syscalls: Add network-interface-running?

* guix/build/syscalls.scm (network-interface-running?): New variable.
Export it.
* tests/syscalls.scm: Add test.

Co-authored-by: John Darrington <jmd@gnu.org>

guix/build/syscalls.scm

@@ -92,6 +92,7 @@
             all-network-interface-names
             network-interface-names
             network-interface-netmask
+            network-interface-running?
             loopback-network-interface?
             network-interface-address
             set-network-interface-netmask
@@ -1156,6 +1157,7 @@ bytes."
 (define-as-needed IFF_UP #x1)                     ;Interface is up
 (define-as-needed IFF_BROADCAST #x2)              ;Broadcast address valid.
 (define-as-needed IFF_LOOPBACK #x8)               ;Is a loopback net.
+(define-as-needed IFF_RUNNING #x40)               ;interface RFC2863 OPER_UP
 
 (define IF_NAMESIZE 16)                           ;maximum interface name size
 
@@ -1330,6 +1332,13 @@ interface NAME."
     (close-port sock)
     (not (zero? (logand flags IFF_LOOPBACK)))))
 
+(define (network-interface-running? name)
+  "Return true if NAME designates a running network interface."
+  (let* ((sock  (socket SOCK_STREAM AF_INET 0))
+         (flags (network-interface-flags sock name)))
+    (close-port sock)
+    (not (zero? (logand flags IFF_RUNNING)))))
+
 (define-as-needed (set-network-interface-flags socket name flags)
   "Set the flag of network interface NAME to FLAGS."
   (let ((req (make-bytevector ifreq-struct-size)))

tests/syscalls.scm

@@ -361,6 +361,16 @@
          (lambda args
            (system-error-errno args)))))
 
+(test-equal "loopback-network-interface-running?"
+  ENODEV
+  (and (network-interface-running? "lo")
+       (catch 'system-error
+         (lambda ()
+           (network-interface-running? "nonexistent")
+           #f)
+         (lambda args
+           (system-error-errno args)))))
+
 (test-skip (if (zero? (getuid)) 1 0))
 (test-assert "set-network-interface-flags"
   (let ((sock (socket AF_INET SOCK_STREAM 0)))