Remove remants of "snapshot too old"

Remove the 'whenTaken' and 'lsn' fields from SnapshotData. After the
removal of the "snapshot too old" feature, they were never set to a
non-zero value.

This largely reverts commit 3e2f3c2e42, which added the
OldestActiveSnapshot tracking, and the init_toast_snapshot()
function. That was only required for setting the 'whenTaken' and 'lsn'
fields. SnapshotToast is now a constant again, like SnapshotSelf and
SnapshotAny. I kept a thin get_toast_snapshot() wrapper around
SnapshotToast though, to check that you have a registered or active
snapshot. That's still a useful sanity check.

Reviewed-by: Nathan Bossart, Andres Freund, Tom Lane
Discussion: https://www.postgresql.org/message-id/cd4b4f8c-e63a-41c0-95f6-6e6cd9b83f6d@iki.fi

contrib/amcheck/verify_heapam.c

@@ -1767,7 +1767,6 @@ check_tuple_attribute(HeapCheckContext *ctx)
 static void
 check_toasted_attribute(HeapCheckContext *ctx, ToastedAttribute *ta)
 {
-	SnapshotData SnapshotToast;
 	ScanKeyData toastkey;
 	SysScanDesc toastscan;
 	bool		found_toasttup;
@@ -1791,10 +1790,9 @@ check_toasted_attribute(HeapCheckContext *ctx, ToastedAttribute *ta)
 	 * Check if any chunks for this toasted object exist in the toast table,
 	 * accessible via the index.
 	 */
-	init_toast_snapshot(&SnapshotToast);
 	toastscan = systable_beginscan_ordered(ctx->toast_rel,
 										   ctx->valid_toast_index,
-										   &SnapshotToast, 1,
+										   get_toast_snapshot(), 1,
 										   &toastkey);
 	found_toasttup = false;
 	while ((toasttup =

src/backend/access/common/toast_internals.c

@@ -393,7 +393,6 @@ toast_delete_datum(Relation rel, Datum value, bool is_speculative)
 	HeapTuple	toasttup;
 	int			num_indexes;
 	int			validIndex;
-	SnapshotData SnapshotToast;
 
 	if (!VARATT_IS_EXTERNAL_ONDISK(attr))
 		return;
@@ -425,9 +424,8 @@ toast_delete_datum(Relation rel, Datum value, bool is_speculative)
 	 * 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);
+										   get_toast_snapshot(), 1, &toastkey);
 	while ((toasttup = systable_getnext_ordered(toastscan, ForwardScanDirection)) != NULL)
 	{
 		/*
@@ -631,41 +629,28 @@ toast_close_indexes(Relation *toastidxs, int num_indexes, LOCKMODE lock)
 }
 
 /* ----------
- * init_toast_snapshot
+ * get_toast_snapshot
  *
- *	Initialize an appropriate TOAST snapshot.  We must use an MVCC snapshot
+ *	Return the TOAST snapshot. Detoasting *must* happen in the same
- *	to initialize the TOAST snapshot; since we don't know which one to use,
+ *	transaction that originally fetched the toast pointer.
- *	just use the oldest one.
  */
-void
+Snapshot
-init_toast_snapshot(Snapshot toast_snapshot)
+get_toast_snapshot(void)
 {
-	Snapshot	snapshot = GetOldestSnapshot();
-
 	/*
-	 * GetOldestSnapshot returns NULL if the session has no active snapshots.
+	 * We cannot directly check that detoasting happens in the same
-	 * We can get that if, for example, a procedure fetches a toasted value
+	 * transaction that originally fetched the toast pointer, but at least
-	 * into a local variable, commits, and then tries to detoast the value.
+	 * check that the session has some active snapshots. It might not if, for
-	 * Such coding is unsafe, because once we commit there is nothing to
+	 * example, a procedure fetches a toasted value into a local variable,
-	 * prevent the toast data from being deleted.  Detoasting *must* happen in
+	 * commits, and then tries to detoast the value. Such coding is unsafe,
-	 * the same transaction that originally fetched the toast pointer.  Hence,
+	 * because once we commit there is nothing to prevent the toast data from
-	 * rather than trying to band-aid over the problem, throw an error.  (This
+	 * being deleted. (This is not very much protection, because in many
-	 * is not very much protection, because in many scenarios the procedure
+	 * scenarios the procedure would have already created a new transaction
-	 * would have already created a new transaction snapshot, preventing us
+	 * snapshot, preventing us from detecting the problem. But it's better
-	 * from detecting the problem.  But it's better than nothing, and for sure
+	 * than nothing.)
-	 * we shouldn't expend code on masking the problem more.)
 	 */
-	if (snapshot == NULL)
+	if (!HaveRegisteredOrActiveSnapshot())
 		elog(ERROR, "cannot fetch toast data without an active snapshot");
 
-	/*
+	return &SnapshotToastData;
-	 * Catalog snapshots can be returned by GetOldestSnapshot() even if not
-	 * registered or active. That easily hides bugs around not having a
-	 * snapshot set up - most of the time there is a valid catalog snapshot.
-	 * So additionally insist that the current snapshot is registered or
-	 * active.
-	 */
-	Assert(HaveRegisteredOrActiveSnapshot());
-
-	InitToastSnapshot(*toast_snapshot, snapshot->lsn, snapshot->whenTaken);
 }

src/backend/access/heap/heaptoast.c

@@ -639,7 +639,6 @@ heap_fetch_toast_slice(Relation toastrel, Oid valueid, int32 attrsize,
 	int			endchunk;
 	int			num_indexes;
 	int			validIndex;
-	SnapshotData SnapshotToast;
 
 	/* Look for the valid index of toast relation */
 	validIndex = toast_open_indexes(toastrel,
@@ -685,9 +684,8 @@ heap_fetch_toast_slice(Relation toastrel, Oid valueid, int32 attrsize,
 	}
 
 	/* Prepare for scan */
-	init_toast_snapshot(&SnapshotToast);
 	toastscan = systable_beginscan_ordered(toastrel, toastidxs[validIndex],
-										   &SnapshotToast, nscankeys, toastkey);
+										   get_toast_snapshot(), nscankeys, toastkey);
 
 	/*
 	 * Read the chunks by index

src/backend/storage/ipc/procarray.c

@@ -2135,8 +2135,6 @@ GetSnapshotDataReuse(Snapshot snapshot)
 	snapshot->active_count = 0;
 	snapshot->regd_count = 0;
 	snapshot->copied = false;
-	snapshot->lsn = InvalidXLogRecPtr;
-	snapshot->whenTaken = 0;
 
 	return true;
 }
@@ -2516,8 +2514,6 @@ GetSnapshotData(Snapshot snapshot)
 	snapshot->active_count = 0;
 	snapshot->regd_count = 0;
 	snapshot->copied = false;
-	snapshot->lsn = InvalidXLogRecPtr;
-	snapshot->whenTaken = 0;
 
 	return snapshot;
 }

src/backend/utils/time/snapmgr.c

@@ -83,6 +83,7 @@ static SnapshotData SecondarySnapshotData = {SNAPSHOT_MVCC};
 SnapshotData CatalogSnapshotData = {SNAPSHOT_MVCC};
 SnapshotData SnapshotSelfData = {SNAPSHOT_SELF};
 SnapshotData SnapshotAnyData = {SNAPSHOT_ANY};
+SnapshotData SnapshotToastData = {SNAPSHOT_TOAST};
 
 /* Pointers to valid snapshots */
 static Snapshot CurrentSnapshot = NULL;
@@ -119,9 +120,6 @@ 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 MyProc->xmin.
@@ -199,8 +197,6 @@ typedef struct SerializedSnapshotData
 	bool		suboverflowed;
 	bool		takenDuringRecovery;
 	CommandId	curcid;
-	TimestampTz whenTaken;
-	XLogRecPtr	lsn;
 } SerializedSnapshotData;
 
 /*
@@ -313,36 +309,6 @@ GetLatestSnapshot(void)
 	return SecondarySnapshot;
 }
 
-/*
- * GetOldestSnapshot
- *
- *		Get the transaction's oldest known snapshot, as judged by the LSN.
- *		Will return NULL if there are no active or registered snapshots.
- */
-Snapshot
-GetOldestSnapshot(void)
-{
-	Snapshot	OldestRegisteredSnapshot = NULL;
-	XLogRecPtr	RegisteredLSN = InvalidXLogRecPtr;
-
-	if (!pairingheap_is_empty(&RegisteredSnapshots))
-	{
-		OldestRegisteredSnapshot = pairingheap_container(SnapshotData, ph_node,
-														 pairingheap_first(&RegisteredSnapshots));
-		RegisteredLSN = OldestRegisteredSnapshot->lsn;
-	}
-
-	if (OldestActiveSnapshot != NULL)
-	{
-		XLogRecPtr	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
@@ -684,8 +650,6 @@ PushActiveSnapshotWithLevel(Snapshot snapshot, int snap_level)
 	newactive->as_snap->active_count++;
 
 	ActiveSnapshot = newactive;
-	if (OldestActiveSnapshot == NULL)
-		OldestActiveSnapshot = ActiveSnapshot;
 }
 
 /*
@@ -756,8 +720,6 @@ PopActiveSnapshot(void)
 
 	pfree(ActiveSnapshot);
 	ActiveSnapshot = newstack;
-	if (ActiveSnapshot == NULL)
-		OldestActiveSnapshot = NULL;
 
 	SnapshotResetXmin();
 }
@@ -902,13 +864,6 @@ xmin_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg)
  * dropped.  For efficiency, we only consider recomputing PGPROC->xmin when
  * the active snapshot stack is empty; this allows us not to need to track
  * which active snapshot is oldest.
- *
- * Note: it's tempting to use GetOldestSnapshot() here so that we can include
- * active snapshots in the calculation.  However, that compares by LSN not
- * xmin so it's not entirely clear that it's the same thing.  Also, we'd be
- * critically dependent on the assumption that the bottommost active snapshot
- * stack entry has the oldest xmin.  (Current uses of GetOldestSnapshot() are
- * not actually critical, but this would be.)
  */
 static void
 SnapshotResetXmin(void)
@@ -980,8 +935,6 @@ AtSubAbort_Snapshot(int level)
 		pfree(ActiveSnapshot);
 
 		ActiveSnapshot = next;
-		if (ActiveSnapshot == NULL)
-			OldestActiveSnapshot = NULL;
 	}
 
 	SnapshotResetXmin();
@@ -1065,7 +1018,6 @@ AtEOXact_Snapshot(bool isCommit, bool resetXmin)
 	 * it'll go away with TopTransactionContext.
 	 */
 	ActiveSnapshot = NULL;
-	OldestActiveSnapshot = NULL;
 	pairingheap_reset(&RegisteredSnapshots);
 
 	CurrentSnapshot = NULL;
@@ -1727,8 +1679,6 @@ SerializeSnapshot(Snapshot snapshot, char *start_address)
 	serialized_snapshot.suboverflowed = snapshot->suboverflowed;
 	serialized_snapshot.takenDuringRecovery = snapshot->takenDuringRecovery;
 	serialized_snapshot.curcid = snapshot->curcid;
-	serialized_snapshot.whenTaken = snapshot->whenTaken;
-	serialized_snapshot.lsn = snapshot->lsn;
 
 	/*
 	 * Ignore the SubXID array if it has overflowed, unless the snapshot was
@@ -1801,8 +1751,6 @@ RestoreSnapshot(char *start_address)
 	snapshot->suboverflowed = serialized_snapshot.suboverflowed;
 	snapshot->takenDuringRecovery = serialized_snapshot.takenDuringRecovery;
 	snapshot->curcid = serialized_snapshot.curcid;
-	snapshot->whenTaken = serialized_snapshot.whenTaken;
-	snapshot->lsn = serialized_snapshot.lsn;
 	snapshot->snapXactCompletionCount = 0;
 
 	/* Copy XIDs, if present. */

src/include/access/genam.h

@@ -14,9 +14,11 @@
 #ifndef GENAM_H
 #define GENAM_H
 
+#include "access/htup.h"
 #include "access/sdir.h"
 #include "access/skey.h"
 #include "nodes/tidbitmap.h"
+#include "storage/buf.h"
 #include "storage/lockdefs.h"
 #include "utils/relcache.h"
 #include "utils/snapshot.h"

src/include/access/toast_internals.h

@@ -58,6 +58,6 @@ extern int	toast_open_indexes(Relation toastrel,
 							   int *num_indexes);
 extern void toast_close_indexes(Relation *toastidxs, int num_indexes,
 								LOCKMODE lock);
-extern void init_toast_snapshot(Snapshot toast_snapshot);
+extern Snapshot get_toast_snapshot(void);
 
 #endif							/* TOAST_INTERNALS_H */

src/include/storage/predicate.h

@@ -14,6 +14,7 @@
 #ifndef PREDICATE_H
 #define PREDICATE_H
 
+#include "storage/itemptr.h"
 #include "storage/lock.h"
 #include "utils/relcache.h"
 #include "utils/snapshot.h"

src/include/utils/snapmgr.h

@@ -27,11 +27,14 @@ extern PGDLLIMPORT TransactionId RecentXmin;
 /* Variables representing various special snapshot semantics */
 extern PGDLLIMPORT SnapshotData SnapshotSelfData;
 extern PGDLLIMPORT SnapshotData SnapshotAnyData;
+extern PGDLLIMPORT SnapshotData SnapshotToastData;
 extern PGDLLIMPORT SnapshotData CatalogSnapshotData;
 
 #define SnapshotSelf		(&SnapshotSelfData)
 #define SnapshotAny			(&SnapshotAnyData)
 
+/* Use get_toast_snapshot() for the TOAST snapshot */
+
 /*
  * We don't provide a static SnapshotDirty variable because it would be
  * non-reentrant.  Instead, users of that snapshot type should declare a
@@ -49,15 +52,6 @@ extern PGDLLIMPORT SnapshotData CatalogSnapshotData;
 	((snapshotdata).snapshot_type = SNAPSHOT_NON_VACUUMABLE, \
 	 (snapshotdata).vistest = (vistestp))
 
-/*
- * Similarly, some initialization is required for SnapshotToast.  We need
- * to set lsn and whenTaken correctly to support snapshot_too_old.
- */
-#define InitToastSnapshot(snapshotdata, l, w)  \
-	((snapshotdata).snapshot_type = SNAPSHOT_TOAST, \
-	 (snapshotdata).lsn = (l),					\
-	 (snapshotdata).whenTaken = (w))
-
 /* This macro encodes the knowledge of which snapshots are MVCC-safe */
 #define IsMVCCSnapshot(snapshot)  \
 	((snapshot)->snapshot_type == SNAPSHOT_MVCC || \
@@ -66,7 +60,6 @@ extern PGDLLIMPORT SnapshotData CatalogSnapshotData;
 extern Snapshot GetTransactionSnapshot(void);
 extern Snapshot GetLatestSnapshot(void);
 extern void SnapshotSetCommandId(CommandId curcid);
-extern Snapshot GetOldestSnapshot(void);
 
 extern Snapshot GetCatalogSnapshot(Oid relid);
 extern Snapshot GetNonHistoricCatalogSnapshot(Oid relid);

src/include/utils/snapshot.h

@@ -13,11 +13,7 @@
 #ifndef SNAPSHOT_H
 #define SNAPSHOT_H
 
-#include "access/htup.h"
-#include "access/xlogdefs.h"
-#include "datatype/timestamp.h"
 #include "lib/pairingheap.h"
-#include "storage/buf.h"
 
 
 /*
@@ -205,9 +201,6 @@ typedef struct SnapshotData
 	uint32		regd_count;		/* refcount on RegisteredSnapshots */
 	pairingheap_node ph_node;	/* link in the RegisteredSnapshots heap */
 
-	TimestampTz whenTaken;		/* timestamp when snapshot was taken */
-	XLogRecPtr	lsn;			/* position in the WAL stream when taken */
-
 	/*
 	 * The transaction completion count at the time GetSnapshotData() built
 	 * this snapshot. Allows to avoid re-computing static snapshots when no