Re: [Libguestfs] [PATCH nbdkit v2] PROPOSED: server: Implement list_exports.

We need something similar for a filter to be able to request a single shared .open of the underlying plugin (shared by all connections into the filter), and which has a lifetime visible between the filter's .after_fork and .unload regardless of how many .open/.prepare/.close client connections come and go, rather than having to repeat a next_ops->open for each filter .open. Kind of the opposite of the reopen filter (which uses multiple calls into next_ops->open from a single .open to the filter).

While filters like tar and ext2 will probably benefit from doing explicit sharing of a single underlying plugin, it would also allow us to create a new --filter=shared that can be thrown in front of any plugin (rendering the existing 'nbdkit nbd --shared' obsolete in favor of 'nbdkit nbd --filter=shared'). Basically, I'm thinking we need the following filter-only functions: int nbdkit_plugin_open(int readonly, struct nbdkit_next_ops **next_ops, void **nxdata); which a filter can call at any point between .after_fork and .unload to open a distinct connection into the plugin independent of any filter clients, and which returns by reference a next_ops/nxdata pair that the filter can then use in place of waiting for a per-connection next_ops/nxdata passed through .prepare/.can_write/.pread/etc. void nbdkit_plugin_close (struct nbdkit_next_ops *next_ops, void *nxdata); which a filter later calls to close its own connection into the underlying plugin. Any plugin that uses these new functions would probably implement a .open that does NOT call next_open(nxdata), because it instead reuses the shared handle that it opened globally.