environment: Turn "lo" up in network-less containers.

This is a followup to 0f53c801b9.

* guix/scripts/environment.scm (launch-environment/container): Add call
to 'set-network-interface-up'.
* tests/guix-environment-container.sh: Add test.

guix/scripts/environment.scm

@@ -34,6 +34,7 @@ (define-module (guix scripts environment)
   #:use-module (guix scripts build)
   #:use-module (gnu build linux-container)
   #:use-module (gnu build accounts)
+  #:use-module ((guix build syscalls) #:select (set-network-interface-up))
   #:use-module (gnu system linux-container)
   #:use-module (gnu system file-systems)
   #:use-module (gnu packages)
@@ -554,7 +555,10 @@ (define (optional-mapping->fs mapping)
               ;; to resolve "localhost".
               (call-with-output-file "/etc/hosts"
                 (lambda (port)
-                  (display "127.0.0.1 localhost\n" port))))
+                  (display "127.0.0.1 localhost\n" port)))
+
+              ;; Allow local AF_INET communications.
+              (set-network-interface-up "lo"))
 
             ;; For convenience, start in the user's current working
             ;; directory or, if unmapped, the home directory.

tests/guix-environment-container.sh

@@ -48,6 +48,17 @@ fi
 guix environment --container --ad-hoc --bootstrap guile-bootstrap \
      -- guile -c '(exit (pair? (getaddrinfo "localhost" "80")))'
 
+# We should get ECONNREFUSED, not ENETUNREACH, which would indicate that "lo"
+# is down.
+guix environment --container --ad-hoc --bootstrap guile-bootstrap \
+     -- guile -c "(exit (= ECONNREFUSED
+  (catch 'system-error
+    (lambda ()
+      (let ((sock (socket AF_INET SOCK_STREAM 0)))
+        (connect sock AF_INET INADDR_LOOPBACK 12345)))
+    (lambda args
+      (pk 'errno (system-error-errno args))))))"
+
 # Make sure '--preserve' is honored.
 result="`FOOBAR=42; export FOOBAR; guix environment -C --ad-hoc --bootstrap \
    guile-bootstrap -E ^FOO -- guile -c '(display (getenv \"FOOBAR\"))'`"