Patch for Multiple XA Storage Engines

While integrating TokuDB into MySQL 5.6, we found that MySQL 5.6 does not support more than one XA storage engine. For example, there is an assert in the ha_recover function that fires when the total number of XA storage engines is too big. After disabling this assert, we found lots of bugs in the MySQL 5.6 implementation of the TC_LOG_MMAP class, which is used when running with the binlog turned off.

There are two alternatives that we know about to fix this problem in MySQL 5.6

First, we could merge code from MariaDB 5.5 into MySQL 5.6. The advantage of this approach is that we have been running this code with TokuDB in MariaDB 5.5 for a long time, so we have confidence in its correctness.

Second, we found that MySQL 5.7.2 has made changes to allow multiple XA storage engines. This is great news for TokuDB since we have one less MySQL patch to have to worry about. Our simple scan of the MySQL 5.7 source showed only one XA storage engine - InnoDB. Where are the others? How did this feature get tested?

We merged MariaDB's implemention into our version of MySQL 5.6.