services: anonip: Bail out when the input is not a FIFO.

* gnu/services/web.scm (anonip-shepherd-service)[start]: Accept zero
arguments.  Define 'spawn'.  Print a message and return #f when #$INPUT
does not denote a FIFO.

gnu/services/web.scm

@@ -1438,10 +1438,8 @@ (define (anonip-shepherd-service config)
       (documentation
        "Anonimyze the given log file location with anonip.")
       (start
-       #~(lambda _
-           (unless (file-exists? #$input)
-             (mknod #$input 'fifo #o600 0))
-           (let ((pid
+       #~(lambda ()
+           (define (spawn)
              (fork+exec-command
               (append
                (list #$(file-append (anonip-configuration-anonip config)
@@ -1462,8 +1460,18 @@ (define (anonip-shepherd-service config)
               #:environment-variables
               (list (string-append "GUIX_LOCPATH=" #$glibc-utf8-locales
                                    "/lib/locale")
-                         "LC_ALL=en_US.utf8"))))
-             pid)))
+                    "LC_ALL=en_US.utf8")))
+
+           (let ((stat (stat #$input #f)))
+             (cond ((not stat)
+                    (mknod #$input 'fifo #o600 0)
+                    (spawn))
+                   ((eq? 'fifo (stat:type stat))
+                    (spawn))
+                   (else
+                    (format #t "'~a' is not a FIFO; bailing out~%"
+                            #$input)
+                    #f)))))
       (stop #~(make-kill-destructor))))))
 
 (define anonip-service-type