RE: Slow catchup of 2PC (twophase) transactions on replica in LR

Dear hackers,

> Looking at this a bit more, maybe rolling back all prepared transactions on the
> subscriber when toggling two_phase from true to false might not be desirable
> for the customer. Maybe we should have an option for customers to control
> whether transactions should be rolled back or not. Maybe transactions should
> only be rolled back if a "force" option is also set. What do people think?

And here is a patch for adds new option "force_alter" (better name is very welcome).
It could be used only when altering two_phase option. Let me share examples.

Assuming that there are logical replication system with two_phase = on, and
there are prepared transactions:

```
subscriber=# SELECT * FROM pg_prepared_xacts ;
transaction | gid | prepared | owner | database
-------------+------------------+-------------------------------+----------+----------
741 | pg_gid_16390_741 | 2024-04-22 08:02:34.727913+00 | postgres | postgres
742 | pg_gid_16390_742 | 2024-04-22 08:02:34.729486+00 | postgres | postgres
(2 rows)
```

At that time, altering two_phase to false alone will be failed:

```
subscriber=# ALTER SUBSCRIPTION sub DISABLE ;
ALTER SUBSCRIPTION
subscriber=# ALTER SUBSCRIPTION sub SET (two_phase = off);
ERROR: cannot alter two_phase = false when there are prepared transactions
```

It succeeds if force_alter is also expressly set. Prepared transactions will be
aborted at that time.

```
subscriber=# ALTER SUBSCRIPTION sub SET (two_phase = off, force_alter = on);
ALTER SUBSCRIPTION
subscriber=# SELECT * FROM pg_prepared_xacts ;
transaction | gid | prepared | owner | database
-------------+-----+----------+-------+----------
(0 rows)
```

Best Regards,
Hayato Kuroda
FUJITSU LIMITED
https://www.fujitsu.com/global/