Description:
When innodb_support_xa == true(always enabled as of MySQL 5.7.10) and log_bin=true, as part of crash recovery, InnoDB will commit transaction which is in the prepared state.
@see Figure 1. Two-Phase Commit Protocol(2PC)
http://mysqlmusings.blogspot.jp/2012/06/binary-log-group-commit-in-mysql-56.html
In the case of InnoDB, at the time of the "prepare", innodb_xa_prepare() is executed, it write MYSQL_XID and mlog to UNDO LOG. Then, the time of "write", the XID is recorded as Xid_log_event to binary log. If the server crashes between "fsync" and "commit", since LOG_EVENT_BINLOG_IN_USE_F flag has been set in the header of the binary log, MYSQL_BIN_LOG::recover() is executed. Then, the transaction has left in a PREPARED state in UNDO LOG, if the XID was in the binary log, and innodb_commit_by_xid() is executed, InnoDB will commit transaction which is in the prepared state.
See Comment:
https://github.com/mysql/mysql-server/blob/mysql-5.7.12/sql/binlog.cc#L7712-L7724
However, there is an sentence
https://dev.mysql.com/doc/refman/5.7/en/innodb-recovery.html
```
As part of crash recovery, InnoDB rolls back any transactions that were not committed or in XA PREPARE state when the server crashed.
```
And,
https://dev.mysql.com/doc/refman/5.7/en/binary-log.html
```
The InnoDB logs are synchronized by default, and sync_binlog=1 can be used to synchronize the binary log. The effect of this option is that at restart after a crash, after doing a rollback of transactions, the MySQL server removes rolled back InnoDB transactions from the binary log.
```
These descriptions are not correct as the behavior of the crash recovery in the case of innodb_support_xa = true.
How to repeat:
01. build MySQL server with -DWITH_DEBUG=1.
02. set log_bin and server_id in my.cnf.
03. start mysqld_safe.
04. create test table.
05. attach mysqld by gdb.
06. rbreak sync_binlog_file and continue.
07. insert test data into test table.
08. return from sync_binlog_file().
09. kill mysqld by gdb.
10. innodb_commit_by_xid() will executed, and complete commit of test data , and write message“commit xid”to error_log.
https://github.com/mysql/mysql-server/blob/mysql-5.7.12/sql/xa.cc#L117
