UNDER THE SAME CONDITIONS.
ISSUE   :
In myisam, Suppose in btree index of varchar type we have
keys with and without trailing spaces example keys 'abc',
'abc ', 'abc '. During index search based on same key,
length of the lastkey get changed if lastkey read is the
one with trailing spaces. If x is the length of key 'abc'.
After reading 'abc ' length will be x + 1. And,
last_rkey_length should be recalculated as accordingly
whenever lastkey changes.
But in function mi_rnext_same during BTREE search when we
tried to copy lastkey to lastkey2 we have used non updated
last_rkey_length even though lastkey and its length might
have been changed as explained above. And, last_rkey_length
is computed only once during mi_rkey. Because of this
invalid length usage compare_key failed and scan got
terminated. And, hence Update command ended before updating
further tuples which will satisfy the condition.

SOLUTION :
In function mi_rnext_same the in...