MDEV-23651 InnoDB: Failing assertion: !space->referenced()
commit de942c9f (MDEV-15983) introduced a race condition that we inadequately fixed in commit 93b69825 (MDEV-16169). Because fil_space_t::release() or fil_space_t::acquire() are not protected by fil_system.mutex like their predecessors, it is possible that stop_new_ops was set between the time a thread checked fil_space_t::is_stopping() and invoked fil_space_t::acquire(). In an execution trace, this happened in fil_system_t::keyrotate_next(), causing an assertion failure in fil_delete_tablespace() in the other thread that seeked to stop new operations. We fix this bug by merging the flag fil_space_t::stop_new_ops and the reference count fil_space_t::n_pending_ops into a single word that is only being accessed by atomic memory operations. fil_space_t::set_stopping(): Accessor for changing the state of the former stop_new_ops flag. fil_space_t::acquire(): Return whether the acquisition succeeded. It would fail between set_stopping(true) and set_stopping(false).
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