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For a primary configured with wait_point=AFTER_SYNC, if two threads T1 (binlogging through MYSQL_BIN_LOG::write()) and T2 were binlogging at the same time, T1 could accidentally wait for its semi-sync ACK using the binlog coordinates of T2. Prior to MDEV-33551, this only resulted in delayed transactions, because all transactions shared the same condition variable for ACK signaling. However, with the MDEV-33551 changes, each thread has its own condition variable to signal. So T1 could wait indefinitely when either: 1) T1's ACK is received but not T2's when T1 goes into wait_after_sync(), because the ACK receiver thread has already notified about the T1 ACK, but T1 was _actually_ waiting on T2's ACK, and therefore tries to wait (in vain). 2) T1 goes to wait_after_sync() before any ACKs have arrived. When T1's ACK comes in, T1 is woken up; however, sees it needs to wait more (because it was actually waiting on T2's ACK), and goes to wait again (this time, in vain). Note that the actual cause of T1 waiting on T2's binlog coordinates is when MYSQL_BIN_LOG::write() would call Repl_semisync_master::wait_after_sync(), the binlog offset parameter was read as the end of MYSQL_BIN_LOG::log_file, which is shared among transactions. So if T2 had updated the binary log _after_ T1 had released LOCK_log, but not yet invoked wait_after_sync(), it would use the end of the binary log file as the binlog offset, which was that of T2 (or any future transaction). The fix in this patch ensures consistency between the binary log coordinates a transaction uses between report_binlog_update() and wait_after_sync(). Reviewed By ============ Kristian Nielsen <knielsen@knielsen-hq.org> Andrei Elkin <andrei.elkin@mariadb.com> |
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