[Libguestfs] Hierarchical local mount

[Richard W.M. Jones]

On Tue, Nov 10, 2015 at 02:06:30PM +0300, Maxim Perevedentsev wrote:
> I've rewritten this script in C, and it works just as expected.
> The only problem is that for unmount, we cannot reuse the same
> handle, and have to call guestunmount.
> 
> There's a dim place:
> what is the difference between guestunmount and guestfs_umount_local?
> As I understood, guestunmount does [not] need guestfs handle, launch etc.
> Does umount_local need it?
> If yes, how should this command be used (assumed that the handle is
> already used in mount_local_run)?
> 
> These questions confuse me a little :-s

guestfs_umount_local just calls guestunmount, so they're the same
thing:

https://github.com/libguestfs/libguestfs/blob/master/src/fuse.c#L1103

Also as you can see from the implementation, guestfs_umount_local
doesn't really use the guestfs_h *g handle -- it only uses it to find
the mountpoint (so it can run guestunmount /mountpoint) and for a bit
of bookkeeping: creating the 'cmd' object and error messages.

We have an example program:

https://github.com/libguestfs/libguestfs/blob/master/examples/mount-local.c

Also an example of using mount-local with threads (which I don't
recommend - I see lots of deadlocks under load which I don't think
have anything to do with libguestfs):

https://github.com/libguestfs/libguestfs/blob/master/tests/mount-local/test-parallel-mount-local.c

I found that pthreads + FUSE is a minefield.  It's easy to end up with
a forgotten file descriptor which makes the mountpoint busy and
unmountable, or get deadlocks.  Better to use fork()-parallelism
instead.

Rich.

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