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The if branch of wsrep_handle_mdl_conflict():
else if (granted_thd->lex->sql_command == SQLCOM_FLUSH ||
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granted_thd->mdl_context.has_explicit_locks())
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{
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WSREP_DEBUG("BF thread waiting for FLUSH");
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doesn't consider that there may be regular transactions having explicit MDL locks.
Example output:
2023-12-28 17:44:17 2 [Note] WSREP: Wsrep_high_priority_service::apply_toi: 1831
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2023-12-28 17:44:17 2 [Note] WSREP: assigned new next query and trx id: 4379
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T@4 : 17:44:17.637771 Query_log_event::do_apply_event: query: TRUNCATE TABLE t1
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T@4 : 17:44:17.637797 reset_current_stmt_binlog_format_row: debug: temporary_tables: no, in_sub_stmt: no, system_thread: SYSTEM_THREAD_SLAVE_SQL
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2023-12-28 17:44:17 2 [Note] WSREP: MDL conflict·
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schema: test
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request: (2 seqno 1831 wsrep (toi, exec, committed) cmd 0 8 TRUNCATE TABLE t1)
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granted: (237 seqno -1 wsrep (local, exec, executing) cmd 3 5 INSERT INTO t1 VALUE (4, 'z'))
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2023-12-28 17:44:17 2 [Note] WSREP: MDL ticket: type: MDL_SHARED_WRITE space: TABLE db: test name: t1 (Waiting for table metadata lock)
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2023-12-28 17:44:17 2 [Note] WSREP: BF thread waiting for FLUSH
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2023-12-28 17:44:17 2 [Note] WSREP: MDL ticket: type: MDL_SHARED_WRITE space: TABLE db: test name: t1 (Waiting for table metadata lock)
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Fixed debug output printing SQL query in the mentioned if branch results in:
2023-12-28 20:09:11 2 [Note] WSREP: Wsrep_high_priority_service::apply_toi: 6599
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2023-12-28 20:09:11 2 [Note] WSREP: assigned new next query and trx id: 16942
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T@4 : 20:09:11.867236 Query_log_event::do_apply_event: query: TRUNCATE TABLE t1
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T@4 : 20:09:11.867266 reset_current_stmt_binlog_format_row: debug: temporary_tables: no, in_sub_stmt: no, system_thread: SYSTEM_THREAD_SLAVE_SQL
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2023-12-28 20:09:11 2 [Note] WSREP: MDL conflict·
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schema: test
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request: (2 seqno 6599 wsrep (toi, exec, committed) cmd 0 8 TRUNCATE TABLE t1)
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granted: (907 seqno -1 wsrep (local, exec, executing) cmd 3 5 INSERT INTO t1 VALUE (4, 'z'))
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2023-12-28 20:09:11 2 [Note] WSREP: MDL ticket: type: MDL_SHARED_WRITE space: TABLE db: test name: t1 (Waiting for table metadata lock)
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2023-12-28 20:09:11 2 [Note] WSREP: BF thread waiting for INSERT INTO t1 VALUE (4, 'z')
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2023-12-28 20:09:11 2 [Note] WSREP: MDL ticket: type: MDL_SHARED_WRITE space: TABLE db: test name: t1 (Waiting for table metadata lock)
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In this case no BF-abort happens as the DML operation INSERT INTO t1 VALUE (4, 'z') holding explicit MDL locks is treated as FLUSH TABLES, which is not the case. This prevents such an operation to be aborted.
The reason why a DML operation may hold explicit locks is an open question.
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backtrace_all
mysqld.1.err