According to https://mariadb.com/kb/en/library/changes-improvements-in-mariadb-101/, Galera (wsrep stroage engine in SHOW PLUGINS output, to be more specific) is a standard part of MariaDB 10.1 (and newer version).
It is not documented anywhere explicitly, but one can conclude from code review and fix for MDEV-9214 that wsrep is XA-capable storage engine. With it we'll have tc.log created in the datadir of 10.1+ and maintained by default, even if wsrep_on is set to OFF and InnoDB is the only XA-capable storage engine listed in SHOW ENGINES:
MariaDB [test]> \! ls -l data/*.log
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-rw-rw---- 1 openxs openxs 24576 чер 8 17:25 data/tc.log
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MariaDB [test]> select @@datadir, @@wsrep_on, @@log_bin;
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+----------------------------------+------------+-----------+
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| @@datadir | @@wsrep_on | @@log_bin |
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+----------------------------------+------------+-----------+
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| /home/openxs/dbs/maria10.1/data/ | 0 | 0 |
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+----------------------------------+------------+-----------+
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1 row in set (0.00 sec)
|
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MariaDB [test]> show engines;
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+--------------------+---------+--------------------------------------------------------------------------------------------------+--------------+------+------------+
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| Engine | Support | Comment | Transactions | XA | Savepoints |
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+--------------------+---------+--------------------------------------------------------------------------------------------------+--------------+------+------------+
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| MRG_MyISAM | YES | Collection of identical MyISAM tables | NO | NO | NO |
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| CSV | YES | CSV storage engine | NO | NO | NO |
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| BLACKHOLE | YES | /dev/null storage engine (anything you write to it disappears) | NO | NO | NO |
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| MyISAM | YES | MyISAM storage engine | NO | NO | NO |
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| SEQUENCE | YES | Generated tables filled with sequential values | YES | NO | YES |
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| Aria | YES | Crash-safe tables with MyISAM heritage | NO | NO | NO |
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| PERFORMANCE_SCHEMA | YES | Performance Schema | NO | NO | NO |
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| InnoDB | DEFAULT | Percona-XtraDB, Supports transactions, row-level locking, foreign keys and encryption for tables | YES | YES | YES |
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| MEMORY | YES | Hash based, stored in memory, useful for temporary tables | NO | NO | NO |
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+--------------------+---------+--------------------------------------------------------------------------------------------------+--------------+------+------------+
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9 rows in set (0.00 sec)
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I found literally nothing in MariaDB KB about the tc.log file and numerous bug reports mentioning failures to create it etc mean that this information is clearly missing. MySQL's reference here, https://dev.mysql.com/doc/refman/5.6/en/server-options.html#option_mysqld_log-tc, refers to multiple storage engines:
"... memory-mapped transaction coordinator log file (for XA transactions that affect multiple storage engines when the binary log is disabled)."
and is not helpful unless we document Galera (wsrep) engine as the one that causes this file to be created in 10.1+.
Bug
Critical