Hemant Dangi 1dd4889d95 MDEV-25039: MDL BF-BF conflict because of foreign key ...

Issue:
The Commit: 0584846 'Add TL_FIRST_WRITE in SQL layer for determining R/W'
limits INSERT statements on write nodes to acquire MDL locks on it's all child
tables and thereby wsrep certification keys added, but on applier nodes it does
acquire MDL locks for all child tables. This can result into MDL BF-BF conflict
on applier node when transactions referring to parent and child tables are
executed concurrently. For example:

Tables with foreign keys: t1<-t2<-t3<-t4
Conflicting transactions: INSERT t1 and DROP TABLE t4

Wsrep certification keys taken on write node:
- for INSERT t1: t1 and t2
- for DROP TABLE t4: t4

On applier node MDL BF-BF conflict happened between two transaction because
MDL locks on t1, t2, t3 and t4 were taken for INSERT t1, which conflicted
with MDL lock on t4 taken by DROP TABLE t4.
The Wsrep certification keys helps in resolving this MDL BF-BF conflict by
prioritizing and scheduling concurrent transactions. But to generate Wsrep
certification keys it needs to open and take MDL locks on all the child tables.

The Commit: 0584846 change limits MDL lock to be taken on all child nodes for
read-only FK checks (INSERT t1). But this doesn't works on applier nodes
because Write_rows_log_event event logged for INSERT is also used to record
update (check Write_rows_log_event::get_trg_event_map()), and
therefore MDL locks is taken for all the child tables on applier node
for update and insert event.

Solution:
Additional keys for the referenced/foreign table needs to be added
to avoid potential MDL conflicts with concurrent update and DDLs.

2025-10-07 14:08:35 +05:30

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