substitute-binary: Gracefully exit upon networking errors.

Suggested by Andreas Enge <andreas@enge.fr>.

* guix/scripts/substitute-binary.scm (with-networking): New macro.
  (guix-substitute-binary): Wrap the body in `with-networking'.

guix/scripts/substitute-binary.scm

@@ -361,6 +361,19 @@ (define %cache-url
   (or (getenv "GUIX_BINARY_SUBSTITUTE_URL")
       "http://hydra.gnu.org"))
 
+(define-syntax with-networking
+  (syntax-rules ()
+    "Catch DNS lookup errors and gracefully exit."
+    ;; Note: no attempt is made to catch other networking errors, because DNS
+    ;; lookup errors are typically the first one, and because other errors are
+    ;; a subset of `system-error', which is harder to filter.
+    ((_ exp ...)
+     (catch 'getaddrinfo-error
+       (lambda () exp ...)
+       (lambda (key error)
+         (leave (_ "host name lookup error: ~a~%")
+                (gai-strerror error)))))))
+
 
 ;;;
 ;;; Entry point.
@@ -370,6 +383,7 @@ (define (guix-substitute-binary . args)
   "Implement the build daemon's substituter protocol."
   (mkdir-p %narinfo-cache-directory)
   (maybe-remove-expired-cached-narinfo)
+  (with-networking
    (match args
      (("--query")
       (let ((cache (delay (open-cache %cache-url))))
@@ -441,6 +455,6 @@ (define (guix-substitute-binary . args)
           (restore-file input destination)
           (every (compose zero? cdr waitpid) pids))))
      (("--version")
-     (show-version-and-exit "guix substitute-binary"))))
+      (show-version-and-exit "guix substitute-binary")))))
 
 ;;; substitute-binary.scm ends here