bug#13149: 24.3.50; Emacs thinks file was changed outside Emacs, but it was not

[Dmitry Gutov]

On 12.12.2012 2:54, Drew Adams wrote:
> > > Anyway, hoping this helps in some way...
> >
> > I've been seeing the same kind of prompts lately when editing files in
> > Emacs in Ubuntu inside a virtual machine.
> >
> > IIRC all those files were were mounted from the host machine
> > (MS Windows 7) using the vboxsf file system type.
>
> Good to know that someone else is seeing something similar.
>
> In my case, I'm using Windows XP and am not using vboxsf or anything else
> special - just an ordinary laptop.

So, I can now reproduce it 100% in the conditions I mentioned above. And 
it's mighty annoying.

0. Open a file.
1. Make some changes. Emacs complies.
2. Press C-x C-s, saved successfully.
3. Try to make a single modification. Emacs instantly prompts "... 
changed on disk; really edit the buffer?".

Answer yes -> goto 4, answer no -> goto 3.
Answer "revert", buffer reverts, goto -> 1.

4. Make modifications, try to save: "... has changed since visited or 
save.  Save anyway?" Answer yes -> goto 2.

If auto-revert-mode is enabled, and you wait the required interval of 
time after 2. without making modifications, the buffer is "reverted", 
also goto -> 1.

All this on the latest trunk. A build from the latest emacs-24 doesn't 
exhibit the problem. Same with not-exactly-latest builds from these 
branches I had a few hours ago, with the possible exception of 
auto-revert-mode, IIRC it was less reliably helpful. Not sure.

Any tips for debugging this?

I'm doing bzr bisect, but it will take a while.

--Dmitry