I've created a workable async framework with the latest nbdkit code and a sample file plugin using aio to service requests. This allows for a single worker thread to issue multiple requests in parallel via a callback mechanism. Existing locking mechanisms are preserved and if an async thread model sets THREAD_MODEL < PARALLEL then nbdkit continues to properly serialize.

The changes are split into 3 commits:

1) backend plumbing changes required to keep the memory for requests around until completed

2) the async plugin backend

3) a sample plugin that uses aio and the async plugin backend

The patches would be large so rather than post them all here I've included a link to the github diff