[PATCH v2 0/2] nbd/server: Quiesce server on drained section

[Sergio Lopez <slp@redhat.com>]

Before switching between AioContexts we need to make sure that we're
fully quiesced ("nb_requests == 0" for every client) when entering the
drained section. Otherwise, coroutines may be run in the wrong context
after the switch, leading to a number of critical issues.

To accomplish this, we add ".drained_poll" to BlockDevOps and use it
in the NBD server, along with ".drained_being" and "drained_end", to
coordinate the quiescing of the server while entering a drained
section.

v2:
 - Use a bool for the value returned by .drained_poll [Kevin]
 - Change .drained_poll comment to reflect that the returned boolean
   value will be true if the device is still busy, or false otherwise
 - Drop yield_co_list and use recv_coroutine and read_yielding [Kevin]
 - Return "true" or "false" in nbd_drained_poll [Kevin]
 - Fix grammar in the commit message of patch 2 [Eric]

Sergio Lopez (2):
  block-backend: add drained_poll
  nbd/server: Use drained block ops to quiesce the server

 block/block-backend.c          |  7 ++-
 include/sysemu/block-backend.h |  4 ++
 nbd/server.c                   | 82 +++++++++++++++++++++++++---------
 3 files changed, 71 insertions(+), 22 deletions(-)

-- 
2.26.2