REPLICATION

Problem: In RBR mode, merge table updates are not successfully applied on a cascading
replication.

Analysis & Fix: Every type of row event is preceded by one or more table_map_log_events
that gives the information about all the tables that are involved in the row
event. Server maintains the list in RPL_TABLE_LIST and it goes through all the
tables and checks for the compatibility between master and slave. Before
checking for the compatibility, it calls 'open_tables()' which takes the list
of all tables that needs to be locked and opened. In RBR, because of the
Table_map_log_event , we already have all the tables including base tables in
the list. But the open_tables() which is generic call takes care of appending
base tables if the list contains merge tables. There is an assumption in the
current replication layer logic that these tables (TABLE_LIST type objects) are always
added in the end of the list. Replication layer maintains the count of
tables(tables_to_lock_count) that needs to be verified for compatibility check
and runs through only those many tables from the list and rest of the objects
in linked list can be skipped. But this assumption is wrong.
open_tables()->..->add_children_to_list() adds base tables to the list immediately
after seeing the merge table in the list.

For eg: If the list passed to open_tables() is t1->t2->t3 where t3 is merge
table (and t1 and t2 are base tables), it adds t1'->t2' to the list after t3.
New table list looks like t1->t2->t3->t1'->t2'. It looks like it added at the
end of the list but that is not correct. If the list passed to open_tables()
is t3->t1->t2 where t3 is merge table (and t1 and t2 are base tables), the new
prepared list will be t3->t1'->t2'->t1->t2. Where t1' and t2' are of
TABLE_LIST objects which were added by add_children_to_list() call and replication
layer should not look into them. Here tables_to_lock_count  will not help as the
objects are added in between the list.

Fix: After investigating add_children_list() logic (which is called from open_tables()),
there is no flag/logic in it to skip adding the children to the list even if the
children are already included in the table list. Hence to fix the issue, a
logic should be added in the replication layer to skip children in the list by
checking whether  'parent_l' is non-null or not. If it is children, we will skip 'compatibility'
check for that table.

Also this patch is not removing 'tables_to_lock_count' logic for the performance issues
if there are any children at the end of the list, those can be easily skipped directly by
stopping the loop with tables_to_lock_count check.