Prevent "snapshot too old" from trying to return pruned TOAST tuples.

Previously, we tested for MVCC snapshots to see whether they were too
old, but not TOAST snapshots, which can lead to complaints about missing
TOAST chunks if those chunks are subject to early pruning.  Ideally,
the threshold lsn and timestamp for a TOAST snapshot would be that of
the corresponding MVCC snapshot, but since we have no way of deciding
which MVCC snapshot was used to fetch the TOAST pointer, use the oldest
active or registered snapshot instead.

Reported by Andres Freund, who also sketched out what the fix should
look like.  Patch by me, reviewed by Amit Kapila.

3 files changed, 71 insertions, 5 deletions

diff --git a/src/backend/access/heap/tuptoaster.c b/src/backend/access/heap/tuptoaster.c
index 4cffd21d18..55b6b41f8c 100644
--- a/src/backend/access/heap/tuptoaster.c
+++ b/src/backend/access/heap/tuptoaster.c
@@ -40,6 +40,7 @@
 #include "utils/expandeddatum.h"
 #include "utils/fmgroids.h"
 #include "utils/rel.h"
+#include "utils/snapmgr.h"
 #include "utils/typcache.h"
 #include "utils/tqual.h"
 
@@ -81,6 +82,7 @@ static int toast_open_indexes(Relation toastrel,
 				   int *num_indexes);
 static void toast_close_indexes(Relation *toastidxs, int num_indexes,
 					LOCKMODE lock);
+static void init_toast_snapshot(Snapshot toast_snapshot);
 
 
 /* ----------
@@ -1665,6 +1667,7 @@ toast_delete_datum(Relation rel, Datum value)
 	HeapTuple	toasttup;
 	int			num_indexes;
 	int			validIndex;
+	SnapshotData	SnapshotToast;
 
 	if (!VARATT_IS_EXTERNAL_ONDISK(attr))
 		return;
@@ -1696,8 +1699,9 @@ toast_delete_datum(Relation rel, Datum value)
 	 * sequence or not, but since we've already locked the index we might as
 	 * well use systable_beginscan_ordered.)
 	 */
+	init_toast_snapshot(&SnapshotToast);
 	toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
-										   SnapshotToast, 1, &toastkey);
+										   &SnapshotToast, 1, &toastkey);
 	while ((toasttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
 	{
 		/*
@@ -1730,6 +1734,7 @@ toastrel_valueid_exists(Relation toastrel, Oid valueid)
 	int			num_indexes;
 	int			validIndex;
 	Relation   *toastidxs;
+	SnapshotData	SnapshotToast;
 
 	/* Fetch a valid index relation */
 	validIndex = toast_open_indexes(toastrel,
@@ -1748,9 +1753,10 @@ toastrel_valueid_exists(Relation toastrel, Oid valueid)
 	/*
 	 * Is there any such chunk?
 	 */
+	init_toast_snapshot(&SnapshotToast);
 	toastscan = systable_beginscan(toastrel,
 								   RelationGetRelid(toastidxs[validIndex]),
-								   true, SnapshotToast, 1, &toastkey);
+								   true, &SnapshotToast, 1, &toastkey);
 
 	if (systable_getnext(toastscan) != NULL)
 		result = true;
@@ -1813,6 +1819,7 @@ toast_fetch_datum(struct varlena * attr)
 	int32		chunksize;
 	int			num_indexes;
 	int			validIndex;
+	SnapshotData	SnapshotToast;
 
 	if (!VARATT_IS_EXTERNAL_ONDISK(attr))
 		elog(ERROR, "toast_fetch_datum shouldn't be called for non-ondisk datums");
@@ -1859,8 +1866,9 @@ toast_fetch_datum(struct varlena * attr)
 	 */
 	nextidx = 0;
 
+	init_toast_snapshot(&SnapshotToast);
 	toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
-										   SnapshotToast, 1, &toastkey);
+										   &SnapshotToast, 1, &toastkey);
 	while ((ttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
 	{
 		/*
@@ -1990,6 +1998,7 @@ toast_fetch_datum_slice(struct varlena * attr, int32 sliceoffset, int32 length)
 	int32		chcpyend;
 	int			num_indexes;
 	int			validIndex;
+	SnapshotData	SnapshotToast;
 
 	if (!VARATT_IS_EXTERNAL_ONDISK(attr))
 		elog(ERROR, "toast_fetch_datum_slice shouldn't be called for non-ondisk datums");
@@ -2082,9 +2091,10 @@ toast_fetch_datum_slice(struct varlena * attr, int32 sliceoffset, int32 length)
 	 *
 	 * The index is on (valueid, chunkidx) so they will come in order
 	 */
+	init_toast_snapshot(&SnapshotToast);
 	nextidx = startchunk;
 	toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
-										 SnapshotToast, nscankeys, toastkey);
+										 &SnapshotToast, nscankeys, toastkey);
 	while ((ttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
 	{
 		/*
@@ -2289,3 +2299,22 @@ toast_close_indexes(Relation *toastidxs, int num_indexes, LOCKMODE lock)
 		index_close(toastidxs[i], lock);
 	pfree(toastidxs);
 }
+
+/* ----------
+ * init_toast_snapshot
+ *
+ *	Initialize an appropriate TOAST snapshot.  We must use an MVCC snapshot
+ *	to initialize the TOAST snapshot; since we don't know which one to use,
+ *	just use the oldest one.  This is safe: at worst, we will get a "snapshot
+ *	too old" error that might have been avoided otherwise.
+ */
+static void
+init_toast_snapshot(Snapshot toast_snapshot)
+{
+	Snapshot	snapshot = GetOldestSnapshot();
+
+	if (snapshot == NULL)
+		elog(ERROR, "no known snapshots");
+
+	InitToastSnapshot(toast_snapshot, snapshot->lsn, snapshot->whenTaken);
+}
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index e1caf01c60..4bddaed33b 100644
--- a/src/backend/utils/time/snapmgr.c
+++ b/src/backend/utils/time/snapmgr.c
@@ -188,6 +188,9 @@ typedef struct ActiveSnapshotElt
 /* Top of the stack of active snapshots */
 static ActiveSnapshotElt *ActiveSnapshot = NULL;
 
+/* Bottom of the stack of active snapshots */
+static ActiveSnapshotElt *OldestActiveSnapshot = NULL;
+
 /*
  * Currently registered Snapshots.  Ordered in a heap by xmin, so that we can
  * quickly find the one with lowest xmin, to advance our MyPgXat->xmin.
@@ -394,6 +397,34 @@ GetLatestSnapshot(void)
 }
 
 /*
+ * GetOldestSnapshot
+ *
+ *		Get the oldest known snapshot, as judged by the LSN.
+ */
+Snapshot
+GetOldestSnapshot(void)
+{
+	Snapshot	OldestRegisteredSnapshot = NULL;
+	XLogRecPtr	RegisteredLSN = InvalidXLogRecPtr;
+	XLogRecPtr	ActiveLSN = InvalidXLogRecPtr;
+
+	if (!pairingheap_is_empty(&RegisteredSnapshots))
+	{
+		OldestRegisteredSnapshot = pairingheap_container(SnapshotData, ph_node,
+									pairingheap_first(&RegisteredSnapshots));
+		RegisteredLSN = OldestRegisteredSnapshot->lsn;
+	}
+
+	if (OldestActiveSnapshot != NULL)
+		ActiveLSN = OldestActiveSnapshot->as_snap->lsn;
+
+	if (XLogRecPtrIsInvalid(RegisteredLSN) || RegisteredLSN > ActiveLSN)
+		return OldestActiveSnapshot->as_snap;
+
+	return OldestRegisteredSnapshot;
+}
+
+/*
  * GetCatalogSnapshot
  *		Get a snapshot that is sufficiently up-to-date for scan of the
  *		system catalog with the specified OID.
@@ -674,6 +705,8 @@ PushActiveSnapshot(Snapshot snap)
 	newactive->as_snap->active_count++;
 
 	ActiveSnapshot = newactive;
+	if (OldestActiveSnapshot == NULL)
+		OldestActiveSnapshot = ActiveSnapshot;
 }
 
 /*
@@ -744,6 +777,8 @@ PopActiveSnapshot(void)
 
 	pfree(ActiveSnapshot);
 	ActiveSnapshot = newstack;
+	if (ActiveSnapshot == NULL)
+		OldestActiveSnapshot = NULL;
 
 	SnapshotResetXmin();
 }
@@ -953,6 +988,8 @@ AtSubAbort_Snapshot(int level)
 		pfree(ActiveSnapshot);
 
 		ActiveSnapshot = next;
+		if (ActiveSnapshot == NULL)
+			OldestActiveSnapshot = NULL;
 	}
 
 	SnapshotResetXmin();
@@ -1037,6 +1074,7 @@ AtEOXact_Snapshot(bool isCommit)
 	 * it'll go away with TopTransactionContext.
 	 */
 	ActiveSnapshot = NULL;
+	OldestActiveSnapshot = NULL;
 	pairingheap_reset(&RegisteredSnapshots);
 
 	CurrentSnapshot = NULL;
diff --git a/src/backend/utils/time/tqual.c b/src/backend/utils/time/tqual.c
index 2960455d6e..d99c847000 100644
--- a/src/backend/utils/time/tqual.c
+++ b/src/backend/utils/time/tqual.c
@@ -78,7 +78,6 @@
 /* Static variables representing various special snapshot semantics */
 SnapshotData SnapshotSelfData = {HeapTupleSatisfiesSelf};
 SnapshotData SnapshotAnyData = {HeapTupleSatisfiesAny};
-SnapshotData SnapshotToastData = {HeapTupleSatisfiesToast};
 
 /* local functions */
 static bool XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot);