When the reconnect in NBD client is in progress, the iochannel used for
NBD connection doesn't exist.  Therefore an attempt to detach it from
the aio_context of the parent BlockDriverState results in a NULL pointer
dereference.

The problem is triggerable, in particular, when an outgoing migration is
about to finish, and stopping the dataplane tries to move the
BlockDriverState from the iothread aio_context to the main loop.  If the
NBD connection is lost before this point, and the NBD client has entered
the reconnect procedure, QEMU crashes:

  #0  qemu_aio_coroutine_enter (ctx=0x5618056c7580, co=0x0)
      at /build/qemu-6MF7tq/qemu-5.0.1/util/qemu-coroutine.c:109
  #1  0x00005618034b1b68 in nbd_client_attach_aio_context_bh (
      opaque=0x561805ed4c00) at /build/qemu-6MF7tq/qemu-5.0.1/block/nbd.c:164
  #2  0x000056180353116b in aio_wait_bh (opaque=0x7f60e1e63700)
      at /build/qemu-6MF7tq/qemu-5.0.1/util/aio-wait.c:55
  #3  0x0000561803530633 in aio_bh_call (bh=0x7f60d40a7e80)
      at /build/qemu-6MF7tq/qemu-5.0.1/util/async.c:136
  #4  aio_bh_poll (ctx=ctx@entry=0x5618056c7580)
      at /build/qemu-6MF7tq/qemu-5.0.1/util/async.c:164
  #5  0x0000561803533e5a in aio_poll (ctx=ctx@entry=0x5618056c7580,
      blocking=blocking@entry=true)
      at /build/qemu-6MF7tq/qemu-5.0.1/util/aio-posix.c:650
  #6  0x000056180353128d in aio_wait_bh_oneshot (ctx=0x5618056c7580,
      cb=<optimized out>, opaque=<optimized out>)
      at /build/qemu-6MF7tq/qemu-5.0.1/util/aio-wait.c:71
  #7  0x000056180345c50a in bdrv_attach_aio_context (new_context=0x5618056c7580,
      bs=0x561805ed4c00) at /build/qemu-6MF7tq/qemu-5.0.1/block.c:6172
  #8  bdrv_set_aio_context_ignore (bs=bs@entry=0x561805ed4c00,
      new_context=new_context@entry=0x5618056c7580,
      ignore=ignore@entry=0x7f60e1e63780)
      at /build/qemu-6MF7tq/qemu-5.0.1/block.c:6237
  #9  0x000056180345c969 in bdrv_child_try_set_aio_context (
      bs=bs@entry=0x561805ed4c00, ctx=0x5618056c7580,
      ignore_child=<optimized out>, errp=<optimized out>)
      at /build/qemu-6MF7tq/qemu-5.0.1/block.c:6332
  #10 0x00005618034957db in blk_do_set_aio_context (blk=0x56180695b3f0,
      new_context=0x5618056c7580, update_root_node=update_root_node@entry=true,
      errp=errp@entry=0x0)
      at /build/qemu-6MF7tq/qemu-5.0.1/block/block-backend.c:1989
  #11 0x00005618034980bd in blk_set_aio_context (blk=<optimized out>,
      new_context=<optimized out>, errp=errp@entry=0x0)
      at /build/qemu-6MF7tq/qemu-5.0.1/block/block-backend.c:2010
  #12 0x0000561803197953 in virtio_blk_data_plane_stop (vdev=<optimized out>)
      at /build/qemu-6MF7tq/qemu-5.0.1/hw/block/dataplane/virtio-blk.c:292
  #13 0x00005618033d67bf in virtio_bus_stop_ioeventfd (bus=0x5618056d9f08)
      at /build/qemu-6MF7tq/qemu-5.0.1/hw/virtio/virtio-bus.c:245
  #14 0x00005618031c9b2e in virtio_vmstate_change (opaque=0x5618056d9f90,
      running=0, state=<optimized out>)
      at /build/qemu-6MF7tq/qemu-5.0.1/hw/virtio/virtio.c:3220
  #15 0x0000561803208bfd in vm_state_notify (running=running@entry=0,
      state=state@entry=RUN_STATE_FINISH_MIGRATE)
      at /build/qemu-6MF7tq/qemu-5.0.1/softmmu/vl.c:1275
  #16 0x0000561803155c02 in do_vm_stop (state=RUN_STATE_FINISH_MIGRATE,
      send_stop=<optimized out>) at /build/qemu-6MF7tq/qemu-5.0.1/cpus.c:1032
  #17 0x00005618033e3765 in migration_completion (s=0x5618056e6960)
      at /build/qemu-6MF7tq/qemu-5.0.1/migration/migration.c:2914
  #18 migration_iteration_run (s=0x5618056e6960)
      at /build/qemu-6MF7tq/qemu-5.0.1/migration/migration.c:3275
  #19 migration_thread (opaque=opaque@entry=0x5618056e6960)
      at /build/qemu-6MF7tq/qemu-5.0.1/migration/migration.c:3439
  #20 0x0000561803536ad6 in qemu_thread_start (args=<optimized out>)
      at /build/qemu-6MF7tq/qemu-5.0.1/util/qemu-thread-posix.c:519
  #21 0x00007f61085d06ba in start_thread ()
     from /lib/x86_64-linux-gnu/libpthread.so.0
  #22 0x00007f610830641d in sysctl () from /lib/x86_64-linux-gnu/libc.so.6
  #23 0x0000000000000000 in ?? ()

Fix it by checking that the iochannel is non-null before trying to
detach it from the aio_context.  If it is null, no detaching is needed,
and it will get reattached in the proper aio_context once the connection
is reestablished.

Signed-off-by: Roman Kagan <rvkagan@yandex-team.ru>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210129073859.683063-2-rvkagan@yandex-team.ru>
Signed-off-by: Eric Blake <eblake@redhat.com>