Post-commit review fixes for 228c370868.

This commit fixes three issues:

1) When a disabled subscription is created with retain_dead_tuples set to true,
the launcher is not woken up immediately, which may lead to delays in creating
the conflict detection slot.

Creating the conflict detection slot is essential even when the subscription is
not enabled. This ensures that dead tuples are retained, which is necessary for
accurately identifying the type of conflict during replication.

2) Conflict-related data was unnecessarily retained when the subscription does
not have a table.

3) Conflict-relevant data could be prematurely removed before applying
prepared transactions on the publisher that are in the commit critical section.

This issue occurred because the backend executing COMMIT PREPARED was not
accounted for during the computation of oldestXid in the commit phase on
the publisher. As a result, the subscriber could advance the conflict
slot's xmin without waiting for such COMMIT PREPARED transactions to
complete.

We fixed this issue by identifying prepared transactions that are in the
commit critical section during computation of oldestXid in commit phase.

Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Reviewed-by: Nisha Moond <nisha.moond412@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/OS9PR01MB16913DACB64E5721872AA5C02943BA@OS9PR01MB16913.jpnprd01.prod.outlook.com
Discussion: https://postgr.es/m/OS9PR01MB16913F67856B0DA2A909788129400A@OS9PR01MB16913.jpnprd01.prod.outlook.com

1 files changed, 55 insertions, 0 deletions

diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
index 7918176fc58..3e20f448787 100644
--- a/src/backend/access/transam/twophase.c
+++ b/src/backend/access/transam/twophase.c
@@ -2809,3 +2809,58 @@ LookupGXactBySubid(Oid subid)
 
 	return found;
 }
+
+/*
+ * TwoPhaseGetXidByLockingProc
+ *		Return the oldest transaction ID from prepared transactions that are
+ *		currently in the commit critical section.
+ *
+ * This function only considers transactions in the currently connected
+ * database. If no matching transactions are found, it returns
+ * InvalidTransactionId.
+ */
+TransactionId
+TwoPhaseGetOldestXidInCommit(void)
+{
+	TransactionId oldestRunningXid = InvalidTransactionId;
+
+	LWLockAcquire(TwoPhaseStateLock, LW_SHARED);
+
+	for (int i = 0; i < TwoPhaseState->numPrepXacts; i++)
+	{
+		GlobalTransaction gxact = TwoPhaseState->prepXacts[i];
+		PGPROC	   *commitproc;
+		TransactionId xid;
+
+		if (!gxact->valid)
+			continue;
+
+		if (gxact->locking_backend == INVALID_PROC_NUMBER)
+			continue;
+
+		/*
+		 * Get the backend that is handling the transaction. It's safe to
+		 * access this backend while holding TwoPhaseStateLock, as the backend
+		 * can only be destroyed after either removing or unlocking the
+		 * current global transaction, both of which require an exclusive
+		 * TwoPhaseStateLock.
+		 */
+		commitproc = GetPGProcByNumber(gxact->locking_backend);
+
+		if (MyDatabaseId != commitproc->databaseId)
+			continue;
+
+		if ((commitproc->delayChkptFlags & DELAY_CHKPT_IN_COMMIT) == 0)
+			continue;
+
+		xid = XidFromFullTransactionId(gxact->fxid);
+
+		if (!TransactionIdIsValid(oldestRunningXid) ||
+			TransactionIdPrecedes(xid, oldestRunningXid))
+			oldestRunningXid = xid;
+	}
+
+	LWLockRelease(TwoPhaseStateLock);
+
+	return oldestRunningXid;
+}