bug#22795: 25.0.91; Can't write read only file on w32

[Eli Zaretskii]

> Date: Mon, 6 Jun 2016 13:46:32 -0700
> CC: <22795@debbugs.gnu.org>
> From: "Ota, Takaaki" <Takaaki.Ota@am.sony.com>
> 
> Mon, 6 Jun 2016 22:10:29 +0300: Eli Zaretskii <eliz@gnu.org> wrote:
> 
> > > Date: Mon, 6 Jun 2016 09:24:23 -0700
> > > CC: <22795@debbugs.gnu.org>
> > > From: "Ota, Takaaki" <Takaaki.Ota@am.sony.com>
> > > 
> > > After opening a read only file, execute read-only-mode, modify the
> > > file and try to save the file under emacs 25.0.94 on Linux and Windows
> > > file-ownership-preserved-p returns non nil value on Linux and nil on
> > > Windows.
> > 
> > What does (file-attributes FILENAME 'integer) return for that file?
> > And what does (user-uid) return?
> 
> On Linux
> 
> (file-attributes "~/memo" 'integer)
> (nil 1 1000 1000 (22351 8780 403703 908000) (22351 6556 219830 864000) (22351 
> 6556 223830 864000) 1827 "-r--r--r--" t 131312 2049)
> (user-uid)
> 1000
> 
> On Windows (both 24.5 and 25.0.94)
> 
> (file-attributes "~/memo" 'integer)
> (nil 1 544 513 (21821 905 0 0) (22357 41339 0 0) (20624 9296 0 0) 289738 
> "-r--r--r--" t (1024 1 . 32777) (50396 . 34426))
> (user-uid)
> 1001

This may be part of the problem: your home directory is not owned by
your user, it is owned by the Administrators group.  That is
definitely why file-ownership-preserved-p returns nil in your case,
but it can also be part of the larger problem we are investigating.
(On my machines, I always take ownership of the home directory and all
of its subdirectories, recursively, to avoid any ownership issues.)