Hi all!
There are problems with nbd driver:
- nbd reconnect is cancelled on drain, which is bad as Roman describes
in his "[PATCH 0/7] block/nbd: decouple reconnect from drain"
- nbd driver is too complicated around drained sections and aio context
switch. It's nearly impossible to follow all the logic, including
abuse of bs->in_flight, which is temporary decreased in some places
(like nbd_read_eof()). Additional reconnect thread and two different
state machines (we have BDRVNBDState::state and
BDRVNBDState::connect_thread->state) doesn't make things simpler :)
So, I have a plan:
1. Move nbd negotiation to connect_thread
2. Do receive NBD replies in request coroutines, not in connection_co
At this point we can drop connection_co, and when we don't have
endless running coroutine, NBD driver becomes a usual block driver,
and we can drop abuse of bs->in_flight, and probably drop most of
complicated logic around drained section and aio context switch in
nbd driver.
3. Still, as Roman describes, with [2] we loose a possibility to
reconnect immediately when connection breaks (with current code we
have endless read in reconnect_co, but actually for this to work
keep-alive should be setup correctly). So, we'll need to reinvent it,
checking connection periodically by timeout, with help of getsockopt
or just sending a kind of PING request (zero-length READ or something
like this).
And this series a kind of preparation. The main point of it is moving
connect-thread to a separate file.