InnoDB: Assertion failure in thread 139995225970432 in file buf0mtflu.cc line 570 InnoDB: Failing assertion: ib_wqueue_is_empty(mtflush_ctx->wq)

Analysis: Real timing bug, we should take the mutex before we try to send those shutdown messages, that would make sure that threads doing a unfinished flush (they have acquired this mutex) have time to do their work before we add shutdown messages to work queue. Currently, we just add those shutdown messages to work queue and code assumes that at flush, there is constant number of items to be processed and thus leading to assertion.