MDEV-25687: Remove trx_active_transactions

MONITOR_TRX_ACTIVE: Remove. The count is not being updated consistently,
and it would also include read-only transactions that are otherwise
fully invisible to any other threads.

If it later turns out that a reliable count of active transactions
is needed, it can be exposed via a different interface.

trx_commit_for_mysql(): If the transaction was not started, return
immediately.

mysql-test/suite/innodb/r/innodb_skip_innodb_is_tables.result

@@ -167,7 +167,6 @@ trx_nl_ro_commits	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	N
 trx_commits_insert_update	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	counter	Number of transactions committed with inserts and updates
 trx_rollbacks	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	counter	Number of transactions rolled back
 trx_rollbacks_savepoint	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	counter	Number of transactions rolled back to savepoint
-trx_active_transactions	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	counter	Number of active transactions
 trx_rseg_history_len	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	value	Length of the TRX_RSEG_HISTORY list
 trx_undo_slots_used	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	counter	Number of undo slots used
 trx_undo_slots_cached	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	counter	Number of undo slots cached

mysql-test/suite/innodb/r/monitor.result

@@ -133,7 +133,6 @@ trx_nl_ro_commits	disabled
 trx_commits_insert_update	disabled
 trx_rollbacks	disabled
 trx_rollbacks_savepoint	disabled
-trx_active_transactions	disabled
 trx_rseg_history_len	disabled
 trx_undo_slots_used	disabled
 trx_undo_slots_cached	disabled
@@ -426,10 +425,9 @@ select name, max_count, min_count, count,
 max_count_reset, min_count_reset, count_reset,
 if(enabled,'enabled','disabled') status
 from information_schema.innodb_metrics
-where name like "trx_rollbacks" or name like "trx_active_transactions";
+where name='trx_rollbacks';
 name	max_count	min_count	count	max_count_reset	min_count_reset	count_reset	status
 trx_rollbacks	1	NULL	1	1	NULL	1	enabled
-trx_active_transactions	1	0	0	1	0	0	enabled
 set global innodb_monitor_disable = module_trx;
 set global innodb_monitor_enable = module_dml;
 insert into monitor_test values(9);

mysql-test/suite/innodb/t/monitor.test

@@ -268,7 +268,7 @@ select name, max_count, min_count, count,
 	max_count_reset, min_count_reset, count_reset,
 	if(enabled,'enabled','disabled') status
 from information_schema.innodb_metrics
-where name like "trx_rollbacks" or name like "trx_active_transactions";
+where name='trx_rollbacks';
 
 set global innodb_monitor_disable = module_trx;
 

storage/innobase/include/srv0mon.h

@@ -2,7 +2,7 @@
 
 Copyright (c) 2010, 2015, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2013, 2020, MariaDB Corporation.
+Copyright (c) 2013, 2021, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it
 under the terms of the GNU General Public License as published by the
@@ -285,7 +285,6 @@ enum monitor_id_t {
 	MONITOR_TRX_COMMIT_UNDO,
 	MONITOR_TRX_ROLLBACK,
 	MONITOR_TRX_ROLLBACK_SAVEPOINT,
-	MONITOR_TRX_ACTIVE,
 	MONITOR_RSEG_HISTORY_LEN,
 	MONITOR_NUM_UNDO_SLOT_USED,
 	MONITOR_NUM_UNDO_SLOT_CACHED,

storage/innobase/srv/srv0mon.cc

@@ -723,11 +723,6 @@ static monitor_info_t	innodb_counter_info[] =
 	 MONITOR_NONE,
 	 MONITOR_DEFAULT_START, MONITOR_TRX_ROLLBACK_SAVEPOINT},
 
-	{"trx_active_transactions", "transaction",
-	 "Number of active transactions",
-	 MONITOR_NONE,
-	 MONITOR_DEFAULT_START, MONITOR_TRX_ACTIVE},
-
 	{"trx_rseg_history_len", "transaction",
 	 "Length of the TRX_RSEG_HISTORY list",
 	 static_cast<monitor_type_t>(

storage/innobase/trx/trx0roll.cc

@@ -141,8 +141,6 @@ inline void trx_t::rollback_low(trx_savept_t *savept)
   }
 
   mem_heap_free(heap);
-
-  MONITOR_DEC(MONITOR_TRX_ACTIVE);
 }
 
 /** Initiate rollback.

storage/innobase/trx/trx0trx.cc

@@ -993,8 +993,6 @@ trx_start_low(
 		: microsecond_interval_timer();
 
 	ut_a(trx->error_state == DB_SUCCESS);
-
-	MONITOR_INC(MONITOR_TRX_ACTIVE);
 }
 
 /** Set the serialisation number for a persistent committed transaction.
@@ -1612,17 +1610,12 @@ trx_commit_for_mysql(
 
 	switch (trx->state) {
 	case TRX_STATE_NOT_STARTED:
-		ut_d(trx->start_file = __FILE__);
-		ut_d(trx->start_line = __LINE__);
-
-		trx_start_low(trx, true);
-		/* fall through */
+		return DB_SUCCESS;
 	case TRX_STATE_ACTIVE:
 	case TRX_STATE_PREPARED:
 	case TRX_STATE_PREPARED_RECOVERED:
 		trx->op_info = "committing";
 		trx->commit();
-		MONITOR_DEC(MONITOR_TRX_ACTIVE);
 		trx->op_info = "";
 		return(DB_SUCCESS);
 	case TRX_STATE_COMMITTED_IN_MEMORY:

storage/rocksdb/mysql-test/rocksdb/r/innodb_i_s_tables_disabled.result

@@ -149,7 +149,6 @@ trx_nl_ro_commits	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	N
 trx_commits_insert_update	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	counter	Number of transactions committed with inserts and updates
 trx_rollbacks	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	counter	Number of transactions rolled back
 trx_rollbacks_savepoint	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	counter	Number of transactions rolled back to savepoint
-trx_active_transactions	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	counter	Number of active transactions
 trx_rseg_history_len	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	value	Length of the TRX_RSEG_HISTORY list
 trx_undo_slots_used	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	counter	Number of undo slots used
 trx_undo_slots_cached	transaction	0	NULL	NULL	NULL	0	NULL	NULL	NULL	NULL	NULL	NULL	NULL	0	counter	Number of undo slots cached