Set ReorderBufferTXN->final_lsn more eagerly

... specifically, set it incrementally as each individual change is
spilled down to disk.  This way, it is set correctly when the
transaction disappears without trace, ie. without leaving an XACT_ABORT
wal record.  (This happens when the server crashes midway through a
transaction.)

Failing to have final_lsn prevents ReorderBufferRestoreCleanup() from
working, since it needs the final_lsn in order to know the endpoint of
its iteration through spilled files.

Commit df9f682c7b already tried to fix the problem, but it didn't set
the final_lsn in all cases.  Revert that, since it's no longer needed.

Author: Vignesh C
Reviewed-by: Amit Kapila, Dilip Kumar
Discussion: https://postgr.es/m/CALDaNm2CLk+K9JDwjYST0sPbGg5AQdvhUt0jbKyX_HdAE0jk3A@mail.gmail.com

src/backend/replication/logical/reorderbuffer.c

@@ -1974,21 +1974,6 @@ ReorderBufferAbortOld(ReorderBuffer *rb, TransactionId oldestRunningXid)
 
 		if (TransactionIdPrecedes(txn->xid, oldestRunningXid))
 		{
-			/*
-			 * We set final_lsn on a transaction when we decode its commit or
-			 * abort record, but we never see those records for crashed
-			 * transactions.  To ensure cleanup of these transactions, set
-			 * final_lsn to that of their last change; this causes
-			 * ReorderBufferRestoreCleanup to do the right thing.
-			 */
-			if (rbtxn_is_serialized(txn) && txn->final_lsn == 0)
-			{
-				ReorderBufferChange *last =
-				dlist_tail_element(ReorderBufferChange, node, &txn->changes);
-
-				txn->final_lsn = last->lsn;
-			}
-
 			elog(DEBUG2, "aborting old transaction %u", txn->xid);
 
 			/* remove potential on-disk data, and deallocate this tx */
@@ -2623,8 +2608,7 @@ ReorderBufferSerializeChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
 
 				sz += sizeof(SnapshotData) +
 					sizeof(TransactionId) * snap->xcnt +
-					sizeof(TransactionId) * snap->subxcnt
-					;
+					sizeof(TransactionId) * snap->subxcnt;
 
 				/* make sure we have enough space */
 				ReorderBufferSerializeReserve(rb, sz);
@@ -2697,6 +2681,17 @@ ReorderBufferSerializeChange(ReorderBuffer *rb, ReorderBufferTXN *txn,
 	}
 	pgstat_report_wait_end();
 
+	/*
+	 * Keep the transaction's final_lsn up to date with each change we send to
+	 * disk, so that ReorderBufferRestoreCleanup works correctly.  (We used to
+	 * only do this on commit and abort records, but that doesn't work if a
+	 * system crash leaves a transaction without its abort record).
+	 *
+	 * Make sure not to move it backwards.
+	 */
+	if (txn->final_lsn < change->lsn)
+		txn->final_lsn = change->lsn;
+
 	Assert(ondisk->change.action == change->action);
 }
 

src/include/replication/reorderbuffer.h

@@ -207,9 +207,10 @@ typedef struct ReorderBufferTXN
 	 * * prepared transaction commit
 	 * * plain abort record
 	 * * prepared transaction abort
-	 * * error during decoding
-	 * * for a crashed transaction, the LSN of the last change, regardless of
-	 *   what it was.
+	 *
+	 * This can also become set to earlier values than transaction end when
+	 * a transaction is spilled to disk; specifically it's set to the LSN of
+	 * the latest change written to disk so far.
 	 * ----
 	 */
 	XLogRecPtr	final_lsn;