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50b0ce44a2
This was the last abuse of trx_sys.mutex, which is now exclusively protecting trx_sys.trx_list. This global acquisition was also potential scalability bottleneck for oltp_read_write benchmark. Although it didn't expose itself as such due to larger scalability issues. Replaced trx_sys.mutex based synchronisation between ReadView creator thread and purge coordinator thread performing latest view clone with ReadView::m_mutex. It also allowed to simplify tri-state view m_state down to boolean m_open flag. For performance reasons trx->read_view.close() is left as atomic relaxed store, so that we don't have to waste resources for otherwise meaningless mutex acquisition.