Apparently, 10.1 somehow wrongly counts the number of XA-capable engines. Here are some scenarios which make me think that:

• start server with only one XA-capable engine, e.g. XtraDB;
• see tc.log is created (already strange, it shouldn't do that, right?);
• sigkill server;
• start server with two XA-capable engines, e.g. XtraDB and TokuDB;
• see the error message in the log:

  [ERROR] Recovery failed! You must enable exactly 2 storage engines that support two-phase commit protocol

  It should be 1, not 2, right?

or

• start server with two XA-capable engines, e.g. XtraDB and TokuDB;
• see tc.log is created;
• sigkill server;
• start server without TokuDB;
• see the error message in the log:

  [ERROR] Recovery failed! You must enable exactly 3 storage engines that support two-phase commit protocol

  It should be 2, not 3, right?