XID_STATE->XID.KEY(),
                                XID_STATE->XID.KEY_LENGTH())==0
  
This bug is a regression of bug#11759534 - 51855: RACE CONDITION
                                           IN XA START.
  
The reason for regression is that the changes that fixes the original
bug wasn't merged from mysql-5.1 into mysql-5.5 and mysql-trunk.
Only null-merge was done for the patch changeset.
  
To incorporate lost changes the manual merge have been done.
  
Additionally the call of trans_rolback() was added into trans_xa_start()
in case if xid_cache_insert is failed() after transaction has been started.
If we don't call trans_rollback() we would never reset the flag
SERVER_STATUS_IN_TRANS in THD::server_status and therefore all subsequent
attempts to execute XA START in the connection where the error was occurred
will be failed since thd->in_active_multi_stmt_transaction() will return
the true every time when trans_xa_start is called.
  
The latest changes were absent in patch for mysql-5.1