Support replication origin advance for xid-less commits

[GremSnoort]

Summary

This patch adds a transaction-level integration point to PostgreSQL commit machinery to support correct replication origin advancement for top-level transactions that reach commit without a heap XID but still have a durable local commit LSN.

The immediate motivation is fixing logical replication failures observed with storage engines that use a non-heap-XID commit model, such as OrioleDB.

Problem

PostgreSQL currently assumes the following model:

• if a transaction has a top-level heap xid, its durable commit point is known through the regular commit WAL path
• if a transaction has no top-level heap xid, it is effectively treated as outside the ordinary commit/origin path

That assumption is too narrow for storage engines that can:

• commit without assigning a top-level heap xid
• still produce a real durable commit effect
• still have a valid local commit LSN in PostgreSQL WAL space

OrioleDB is one such case.

As a result, a logical replication apply worker can successfully apply changes and reach commit, but if the transaction has no top-level heap xid, PostgreSQL does not treat it as a participant in normal replication-origin advancement.

This breaks restart/replay correctness for subscriber-side logical apply.

Root Cause

RecordTransactionCommit() currently ties replication-origin advancement to the ordinary heap-XID commit path.

Conceptually:

• markXidCommitted == true
  • regular commit WAL path exists
  • commit LSN is known
  • replication origin is advanced normally
• markXidCommitted == false
  • no regular commit record is produced
  • no generic way exists for PostgreSQL to ask whether the transaction still has a durable local commit point
  • replication origin is not advanced through the same commit/origin protocol

This means PostgreSQL currently has no general abstraction for:

• xid-less top-level transaction
• with durable commit effect
• with valid local commit LSN
• requiring correct replication-origin handling

Solution

Introduce a new transaction-level hook: get_xidless_commit_lsn_hook.

Its contract is narrow:

• it is consulted only from RecordTransactionCommit()
• only for top-level transactions that reach commit without a heap xid
• if the extension can provide a durable local commit LSN for the current transaction, it returns that LSN

This lets PostgreSQL remain the owner of commit/origin policy, while allowing a storage engine to provide the missing durable anchor.