syscalls: Add 'all-network-interfaces'.

* guix/build/syscalls.scm (network-interfaces): Update docstring.
  (%interface-line): New variable.
  (all-network-interfaces): New procedure.
* tests/syscalls.scm ("all-network-interfaces"): New test.
  ("network-interfaces"): Change to make sure the result is a subset
  of (all-network-interfaces).

guix/build/syscalls.scm

@@ -21,6 +21,7 @@
   #:use-module (rnrs bytevectors)
   #:use-module (srfi srfi-1)
   #:use-module (ice-9 rdelim)
+  #:use-module (ice-9 regex)
   #:use-module (ice-9 match)
   #:use-module (ice-9 ftw)
   #:export (errno
@@ -35,6 +36,7 @@
             IFF_UP
             IFF_BROADCAST
             IFF_LOOPBACK
+            all-network-interfaces
             network-interfaces
             network-interface-flags
             loopback-network-interface?))
@@ -244,7 +246,8 @@ most LEN bytes from BV."
                    result))))))
 
 (define* (network-interfaces #:optional sock)
-  "Return the list of existing network interfaces."
+  "Return the list of existing network interfaces.  This is typically limited
+to interfaces that are currently up."
   (let* ((close? (not sock))
          (sock   (or sock (socket SOCK_STREAM AF_INET 0)))
          (len    (* ifreq-struct-size 10))
@@ -264,6 +267,26 @@ most LEN bytes from BV."
                (list (strerror err))
                (list err)))))
 
+(define %interface-line
+  ;; Regexp matching an interface line in Linux's /proc/net/dev.
+  (make-regexp "^[[:blank:]]*([[:alnum:]]+): .*$"))
+
+(define (all-network-interfaces)
+  "Return all the registered network interfaces, including those that are not
+up."
+  (call-with-input-file "/proc/net/dev"           ;XXX: Linux-specific
+    (lambda (port)
+      (let loop ((interfaces '()))
+        (let ((line (read-line port)))
+          (cond ((eof-object? line)
+                 (reverse interfaces))
+                ((regexp-exec %interface-line line)
+                 =>
+                 (lambda (match)
+                   (loop (cons (match:substring match 1) interfaces))))
+                (else
+                 (loop interfaces))))))))
+
 (define (network-interface-flags socket name)
   "Return a number that is the bit-wise or of 'IFF*' flags for network
 interface NAME."

tests/syscalls.scm

@@ -18,6 +18,7 @@
 
 (define-module (test-syscalls)
   #:use-module (guix build syscalls)
+  #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-64)
   #:use-module (ice-9 match))
 
@@ -43,10 +44,15 @@
       ;; Both return values have been encountered in the wild.
       (memv (system-error-errno args) (list EPERM ENOENT)))))
 
+(test-assert "all-network-interfaces"
+  (match (all-network-interfaces)
+    (((? string? names) ..1)
+     (member "lo" names))))
+
 (test-assert "network-interfaces"
   (match (network-interfaces)
     (((? string? names) ..1)
-     (member "lo" names))))
+     (lset<= string=? names (all-network-interfaces)))))
 
 (test-assert "network-interface-flags"
   (let* ((sock  (socket SOCK_STREAM AF_INET 0))