing ~a on ~a, port ~d~%")
+        (if (eq? style 'systemd)
-              %store-directory
+            (info (G_ "publishing (started via socket activation)~%"))
-              (inet-ntop (sockaddr:fam address) (sockaddr:addr address))
+            (info (G_ "publishing ~a on ~a, port ~d~%")
-              (sockaddr:port address))
+                  %store-directory
+                  (inet-ntop (sockaddr:fam address) (sockaddr:addr address))
+                  (sockaddr:port address)))
 
         (for-each (lambda (compression)
                     (info (G_ "using '~a' compression method, level ~a~%")