RelationNeedsWAL-to-relpersistence-sto-v8nm.patch

Author:     Noah Misch <noah@leadboat.com>
Commit:     Noah Misch <noah@leadboat.com>

    Revive "snapshot too old" with wal_level=minimal and SET TABLESPACE.
    
    Given a permanent relation rewritten in the current transaction, the
    old_snapshot_threshold mechanism assumed the relation had never been
    subject to early pruning.  Hence, a query could fail to report "snapshot
    too old" when the rewrite followed an early truncation.  ALTER TABLE SET
    TABLESPACE is probably the only rewrite mechanism capable of exposing
    this bug.  REINDEX sets indcheckxmin, avoiding the problem.  CLUSTER has
    zeroed page LSNs since before old_snapshot_threshold existed, so
    old_snapshot_threshold has never cooperated with it.  ALTER TABLE
    ... SET DATA TYPE makes the table look empty to every past snapshot,
    which is strictly worse.  Back-patch to v13, where commit
    c6b92041d38512a4176ed76ad06f713d2e6c01a8 broke this.
    
    Kyotaro Horiguchi and Noah Misch
    
    Discussion: https://postgr.es/m/20210113.160705.2225256954956139776.horikyota.ntt@gmail.com

diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index ae16c3e..9570426 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -1764,7 +1764,11 @@ TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
 	Assert(OldSnapshotThresholdActive());
 	Assert(limit_ts != NULL && limit_xid != NULL);
 
-	if (!RelationAllowsEarlyPruning(relation))
+	/*
+	 * TestForOldSnapshot() assumes early pruning advances the page LSN, so we
+	 * can't prune early when skipping WAL.
+	 */
+	if (!RelationAllowsEarlyPruning(relation) || !RelationNeedsWAL(relation))
 		return false;
 
 	ts = GetSnapshotCurrentTimestamp();
diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index 579be35..c21ee3c 100644
--- a/src/include/utils/snapmgr.h
+++ b/src/include/utils/snapmgr.h
@@ -37,7 +37,7 @@
  */
 #define RelationAllowsEarlyPruning(rel) \
 ( \
-	 RelationNeedsWAL(rel) \
+	 (rel)->rd_rel->relpersistence == RELPERSISTENCE_PERMANENT	\
   && !IsCatalogRelation(rel) \
   && !RelationIsAccessibleInLogicalDecoding(rel) \
 )