[Libguestfs] [PATCH libnbd v3] lib: Atomically update h->state when leaving the locked region.

[Richard W.M. Jones]

v1 was here:

https://www.redhat.com/archives/libguestfs/2019-June/thread.html#00055

v2 was here:

https://www.redhat.com/archives/libguestfs/2019-June/thread.html#00067

v3:

 - Fix atomicly -> atomically in commit message.

 - Fix a comment.

 - Fix TOCTTOU: There is now an inline function generated called
   <name>_is_permitted_state, and this is called twice, first outside
   the lock for the quick rejection check, and again inside the lock
   in case the state changed.

I think an improvement would be to _drop_ the first check (outside the
lock).  This is an error check, so not performance critical since we
are definitely going to acquire the lock on the hot path.

Rich.