GNU bug report logs - #13149
24.3.50; Emacs thinks file was changed outside Emacs, but it was not

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Message #67 received at 13149 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Dmitry Gutov <raaahh <at> gmail.com>
Cc: 13149 <at> debbugs.gnu.org, eggert <at> cs.ucla.edu
Subject: Re: bug#13149: 24.3.50;
	Emacs thinks file was changed outside Emacs, but it was not
Date: Thu, 17 Jan 2013 19:05:55 +0200

> Date: Thu, 17 Jan 2013 14:32:56 +0400
> From: Dmitry Gutov <raaahh <at> gmail.com>
> CC: 13149 <at> debbugs.gnu.org, Eli Zaretskii <eliz <at> gnu.org>
> 
> Looked at the VirtualBox bug reports, found just this one: 
> https://www.virtualbox.org/ticket/10986, not much information there.
> 
> Some more about space rays:
> 1) I now have a version of Emacs compiled on a brand-new Fedora virtual 
> machine from emacs-24 branch (revno 111185) that exhibits the problem. 
> Just tested it simultaneously with Ubuntu, emacs-24 on Fedora is buggy, 
> Ubuntu's is not. The following items (2 and 3) are from a few hours ago, 
> when I tested Fedora machine exclusively.
> 2) Editing the file on a different disk on the host system (HDD vs SSD) 
> makes no difference, the bug is present.
> 3) Process Monitor doesn't show any other processes accessing the file 
> on the host machine other than VirtualBox.exe, SYSTEM and 
> SearchProtocolHost.exe. The last one goes away if I stop the Windows 
> Search service, but the problem stays.

I suspect that what you see is some bad interaction between VirtualBox
and the nasty way Windows disk cache optimizes its disk I/O.  It is
known to update the file attributes lazily, so data could be written
to a file without the file's directory entry updated at the same time,
until some time later.

> I'm attaching the exported event log for the open-modify-save
> scenario (file-access-log.csv) in case someone knowledgeable can see
> anything weird there (Eli?).

I don't see anything interesting there, although I cannot pretend that
I've studied every single entry.

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