qemu-io: Drop write permissions before read-only reopen
qemu-io provides a 'reopen' command that allows switching from writable to read-only access. We need to make sure that we don't try to keep write permissions to a BlockBackend that becomes read-only, otherwise things are going to fail. This requires a bdrv_drain() call because otherwise in-flight AIO write requests could issue new internal requests while the permission has already gone away, which would cause assertion failures. Draining the queue doesn't break AIO requests in any new way, bdrv_reopen() would drain it anyway only a few lines later. Signed-off-by: NKevin Wolf <kwolf@redhat.com> Reviewed-by: NFam Zheng <famz@redhat.com>
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