a REPLACE command. 


innobase/lock/lock0lock.c:
  Gap type locks without LOCK_INSERT_INTENSION flag do not need to wait for
  anything. This is because different users can have conflicting lock types
  on gaps.
innobase/row/row0ins.c:
  The manual defines the REPLACE semantics that it is either an INSERT or 
  DELETE(s) for duplicate key + INSERT. Therefore, we take X-lock for 
  duplicates.