bug#6324: 24.0.50; vc modeline is not updated when buffer mode is changed

[Alex Harsanyi <harsanyi@mac.com>]

There is a problem with updating the mode line for VC operations when
the buffer mode is changed.  The easiest way to reproduce using
mercurial is as follows (the problem should exist for every version
control):

The VC mode uses the `mode-line-hook' variable to update the mode line.
This variable is set up in `vc-find-file-hook' and it is made buffer
local.  The problem is that this variable should have a 'permanent-local
property set.  If it does not, changing the buffer mode runs
`kill-all-local-variables' which removes it from the buffer.  From than
on, the VC mode line will no longer be updated when the file is checked
in, etc.

I found out this problem when I opened a text file (.txt extension) and
switched the major mode to rst-mode.

Here is a patch that corrects the problem:

--- lisp/vc-hooks.el	2010-04-09 17:19:53 +0000
+++ lisp/vc-hooks.el	2010-06-01 12:24:32 +0000
@@ -877,6 +877,7 @@ current, and kill the buffer that visits
   (when buffer-file-name
     (vc-file-clearprops buffer-file-name)
     (add-hook 'mode-line-hook 'vc-mode-line nil t)
+    (put 'mode-line-hook 'permanent-local t)
     (let (backend)
       (cond
        ((setq backend (with-demoted-errors (vc-backend buffer-file-name)))


Here are some detailed steps to reproduce the problem:

1/  First create a test repository:

    $ mkdir test.hg
    $ cd test.hg/
    $ hg init
    $ echo "Hello World" > hello.txt
    $ hg add hello.txt 
    $ hg commit -m "Initial" hello.txt 

2/  In emacs, open the file hello.txt, than execute "M-x normal-mode RET"

3/ Note the VC version in the mode line, modify the file and check it
in.

4/ Notice that the VC version in the mode line does not change.




In GNU Emacs 24.0.50.1 (x86_64-apple-darwin10.3.0, NS apple-appkit-1038.29)
of 2010-06-01 on karinji-2.local
Windowing system distributor `Apple', version 10.3.1038
configured using `configure  '--with-ns''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_AU.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Text

Minor modes in effect:
  show-paren-mode: t
  global-cwarn-mode: t
  display-time-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  size-indication-mode: t
  line-number-mode: t

Recent input:
<help-echo> <help-echo> C-x 1 C-x C-f P r o <tab> E 
M <tab> t e <tab> <return> <down> <return> M-x n o 
r m <tab> m o <tab> <return> <down> <down> <up> <down> 
<return> k j f d s <return> C-x C-s C-x v v C-c C-c 
C-x v l C-x 0 M-x r e n v e <backspace> <backspace> 
<backspace> v e <tab> r <tab> t <tab> <return> y e 
s <return> M-x r e p o <tab> r <tab> <return>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Loading vc-hg...done
byte-code: End of buffer
Saving file /Users/haral/Projects/Emacs/test.hg/hello.txt...
Wrote /Users/haral/Projects/Emacs/test.hg/hello.txt
Mark set
Press C-c C-c when you are done editing.
Enter a change comment.  Type C-c C-c when done
Checking in /Users/haral/Projects/Emacs/test.hg/hello.txt...done
Making completion list... [3 times]

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