[PATCH] Extending FK check skipping on replicas to ADD FK and TRUNCATE

Hello Everybody,

Currently setting `session_replication_role` to `replica` disables
foreign key checks allowing, among other things, free table copy order
and faster CDC apply in logical replication.

But two other cases of foreign keys are still restricted or blocked
even with this setting

1. Foreign Key checks during `ALTER TABLE <fkt> ADD FOREIGN KEY
(<fkcol>) REFERENCES <pkt>(<pkcol>);`

These can be really, Really, REALLY slow when the PK table is huge
(hundreds of billions of rows). And here I mean taking-tens-of-days
slow in extreme cases.

And they are also completely unnecessary when the table data comes
from a known good source.

2. `TRUNCATE <referenced table>` is blocked when there are any foreign
keys referencing the <referenced table>

But you still can mess up your database in exactly the same way by
running `DELETE FROM <referenced table>`, just a little (or a lot)
slower.

The attached patch fixes this, allowing both cases to work when `SET
session_replication_role=replica;` is in force:

### Test for `ALTER TABLE <fkt> ADD FOREIGN KEY (<fkcol>) REFERENCES
<pkt>(<pkcol>);`

CREATE TABLE pkt(id int PRIMARY KEY);
CREATE TABLE fkt(fk int);
INSERT INTO fkt VALUES(1);

ALTER TABLE fkt ADD FOREIGN KEY (fk) REFERENCES pkt(id); -- fails

SET session_replication_role=replica;
ALTER TABLE fkt ADD FOREIGN KEY (fk) REFERENCES pkt(id); -- now succeeds

### Test for `TRUNCATE <referenced table>`

CREATE TABLE pkt(id int PRIMARY KEY);
CREATE TABLE fkt(fk int REFERENCES pkt(id));

TRUNCATE pkt; -- fails

SET session_replication_role=replica;
TRUNCATE pkt; -- now succeeds

The patch just wraps two sections of code in

if (SessionReplicationRole != SESSION_REPLICATION_ROLE_REPLICA) {
...
}

and the rest is added indentation for the wrapped.

Plus I added tests, including the until now missing test for
explicitly the foreign key checks (which can be argued to already be
covered by generic trigger tests)

I am not sure if we need to add anything to current documentation[*]
which says just this about foreign keys and
`session_replication_role=replica`

> Since foreign keys are implemented as triggers, setting this parameter to replica also disables all foreign key checks, which can leave data in an inconsistent state if improperly used.

[*] https://www.postgresql.org/docs/17/runtime-config-client.html#GUC-SESSION-REPLICATION-ROLE

Any comments and suggestions are welcome