Before, bdrv_aio_cancel will either complete the request (like normal)
and call CB with an actual return code, or skip calling the request (for
example when the IO req is not submitted by thread pool yet).
We will change bdrv_aio_cancel to do it differently: always call CB
before return, with either [1] a normal req completion ret code, or [2]
ret == -ECANCELED. So the callers' callback must accept both cases. The
existing logic works with case [1], but not [2].
The simplest transition of callback code is do nothing in case [2], just
as if the CB is not called by the bdrv_aio_cancel() call.
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
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