services: nginx: Ensure ‘stop’ is synchronous.

Fixes guix/maintenance#17.

Fixes a bug whereby ‘nginx -s stop’ might return before the main nginx process
has actually stopped, which in turn would lead ‘herd restart nginx’ to hang.

* gnu/services/web.scm (nginx-shepherd-service): Change ‘stop’ to use
‘make-kill-destructor’.

Reported-by: Maxim Cournoyer <maxim@guixotic.coop>
Change-Id: I69e8626aab07f806ddc8b1d09d9f0e052a85cf47

gnu/services/web.scm

@@ -907,7 +907,12 @@ of index files."
             (modules `((ice-9 match)
                        ,@%default-modules))
             (start (nginx-action "-p" run-directory))
-            (stop (nginx-action "-s" "stop"))
+
+            ;; Instead of invoking "nginx -s stop", use
+            ;; 'make-kill-destructor', which waits for the main process to
+            ;; actually terminate.
+            (stop #~(make-kill-destructor))
+
             (actions
               (list
                (shepherd-configuration-action config-file)