services: nginx: Get the Shepherd to respawn NGINX.

* gnu/services/web.scm (nginx-shepherd-service): Change 'start' (that is, all
actions that don't send a signal to the master process) to return the PID.
Wait until the PID file is created and contains an integer because it might be
created after the parent process exits.

gnu/services/web.scm

@@ -599,19 +599,33 @@ of index files."
                 <nginx-configuration>
                 (nginx file run-directory)
    (let* ((nginx-binary (file-append nginx "/sbin/nginx"))
+          (pid-file (in-vicinity run-directory "pid"))
           (nginx-action
            (lambda args
              #~(lambda _
                  (invoke #$nginx-binary "-c"
                          #$(or file
                                (default-nginx-config config))
-                         #$@args)))))
+                         #$@args)
+                 (match '#$args
+                   (("-s" . _) #t)
+                   (_
+                    (let loop ((duration 0))
+                      ;; https://bugs.launchpad.net/ubuntu/+source/nginx/+bug/1581864/comments/7
+                      (sleep duration)
+                      (if (file-exists? #$pid-file)
+                          (let ((pid (call-with-input-file #$pid-file read)))
+                            ;; it could be #<eof>
+                            (if (integer? pid) pid (loop 1)))
+                          (loop 1)))))))))
 
      ;; TODO: Add 'reload' action.
      (list (shepherd-service
             (provision '(nginx))
             (documentation "Run the nginx daemon.")
             (requirement '(user-processes loopback))
+            (modules `((ice-9 match)
+                       ,@%default-modules))
             (start (nginx-action "-p" run-directory))
             (stop (nginx-action "-s" "stop")))))))