1 files changed, 40 insertions, 0 deletions
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 9874c3eaa04..c664984d0a1 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -4114,3 +4114,43 @@ IssuePendingWritebacks(WritebackContext *context)
 
 	context->nr_pending = 0;
 }
+
+
+/*
+ * Check whether the given snapshot is too old to have safely read the given
+ * page from the given table.  If so, throw a "snapshot too old" error.
+ *
+ * This test generally needs to be performed after every BufferGetPage() call
+ * that is executed as part of a scan.  It is not needed for calls made for
+ * modifying the page (for example, to position to the right place to insert a
+ * new index tuple or for vacuuming).  To minimize errors of omission, the
+ * BufferGetPage() macro accepts parameters to specify whether the test should
+ * be run, and supply the necessary snapshot and relation parameters.  See the
+ * declaration of BufferGetPage() for more details.
+ *
+ * Note that a NULL snapshot argument is allowed and causes a fast return
+ * without error; this is to support call sites which can be called from
+ * either scans or index modification areas.
+ *
+ * For best performance, keep the tests that are fastest and/or most likely to
+ * exclude a page from old snapshot testing near the front.
+ */
+extern Page
+TestForOldSnapshot(Snapshot snapshot, Relation relation, Page page)
+{
+	Assert(relation != NULL);
+
+	if (old_snapshot_threshold >= 0
+	 && (snapshot) != NULL
+	 && (snapshot)->satisfies == HeapTupleSatisfiesMVCC
+	 && !XLogRecPtrIsInvalid((snapshot)->lsn)
+	 && PageGetLSN(page) > (snapshot)->lsn
+	 && !IsCatalogRelation(relation)
+	 && !RelationIsAccessibleInLogicalDecoding(relation)
+	 && (snapshot)->whenTaken < GetOldSnapshotThresholdTimestamp())
+		ereport(ERROR,
+				(errcode(ERRCODE_SNAPSHOT_TOO_OLD),
+				 errmsg("snapshot too old")));
+
+	return page;
+}