bug#37884: 27.0.50; Cannot write to a file in VirtualBox shared directory

[Robert Pluim]

>>>>> On Wed, 23 Oct 2019 21:28:06 +1100, Bernardo <bernardo.bacic@pobox.com> 
>>>>> said:

    Bernardo> When Emacs is started with strace ( $ strace emacs -Q ) it can be 
seen
    Bernardo> that there are issues with the lock file that's created -
    Bernardo> /media/sf_Home/.#zz_stuff

    Bernardo> The strace messages are in the attachement.

Weʼre getting EPERM when doing unlink on the lock file, which is very
strange.

    Bernardo> The 'shared directory' is a feature of Virtual Box which allows 
both the
    Bernardo> host and applications running in a virtual machine to have access 
to
    Bernardo> files in the same directory.

    Bernardo> The file system type of the shared directory is vboxsf which 
seems to be
    Bernardo> a layer on top of NTFS.
    Bernardo>   $ mount | grep vbox
    Bernardo>   Home on /media/sf_Home type vboxsf (rw,nodev,relatime,<snip>

    Bernardo> It looks like symbolic links are not allowed on
    Bernardo> this FS type; from terminal:
    Bernardo>   $ ln -s zz_stuff my_link
    Bernardo>   ln: failed to create symbolic link 'my_link': Operation not 
permitted

Right, so emacs switches to a regular file instead:

symlink fails:
    symlink("mk7@deb.2246:1571819801", "/media/sf_Home/.#zz_stuff") = -1 EPERM 
(Operation not permitted)

so use a regular file:

    open("/media/sf_Home/.#-emacsvxLcB2", O_RDWR|O_CREAT|O_EXCL|O_CLOEXEC, 
0600) = 13

and put the lock information inside the file, and make it read-only:

    write(13, "mk7@deb.2246:1571819801", 23) = 23
    fchmod(13, 0444)                        = 0

and then rename it:

    "/media/sf_Home/.#zz_stuff", RENAME_NOREPLACE) = 0

Later on when emacs saves the file, it tries to get rid of the lock
file, which fails:

    unlink("/media/sf_Home/.#zz_stuff")     = -1 EPERM (Operation not permitted)

So your filesystem doesnʼt let you call unlink() on read-only files? I
guess we could try adjusting the chmod to 664 or similar if that helps here.

Robert