touched but not actually changed.

The LAST_INSERT_ID() is reset to 0 if no rows were inserted or changed.
This is the case when an INSERT ... ON DUPLICATE KEY UPDATE updates a row
with the same values as the row contains.

Now the LAST_INSERT_ID() values is reset to 0 only if there were no rows
successfully inserted or touched.
The new 'touched' field is added to the COPY_INFO structure. It holds the
number of rows that were touched no matter whether they were actually
changed or not.