[Libguestfs] [PATCH nbdkit filters-v2 0/5] Introduce filters.

[Richard W.M. Jones]

Rebased filters patch.  Requires current git master + the locks /
thread model fix
(https://www.redhat.com/archives/libguestfs/2018-January/msg00128.html)

So a few changes here since last time:

The "introduce filters" and "implement filters" patches are
squashed together.

I introduced a concept of .prepare and .finalize.  These run before
and after the data serving phase respectively.  They're not a lot of
use for plugins because you can just use .open and .close instead, and
therefore I didn't expose them to plugins (only internally in the
backend).  However for filters they make more sense since they give
you a place to prepare and finalize.  They're necessary also for
filters because filter.open could not call the next open method, save
the (plugin) handle, then construct an nbdkit_next_ops object, and
call pread.  It _has_ to be split into two operations.

This patch series is incomplete in at least three ways:

 - We still need a way for filter.open to call down to the next open
   function, so it can adjust the readonly flag (for copyonwrite
   filter, TBD).

 - Partition filter is obviously not complete.

 - Need to write a copyonwrite filter.

Rich.