auto-revert-tail-mode misses some appends in Emacs 22.1

[Joe Wells]

To reproduce the bug:

1. Visit a log file that is actively being extended.
2. Wait for the file to grow.
3. Enable auto-revert-tail-mode in that file's buffer.
4. Watch auto-revert-tail-mode only grab appends that happen _after_
   it is enabled.

This behavior is problematic because the user is likely to expect that
step 3 above will grab any appends that have been written to disk and
are not yet reflected in the buffer.  If the user modifies and saves
the file after step 4, the appends that happened during step 2 will be
lost.

I suggest that it would be better if enabling auto-revert-tail-mode
noticed if the file on disk has changed since Emacs loaded the
contents into the buffer and asked the user what to do.

At the very least, there should be a big warning in the documentation
about this problem.

Joe

----------------------------------------------------------------------
In GNU Emacs 22.1.1 (i686-pc-linux-gnu, GTK+ Version 2.8.20)
 of 2007-06-27 on artemis
Windowing system distributor `The X.Org Foundation', version 11.0.70000000
configured using `configure  '--prefix=/home/jbw/local2' '--enable-debug' 
'--disable-nls' '--with-x-toolkit=gtk' 'CFLAGS=-O0 -g3 -ggdb''

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

Major mode: Fundamental

Minor modes in effect:
  shell-dirtrack-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  unify-8859-on-encoding-mode: t
  utf-translate-cjk-mode: t
  auto-compression-mode: t
  temp-buffer-resize-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
C-n C-n v C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-u C-l C-x 4 f / t m p / k i <tab> 
<tab> <return> ESC x a u t o - r e v <tab> <tab> t 
a i l <tab> C-a C-g C-h f a u t o - r e v e r <tab> 
t a i <tab> <return> C-x o C-e C-b C-b C-b C-b <return> 
C-x 1 ESC < C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-u C-l C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-u C-l C-x C-b C-x o C-n C-n 
v ESC x t <backspace> a u t o - r e v e r <tab> t a 
i <tab> <return> ESC > <up> <up> <up> <up> <up> <up> 
<up> <up> <up> <up> C-u C-l C-x C-v <return> ESC > 
ESC v ESC v C-u C-l C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-u C-l C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n 
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-u C-l 
C-v ESC x r e p o r <tab> <return>

Recent messages:
Quit
Type C-x 4 C-o RET to restore the other window.  
uncompressing autorevert.el.gz...done
Mark set
Loading autorevert...done
Reverting buffer `kill-it'.
Auto-Revert-Tail mode enabled
Mark set
Reverting buffer `kill-it'.
Mark set