A REPLACE in the MERGE engine is actually a REPLACE
into one (FIRST or LAST) of the underlying MyISAM
tables. So in effect the server works on the meta
data of the MERGE table, while the real insert happens
in the MyISAM table.

The MERGE table has no index, while MyISAM has a
unique index. When a REPLACE into a MERGE table (
and the REPLACE conflicts with a duplicate in a
child table) is done, we try to access the duplicate
key information for the MERGE table. This information
actually does not exist, hence this results in a crash.

The problem can be resolved by modifying the MERGE
engine to provide us the duplicate key information
directly, instead of just returning the MyISAM index
number as the error key. Then the SQL layer (or "the
server") does not try to access the key_info of the
MERGE table, which does not exist.

The current patch modifies the MERGE engine to provide
the position for a record where a unique key violation
occurs.

include/myisammrg.h:
  Bug#45800 crash when replacing into a merge table and there is a duplicate
  
  Add a member to the st_mymerge_info structure that will
  store the duplicate key offset in the MERGE table. This
  offset will be the sum of the record offset of the MyISAM
  table within the MERGE table and the offset of the record
  within the MyISAM table.
mysql-test/r/merge.result:
  Bug#45800 crash when replacing into a merge table and there is a duplicate
  
  Result file for the test case.
mysql-test/t/merge.test:
  Bug#45800 crash when replacing into a merge table and there is a duplicate
  
  Added test case for both REPLACE and INSERT...ON DUPLICATE UPDATE.
storage/myisammrg/ha_myisammrg.cc:
  Bug#45800 crash when replacing into a merge table and there is a duplicate
  
  The info method now will process the HA_STATUS_ERRKEY flag
  and will return the index and the offset of the duplicate
  key.
storage/myisammrg/ha_myisammrg.h:
  Bug#45800 crash when replacing into a merge table and there is a duplicate
  
  Set the HA_DUPLICATE_POS flag to indicate that the duplicate
  key information is now available in the MERGE storage engine.
storage/myisammrg/myrg_info.c:
  Bug#45800 crash when replacing into a merge table and there is a duplicate
  
  We modify the myrg_status function to return the position of the
  duplicate key. The duplicate key position in the MERGE table will
  be the MyISAM file_offset and the offset within the MyISAM table
  of the start position of the records.