syscalls: Add 'mount-points'.

* guix/build/syscalls.scm (mount-points): New procedure.
* tests/syscalls.scm ("mount-points"): New test.

guix/build/syscalls.scm

@@ -31,6 +31,7 @@ (define-module (guix build syscalls)
             MS_MOVE
             mount
             umount
+            mount-points
             swapon
             swapoff
             processes
@@ -166,6 +167,18 @@ (define umount
         (when update-mtab?
           (remove-from-mtab target))))))
 
+(define (mount-points)
+  "Return the mounts points for currently mounted file systems."
+  (call-with-input-file "/proc/mounts"
+    (lambda (port)
+      (let loop ((result '()))
+        (let ((line (read-line port)))
+          (if (eof-object? line)
+              (reverse result)
+              (match (string-tokenize line)
+                ((source mount-point _ ...)
+                 (loop (cons mount-point result))))))))))
+
 (define swapon
   (let* ((ptr  (dynamic-func "swapon" (dynamic-link)))
          (proc (pointer->procedure int ptr (list '* int))))

tests/syscalls.scm

@@ -44,6 +44,9 @@ (define-module (test-syscalls)
       ;; Both return values have been encountered in the wild.
       (memv (system-error-errno args) (list EPERM ENOENT)))))
 
+(test-assert "mount-points"
+  (member "/" (mount-points)))
+
 (test-assert "swapon, ENOENT/EPERM"
   (catch 'system-error
     (lambda ()