linux-container: Work around EBADF errors upon exit.

Typically 'read-pid-file/container' would fail when starting services in
containers such as BitlBee.

* gnu/build/linux-container.scm (call-with-clean-exit): Use
'primitive-_exit' instead of 'primitive-exit'.
(container-excursion*): Close OUT.

gnu/build/linux-container.scm

 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2015 David Thompson <davet@gnu.org>
-;;; Copyright © 2017 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2017, 2018 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -61,9 +61,14 @@ (define (call-with-clean-exit thunk)
     (const #t)
     (lambda ()
       (thunk)
-      (primitive-exit 0))
+
+      ;; XXX: Somehow we sometimes get EBADF from write(2) or close(2) upon
+      ;; exit (coming from fd finalizers) when used by the Shepherd.  To work
+      ;; around that, exit forcefully so fd finalizers don't have a chance to
+      ;; run and fail.
+      (primitive-_exit 0))
     (lambda ()
-      (primitive-exit 1))))
+      (primitive-_exit 1))))
 
 (define (purify-environment)
   "Unset all environment variables."
@@ -335,7 +340,8 @@ (define (container-excursion* pid thunk)
      (match (container-excursion pid
               (lambda ()
                 (close-port in)
-                (write (thunk) out)))
+                (write (thunk) out)
+                (close-port out)))
        (0
         (close-port out)
         (let ((result (read in)))