Change snapshot type to be determined by enum rather than callback.

This is in preparation for allowing the same snapshot be used for
different table AMs. With the current callback based approach we would
need one callback for each supported AM, which clearly would not be
extensible.  Thus add a new Snapshot->snapshot_type field, and move
the dispatch into HeapTupleSatisfiesVisibility() (which is now a
function). Later work will then dispatch calls to
HeapTupleSatisfiesVisibility() and other AMs visibility functions
depending on the type of the table.  The central SnapshotType enum
also seems like a good location to centralize documentation about the
intended behaviour of various types of snapshots.

As tqual.h isn't included by bufmgr.h any more (as HeapTupleSatisfies*
isn't referenced by TestForOldSnapshot() anymore) a few files now need
to include it directly.

Author: Andres Freund, loosely based on earlier work by Haribabu Kommi
Discussion:
    https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de
    https://postgr.es/m/20160812231527.GA690404@alvherre.pgsql

Author: anarazel

contrib/amcheck/verify_nbtree.c

@@ -36,6 +36,7 @@
 #include "storage/lmgr.h"
 #include "utils/memutils.h"
 #include "utils/snapmgr.h"
+#include "utils/tqual.h"
 
 
 PG_MODULE_MAGIC;

contrib/pg_visibility/pg_visibility.c

@@ -21,6 +21,8 @@
 #include "storage/procarray.h"
 #include "storage/smgr.h"
 #include "utils/rel.h"
+#include "utils/snapmgr.h"
+#include "utils/tqual.h"
 
 PG_MODULE_MAGIC;
 

src/backend/access/nbtree/nbtsort.c

@@ -72,6 +72,7 @@
 #include "utils/rel.h"
 #include "utils/sortsupport.h"
 #include "utils/tuplesort.h"
+#include "utils/tqual.h"
 
 
 /* Magic numbers for parallel state sharing */

src/backend/replication/logical/snapbuild.c

@@ -376,7 +376,7 @@ static void
 SnapBuildFreeSnapshot(Snapshot snap)
 {
 	/* make sure we don't get passed an external snapshot */
-	Assert(snap->satisfies == HeapTupleSatisfiesHistoricMVCC);
+	Assert(snap->snapshot_type == SNAPSHOT_HISTORIC_MVCC);
 
 	/* make sure nobody modified our snapshot */
 	Assert(snap->curcid == FirstCommandId);
@@ -434,7 +434,7 @@ void
 SnapBuildSnapDecRefcount(Snapshot snap)
 {
 	/* make sure we don't get passed an external snapshot */
-	Assert(snap->satisfies == HeapTupleSatisfiesHistoricMVCC);
+	Assert(snap->snapshot_type == SNAPSHOT_HISTORIC_MVCC);
 
 	/* make sure nobody modified our snapshot */
 	Assert(snap->curcid == FirstCommandId);
@@ -476,7 +476,7 @@ SnapBuildBuildSnapshot(SnapBuild *builder)
 
 	snapshot = MemoryContextAllocZero(builder->context, ssize);
 
-	snapshot->satisfies = HeapTupleSatisfiesHistoricMVCC;
+	snapshot->snapshot_type = SNAPSHOT_HISTORIC_MVCC;
 
 	/*
 	 * We misuse the original meaning of SnapshotData's xip and subxip fields

src/backend/utils/time/snapmgr.c

@@ -141,9 +141,9 @@ static volatile OldSnapshotControlData *oldSnapshotControl;
  * These SnapshotData structs are static to simplify memory allocation
  * (see the hack in GetSnapshotData to avoid repeated malloc/free).
  */
-static SnapshotData CurrentSnapshotData = {HeapTupleSatisfiesMVCC};
-static SnapshotData SecondarySnapshotData = {HeapTupleSatisfiesMVCC};
-SnapshotData CatalogSnapshotData = {HeapTupleSatisfiesMVCC};
+static SnapshotData CurrentSnapshotData = {SNAPSHOT_MVCC};
+static SnapshotData SecondarySnapshotData = {SNAPSHOT_MVCC};
+SnapshotData CatalogSnapshotData = {SNAPSHOT_MVCC};
 
 /* Pointers to valid snapshots */
 static Snapshot CurrentSnapshot = NULL;
@@ -2046,7 +2046,7 @@ EstimateSnapshotSpace(Snapshot snap)
 	Size		size;
 
 	Assert(snap != InvalidSnapshot);
-	Assert(snap->satisfies == HeapTupleSatisfiesMVCC);
+	Assert(snap->snapshot_type == SNAPSHOT_MVCC);
 
 	/* We allocate any XID arrays needed in the same palloc block. */
 	size = add_size(sizeof(SerializedSnapshotData),
@@ -2143,7 +2143,7 @@ RestoreSnapshot(char *start_address)
 
 	/* Copy all required fields */
 	snapshot = (Snapshot) MemoryContextAlloc(TopTransactionContext, size);
-	snapshot->satisfies = HeapTupleSatisfiesMVCC;
+	snapshot->snapshot_type = SNAPSHOT_MVCC;
 	snapshot->xmin = serialized_snapshot.xmin;
 	snapshot->xmax = serialized_snapshot.xmax;
 	snapshot->xip = NULL;

src/backend/utils/time/tqual.c

@@ -78,8 +78,8 @@
 
 
 /* Static variables representing various special snapshot semantics */
-SnapshotData SnapshotSelfData = {HeapTupleSatisfiesSelf};
-SnapshotData SnapshotAnyData = {HeapTupleSatisfiesAny};
+SnapshotData SnapshotSelfData = {SNAPSHOT_SELF};
+SnapshotData SnapshotAnyData = {SNAPSHOT_ANY};
 
 
 /*
@@ -152,10 +152,7 @@ HeapTupleSetHintBits(HeapTupleHeader tuple, Buffer buffer,
  * HeapTupleSatisfiesSelf
  *		True iff heap tuple is valid "for itself".
  *
- *	Here, we consider the effects of:
- *		all committed transactions (as of the current instant)
- *		previous commands of this transaction
- *		changes made by the current command
+ * See SNAPSHOT_MVCC's definition for the intended behaviour.
  *
  * Note:
  *		Assumes heap tuple is valid.
@@ -172,7 +169,7 @@ HeapTupleSetHintBits(HeapTupleHeader tuple, Buffer buffer,
  *			(Xmax != my-transaction &&			the row was deleted by another transaction
  *			 Xmax is not committed)))			that has not been committed
  */
-bool
+static bool
 HeapTupleSatisfiesSelf(HeapTuple htup, Snapshot snapshot, Buffer buffer)
 {
 	HeapTupleHeader tuple = htup->t_data;
@@ -342,7 +339,7 @@ HeapTupleSatisfiesSelf(HeapTuple htup, Snapshot snapshot, Buffer buffer)
  * HeapTupleSatisfiesAny
  *		Dummy "satisfies" routine: any tuple satisfies SnapshotAny.
  */
-bool
+static bool
 HeapTupleSatisfiesAny(HeapTuple htup, Snapshot snapshot, Buffer buffer)
 {
 	return true;
@@ -352,6 +349,8 @@ HeapTupleSatisfiesAny(HeapTuple htup, Snapshot snapshot, Buffer buffer)
  * HeapTupleSatisfiesToast
  *		True iff heap tuple is valid as a TOAST row.
  *
+ * See SNAPSHOT_TOAST's definition for the intended behaviour.
+ *
  * This is a simplified version that only checks for VACUUM moving conditions.
  * It's appropriate for TOAST usage because TOAST really doesn't want to do
  * its own time qual checks; if you can see the main table row that contains
@@ -362,7 +361,7 @@ HeapTupleSatisfiesAny(HeapTuple htup, Snapshot snapshot, Buffer buffer)
  * Among other things, this means you can't do UPDATEs of rows in a TOAST
  * table.
  */
-bool
+static bool
 HeapTupleSatisfiesToast(HeapTuple htup, Snapshot snapshot,
 						Buffer buffer)
 {
@@ -716,10 +715,7 @@ HeapTupleSatisfiesUpdate(HeapTuple htup, CommandId curcid,
  * HeapTupleSatisfiesDirty
  *		True iff heap tuple is valid including effects of open transactions.
  *
- *	Here, we consider the effects of:
- *		all committed and in-progress transactions (as of the current instant)
- *		previous commands of this transaction
- *		changes made by the current command
+ * See SNAPSHOT_DIRTY's definition for the intended behaviour.
  *
  * This is essentially like HeapTupleSatisfiesSelf as far as effects of
  * the current transaction and committed/aborted xacts are concerned.
@@ -735,7 +731,7 @@ HeapTupleSatisfiesUpdate(HeapTuple htup, CommandId curcid,
  * on the insertion without aborting the whole transaction, the associated
  * token is also returned in snapshot->speculativeToken.
  */
-bool
+static bool
 HeapTupleSatisfiesDirty(HeapTuple htup, Snapshot snapshot,
 						Buffer buffer)
 {
@@ -934,14 +930,7 @@ HeapTupleSatisfiesDirty(HeapTuple htup, Snapshot snapshot,
  * HeapTupleSatisfiesMVCC
  *		True iff heap tuple is valid for the given MVCC snapshot.
  *
- *	Here, we consider the effects of:
- *		all transactions committed as of the time of the given snapshot
- *		previous commands of this transaction
- *
- *	Does _not_ include:
- *		transactions shown as in-progress by the snapshot
- *		transactions started after the snapshot was taken
- *		changes made by the current command
+ * See SNAPSHOT_MVCC's definition for the intended behaviour.
  *
  * Notice that here, we will not update the tuple status hint bits if the
  * inserting/deleting transaction is still running according to our snapshot,
@@ -959,7 +948,7 @@ HeapTupleSatisfiesDirty(HeapTuple htup, Snapshot snapshot,
  * inserting/deleting transaction was still running --- which was more cycles
  * and more contention on the PGXACT array.
  */
-bool
+static bool
 HeapTupleSatisfiesMVCC(HeapTuple htup, Snapshot snapshot,
 					   Buffer buffer)
 {
@@ -1390,11 +1379,13 @@ HeapTupleSatisfiesVacuum(HeapTuple htup, TransactionId OldestXmin,
  *	True if tuple might be visible to some transaction; false if it's
  *	surely dead to everyone, ie, vacuumable.
  *
- *	This is an interface to HeapTupleSatisfiesVacuum that meets the
- *	SnapshotSatisfiesFunc API, so it can be used through a Snapshot.
+ *	See SNAPSHOT_TOAST's definition for the intended behaviour.
+ *
+ *	This is an interface to HeapTupleSatisfiesVacuum that's callable via
+ *	HeapTupleSatisfiesSnapshot, so it can be used through a Snapshot.
  *	snapshot->xmin must have been set up with the xmin horizon to use.
  */
-bool
+static bool
 HeapTupleSatisfiesNonVacuumable(HeapTuple htup, Snapshot snapshot,
 								Buffer buffer)
 {
@@ -1659,7 +1650,7 @@ TransactionIdInArray(TransactionId xid, TransactionId *xip, Size num)
  * dangerous to do so as the semantics of doing so during timetravel are more
  * complicated than when dealing "only" with the present.
  */
-bool
+static bool
 HeapTupleSatisfiesHistoricMVCC(HeapTuple htup, Snapshot snapshot,
 							   Buffer buffer)
 {
@@ -1796,3 +1787,44 @@ HeapTupleSatisfiesHistoricMVCC(HeapTuple htup, Snapshot snapshot,
 	else
 		return true;
 }
+
+/*
+ * HeapTupleSatisfiesVisibility
+ *		True iff heap tuple satisfies a time qual.
+ *
+ * Notes:
+ *	Assumes heap tuple is valid, and buffer at least share locked.
+ *
+ *	Hint bits in the HeapTuple's t_infomask may be updated as a side effect;
+ *	if so, the indicated buffer is marked dirty.
+ */
+bool
+HeapTupleSatisfiesVisibility(HeapTuple tup, Snapshot snapshot, Buffer buffer)
+{
+	switch (snapshot->snapshot_type)
+	{
+		case SNAPSHOT_MVCC:
+			return HeapTupleSatisfiesMVCC(tup, snapshot, buffer);
+			break;
+		case SNAPSHOT_SELF:
+			return HeapTupleSatisfiesSelf(tup, snapshot, buffer);
+			break;
+		case SNAPSHOT_ANY:
+			return HeapTupleSatisfiesAny(tup, snapshot, buffer);
+			break;
+		case SNAPSHOT_TOAST:
+			return HeapTupleSatisfiesToast(tup, snapshot, buffer);
+			break;
+		case SNAPSHOT_DIRTY:
+			return HeapTupleSatisfiesDirty(tup, snapshot, buffer);
+			break;
+		case SNAPSHOT_HISTORIC_MVCC:
+			return HeapTupleSatisfiesHistoricMVCC(tup, snapshot, buffer);
+			break;
+		case SNAPSHOT_NON_VACUUMABLE:
+			return HeapTupleSatisfiesNonVacuumable(tup, snapshot, buffer);
+			break;
+	}
+
+	return false;				/* keep compiler quiet */
+}

src/include/storage/bufmgr.h

@@ -20,7 +20,6 @@
 #include "storage/relfilenode.h"
 #include "utils/relcache.h"
 #include "utils/snapmgr.h"
-#include "utils/tqual.h"
 
 typedef void *Block;
 
@@ -268,8 +267,8 @@ TestForOldSnapshot(Snapshot snapshot, Relation relation, Page page)
 
 	if (old_snapshot_threshold >= 0
 		&& (snapshot) != NULL
-		&& ((snapshot)->satisfies == HeapTupleSatisfiesMVCC
-			|| (snapshot)->satisfies == HeapTupleSatisfiesToast)
+		&& ((snapshot)->snapshot_type == SNAPSHOT_MVCC
+			|| (snapshot)->snapshot_type == SNAPSHOT_TOAST)
 		&& !XLogRecPtrIsInvalid((snapshot)->lsn)
 		&& PageGetLSN(page) > (snapshot)->lsn)
 		TestForOldSnapshot_impl(snapshot, relation);

src/include/utils/snapshot.h

@@ -20,19 +20,89 @@
 #include "storage/buf.h"
 
 
+/*
+ * The different snapshot types.  We use SnapshotData structures to represent
+ * both "regular" (MVCC) snapshots and "special" snapshots that have non-MVCC
+ * semantics.  The specific semantics of a snapshot are encoded by its type.
+ *
+ * The behaviour of each type of snapshot should be documented alongside its
+ * enum value, best in terms that are not specific to an individual table AM.
+ *
+ * The reason the snapshot type rather than a callback as it used to be is
+ * that that allows to use the same snapshot for different table AMs without
+ * having one callback per AM.
+ */
+typedef enum SnapshotType
+{
+	/*-------------------------------------------------------------------------
+	 * A tuple is visible iff the tuple is valid for the given MVCC snapshot.
+	 *
+	 * Here, we consider the effects of:
+	 * - all transactions committed as of the time of the given snapshot
+	 * - previous commands of this transaction
+	 *
+	 * Does _not_ include:
+	 * - transactions shown as in-progress by the snapshot
+	 * - transactions started after the snapshot was taken
+	 * - changes made by the current command
+	 * -------------------------------------------------------------------------
+	 */
+	SNAPSHOT_MVCC = 0,
+
+	/*-------------------------------------------------------------------------
+	 * A tuple is visible iff the tuple is valid including effects of open
+	 * transactions.
+	 *
+	 * Here, we consider the effects of:
+	 * - all committed and in-progress transactions (as of the current instant)
+	 * - previous commands of this transaction
+	 * - changes made by the current command
+	 * -------------------------------------------------------------------------
+	 */
+	SNAPSHOT_SELF,
+
+	/*
+	 * Any tuple is visible.
+	 */
+	SNAPSHOT_ANY,
+
+	/*
+	 * A tuple is visible iff the tuple tuple is valid as a TOAST row.
+	 */
+	SNAPSHOT_TOAST,
+
+	/*-------------------------------------------------------------------------
+	 * A tuple is visible iff the tuple is valid including effects of open
+	 * transactions.
+	 *
+	 * Here, we consider the effects of:
+	 * - all committed and in-progress transactions (as of the current instant)
+	 * - previous commands of this transaction
+	 * - changes made by the current command
+	 * -------------------------------------------------------------------------
+	 */
+	SNAPSHOT_DIRTY,
+
+	/*
+	 * A tuple is visible iff it follows the rules of SNAPSHOT_MVCC, but
+	 * supports being called in timetravel context (for decoding catalog
+	 * contents in the context of logical decoding).
+	 */
+	SNAPSHOT_HISTORIC_MVCC,
+
+	/*
+	 * A tuple is visible iff the tuple might be visible to some transaction;
+	 * false if it's surely dead to everyone, ie, vacuumable.
+	 *
+	 * Snapshot.xmin must have been set up with the xmin horizon to use.
+	 */
+	SNAPSHOT_NON_VACUUMABLE
+} SnapshotType;
+
 typedef struct SnapshotData *Snapshot;
 
 #define InvalidSnapshot		((Snapshot) NULL)
 
-/*
- * We use SnapshotData structures to represent both "regular" (MVCC)
- * snapshots and "special" snapshots that have non-MVCC semantics.
- * The specific semantics of a snapshot are encoded by the "satisfies"
- * function.
- */
-typedef bool (*SnapshotSatisfiesFunc) (HeapTuple htup,
-									   Snapshot snapshot, Buffer buffer);
-
 /*
  * Struct representing all kind of possible snapshots.
  *
@@ -52,7 +122,7 @@ typedef bool (*SnapshotSatisfiesFunc) (HeapTuple htup,
  */
 typedef struct SnapshotData
 {
-	SnapshotSatisfiesFunc satisfies;	/* tuple test function */
+	SnapshotType snapshot_type; /* type of snapshot */
 
 	/*
 	 * The remaining fields are used only for MVCC snapshots, and are normally