Re: Emacs cannot write file in a Virtual Box shared folder

[Bernardo <bernardo.bacic@pobox.com>]

Robert Pluim <rpluim@gmail.com> writes:

>>>>>> On Mon, 21 Oct 2019 20:22:04 +1100, Bernardo <bernardo.bacic@pobox.com> said:
>
>     Bernardo> Greetings,
>
>     Bernardo> Running GNU Emacs in Debian (9.11) VM guest under VirtualBox host (6.0.14) running on Windows 10 box.
>     Bernardo> (hope all this makes sense)
>
>     Bernardo> Am having problems editing files located in a Virtual Box shared
>     Bernardo> directory. The directory is owned by root but members of 'vboxsf'
>     Bernardo> group have read/write/execute rights.  Emacs user is a member of
>     Bernardo> 'vboxsf' group.
>
>     Bernardo>  $ groups
>     Bernardo>   some_user cdrom sudo audio <snip> bluetooth vboxsf
>
>     Bernardo> The (old) version of Emacs that comes with Debian reads/writes files ok;
>     Bernardo>   GNU Emacs 24.5.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.11) of 2017-09-12 on hullmann, modified by Debian
>
>     Bernardo> The version of Emacs that's built from Git repos cannot write
>     Bernardo> file. Also by default it opens the file in read-only mode.
>     Bernardo>   GNU Emacs 27.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.11) of 2019-10-21
>
>     Bernardo> Debugger entered--Lisp error: (file-error "Unlocking file" "Operation not permitted" "/media/sf_Home/zz_stuff")
>     Bernardo>   write-region(nil nil "/media/sf_Home/zz_stuff" nil t "/media/sf_Home/zz_stuff")
>     Bernardo>   basic-save-buffer-2()
>     Bernardo>   basic-save-buffer-1()
>     Bernardo>   basic-save-buffer(t)
>     Bernardo>   save-buffer(1)
>     Bernardo>   funcall-interactively(save-buffer 1)
>     Bernardo>   call-interactively(save-buffer nil nil)
>     Bernardo>   command-execute(save-buffer)
>
> Looks like emacs is trying to create/delete a lockfile, and
> failing. Does /media/sf_Home support symlinks?
>
> You can work around this temporarily by customizing 'create-lockfiles'
> to nil, but then emacs won't detect if youʼre editing the same file
> from a different emacs session.
>
> Robert
>

Thanks Robert and Eli.

You were spot on, the symlinks cannot be created on this file system,
vboxsf, which seems to be a layer on top of NTFS.

  $ mount | grep sf_Home
  Home on /media/sf_Home type vboxsf (rw,nodev,relatime,<snip>

  $ ln -s zz_stuff my_link
  ln: failed to create symbolic link 'my_link': Operation not permitted

Will be reporting this as a bug, the manual says:
   (1) If your file system does not support symbolic links, a regular
       file is used.
       
 M-: (info "(emacs) Interlocking") RET

It looks like the old version of Emacs (24.5.1) was creating lock files
elsewhere and not in the directory where the edited file is located.


--
cheers