linux-container: Add 'container-excursion*'.

* gnu/build/linux-container.scm (container-excursion*): New procedure.
* tests/containers.scm ("container-excursion*")
("container-excursion*, same namespaces"): New tests.

gnu/build/linux-container.scm

@@ -1,5 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2015 David Thompson <davet@gnu.org>
+;;; Copyright © 2017 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -32,7 +33,8 @@ (define-module (gnu build linux-container)
             %namespaces
             run-container
             call-with-container
-            container-excursion))
+            container-excursion
+            container-excursion*))
 
 (define (user-namespace-supported?)
   "Return #t if user namespaces are supported on this system."
@@ -326,3 +328,21 @@ (define (namespace-file pid namespace)
      (match (waitpid pid)
        ((_ . status)
         (status:exit-val status))))))
+
+(define (container-excursion* pid thunk)
+  "Like 'container-excursion', but return the return value of THUNK."
+  (match (pipe)
+    ((in . out)
+     (match (container-excursion pid
+              (lambda ()
+                (close-port in)
+                (write (thunk) out)))
+       (0
+        (close-port out)
+        (let ((result (read in)))
+          (close-port in)
+          result))
+       (_                                         ;maybe PID died already
+        (close-port out)
+        (close-port in)
+        #f)))))

tests/containers.scm

@@ -180,4 +180,31 @@ (define (namespaces pid)
     (lambda ()
       (primitive-exit 42))))
 
+(skip-if-unsupported)
+(test-assert "container-excursion*"
+  (call-with-temporary-directory
+   (lambda (root)
+     (define (namespaces pid)
+       (let ((pid (number->string pid)))
+         (map (lambda (ns)
+                (readlink (string-append "/proc/" pid "/ns/" ns)))
+              '("user" "ipc" "uts" "net" "pid" "mnt"))))
+
+     (let* ((pid    (run-container root '()
+                                   %namespaces 1
+                                   (lambda ()
+                                     (sleep 100))))
+            (result (container-excursion* pid
+                      (lambda ()
+                        (namespaces 1)))))
+       (kill pid SIGKILL)
+       (equal? result (namespaces pid))))))
+
+(skip-if-unsupported)
+(test-equal "container-excursion*, same namespaces"
+  42
+  (container-excursion* (getpid)
+    (lambda ()
+      (* 6 7))))
+
 (test-end)