07/08: services: virtual-terminal: Write to "default_utf8" only if necessary.

[Ludovic Courtès]

civodul pushed a commit to branch master
in repository guix.

commit 09b7300c01a8e7100467c6caae9b2c8d8e673971
Author: Ludovic Courtès <address@hidden>
Date:   Wed Sep 26 23:01:33 2018 +0200

    services: virtual-terminal: Write to "default_utf8" only if necessary.
    
    Fixes a bug in containers whereby 'virtual-terminal' would always fail
    to start because writing to /sys/…/default_utf8 would fail with EROFS.
    
    * gnu/services/base.scm (virtual-terminal-service-type): Read from
    "default_utf8" before attempting to write to it.
---
 gnu/services/base.scm | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/gnu/services/base.scm b/gnu/services/base.scm
index 9dfabd9..47c7d8b 100644
--- a/gnu/services/base.scm
+++ b/gnu/services/base.scm
@@ -685,17 +685,20 @@ to add @var{device} to the kernel's entropy pool.  The 
service will fail if
   (shepherd-service-type
    'virtual-terminal
    (lambda (utf8?)
-     (shepherd-service
-      (documentation "Set virtual terminals in UTF-8 module.")
-      (provision '(virtual-terminal))
-      (requirement '(root-file-system))
-      (start #~(lambda _
-                 (call-with-output-file
-                     "/sys/module/vt/parameters/default_utf8"
-                   (lambda (port)
-                     (display 1 port)))
-                 #t))
-      (stop #~(const #f))))
+     (let ((knob "/sys/module/vt/parameters/default_utf8"))
+       (shepherd-service
+        (documentation "Set virtual terminals in UTF-8 module.")
+        (provision '(virtual-terminal))
+        (requirement '(root-file-system))
+        (start #~(lambda _
+                   ;; In containers /sys is read-only so don't insist on
+                   ;; writing to this file.
+                   (unless (= 1 (call-with-input-file #$knob read))
+                     (call-with-output-file #$knob
+                       (lambda (port)
+                         (display 1 port))))
+                   #t))
+        (stop #~(const #f)))))
    #t))                                           ;default to UTF-8
 
 (define console-keymap-service-type