[PATCH] shadow_copy2: mount_point is valid only for locally mountable file-system

[Michael Adam]

On 2016-10-10 at 17:11 -0700, Jeremy Allison wrote:
> On Fri, Sep 30, 2016 at 01:31:58PM +0530, Rajesh Joseph wrote:
> > shadow_copy2 currently assumes that all the underlying file-system are
> > mountable and
> > are mounted on samba server. But this may not be true for all cases, e.g.
> > GlusterFS is
> > accessed via libgfapi and is not mounted.
> > 
> > This can lead to connection failure if such file-systems are used along
> > with shadow_copy2.
> > The attached patch assume "/" as the default mount-point in such cases
> > instead of failing
> > the connect.
> > 
> > This may not be a complete fix because shadow_copy2_find_mount_point can
> > return wrong
> > mount_point if by coincidence the "connectpath" is actually present in the
> > local file-system.
> > 
> > I had a brief discussion on this with Michael, and we think that it might
> > be good if VFS layer
> > can tell if the underlying file-system is mountable or not.
> > 
> > It would be great to know what others think on this problem.
> 
> Yeah, setting it to "/" can also cause problems here:
> 
> 2613         } else {
> 2614                 config->snapshot_basepath = talloc_asprintf(config, "%s/%s",
> 2615                                 config->mount_point, config->snapdir);
> 2616                 if (config->snapshot_basepath == NULL) {
> 2617                         DEBUG(0, ("talloc_asprintf() failed\n"));
> 2618                         errno = ENOMEM;
> 2619                         return -1;
> 2620                 }
> 2621         }
> 
> Let me think some more about what "connectpath" should be in
> this case.
> 
> We have an existing VFS function SMB_VFS_CONNECTPATH(), maybe
> this should be expanded to have a different meaning when
> we're dealing with a completely virtual underlying FS.

Hm,

the connectpath is already set to the path relative to the
volume by vfs_glusterfs/config. (Starting with "/" for the
volume root.)

The problem Rajesh was pointing out was that the path
of the share might be set to something like
"path = /usr/local" (signifying a subdir of the gluster volume)
which conincides to be a local path too, and might be
a mount point. So the find-mountpoint function would
return this as the mountpoint, which is wrong.
In gluster's case we would always have to end up with "/".
If there was a flag inside the vfs that indicates whether
the filsystem is a local / mounted or mountable filesystem
or not, then shadow_copy2_find_mount_point and possibly
other places could behave accordingly.

But maybe there are better options.
For now we have the config workaround of setting
"shadow:mountpoint = /" if glusterfs vfs-module is used.

Cheers - Michael

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