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diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
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+/*-------------------------------------------------------------------------
+ *
+ * snapmgr.h
+ *	  POSTGRES snapshot manager
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/utils/snapmgr.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef SNAPMGR_H
+#define SNAPMGR_H
+
+#include "access/transam.h"
+#include "utils/relcache.h"
+#include "utils/resowner.h"
+#include "utils/snapshot.h"
+
+
+/*
+ * The structure used to map times to TransactionId values for the "snapshot
+ * too old" feature must have a few entries at the tail to hold old values;
+ * otherwise the lookup will often fail and the expected early pruning or
+ * vacuum will not usually occur.  It is best if this padding is for a number
+ * of minutes greater than a thread would normally be stalled, but it's OK if
+ * early vacuum opportunities are occasionally missed, so there's no need to
+ * use an extreme value or get too fancy.  10 minutes seems plenty.
+ */
+#define OLD_SNAPSHOT_PADDING_ENTRIES 10
+#define OLD_SNAPSHOT_TIME_MAP_ENTRIES (old_snapshot_threshold + OLD_SNAPSHOT_PADDING_ENTRIES)
+
+/*
+ * Common definition of relation properties that allow early pruning/vacuuming
+ * when old_snapshot_threshold >= 0.
+ */
+#define RelationAllowsEarlyPruning(rel) \
+( \
+	 (rel)->rd_rel->relpersistence == RELPERSISTENCE_PERMANENT	\
+  && !IsCatalogRelation(rel) \
+  && !RelationIsAccessibleInLogicalDecoding(rel) \
+)
+
+#define EarlyPruningEnabled(rel) (old_snapshot_threshold >= 0 && RelationAllowsEarlyPruning(rel))
+
+/* GUC variables */
+extern PGDLLIMPORT int old_snapshot_threshold;
+
+
+extern Size SnapMgrShmemSize(void);
+extern void SnapMgrInit(void);
+extern TimestampTz GetSnapshotCurrentTimestamp(void);
+extern TimestampTz GetOldSnapshotThresholdTimestamp(void);
+
+extern bool FirstSnapshotSet;
+
+extern PGDLLIMPORT TransactionId TransactionXmin;
+extern PGDLLIMPORT TransactionId RecentXmin;
+extern PGDLLIMPORT TransactionId RecentGlobalXmin;
+extern PGDLLIMPORT TransactionId RecentGlobalDataXmin;
+
+/* Variables representing various special snapshot semantics */
+extern PGDLLIMPORT SnapshotData SnapshotSelfData;
+extern PGDLLIMPORT SnapshotData SnapshotAnyData;
+extern PGDLLIMPORT SnapshotData CatalogSnapshotData;
+
+#define SnapshotSelf		(&SnapshotSelfData)
+#define SnapshotAny			(&SnapshotAnyData)
+
+/*
+ * We don't provide a static SnapshotDirty variable because it would be
+ * non-reentrant.  Instead, users of that snapshot type should declare a
+ * local variable of type SnapshotData, and initialize it with this macro.
+ */
+#define InitDirtySnapshot(snapshotdata)  \
+	((snapshotdata).snapshot_type = SNAPSHOT_DIRTY)
+
+/*
+ * Similarly, some initialization is required for a NonVacuumable snapshot.
+ * The caller must supply the xmin horizon to use (e.g., RecentGlobalXmin).
+ */
+#define InitNonVacuumableSnapshot(snapshotdata, xmin_horizon)  \
+	((snapshotdata).snapshot_type = SNAPSHOT_NON_VACUUMABLE, \
+	 (snapshotdata).xmin = (xmin_horizon))
+
+/*
+ * 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 || \
+	 (snapshot)->snapshot_type == SNAPSHOT_HISTORIC_MVCC)
+
+
+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);
+extern void InvalidateCatalogSnapshot(void);
+extern void InvalidateCatalogSnapshotConditionally(void);
+
+extern void PushActiveSnapshot(Snapshot snapshot);
+extern void PushCopiedSnapshot(Snapshot snapshot);
+extern void UpdateActiveSnapshotCommandId(void);
+extern void PopActiveSnapshot(void);
+extern Snapshot GetActiveSnapshot(void);
+extern bool ActiveSnapshotSet(void);
+
+extern Snapshot RegisterSnapshot(Snapshot snapshot);
+extern void UnregisterSnapshot(Snapshot snapshot);
+extern Snapshot RegisterSnapshotOnOwner(Snapshot snapshot, ResourceOwner owner);
+extern void UnregisterSnapshotFromOwner(Snapshot snapshot, ResourceOwner owner);
+
+extern FullTransactionId GetFullRecentGlobalXmin(void);
+
+extern void AtSubCommit_Snapshot(int level);
+extern void AtSubAbort_Snapshot(int level);
+extern void AtEOXact_Snapshot(bool isCommit, bool resetXmin);
+
+extern void ImportSnapshot(const char *idstr);
+extern bool XactHasExportedSnapshots(void);
+extern void DeleteAllExportedSnapshotFiles(void);
+extern bool ThereAreNoPriorRegisteredSnapshots(void);
+extern TransactionId TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
+														 Relation relation);
+extern void MaintainOldSnapshotTimeMapping(TimestampTz whenTaken,
+										   TransactionId xmin);
+
+extern char *ExportSnapshot(Snapshot snapshot);
+
+/*
+ * Utility functions for implementing visibility routines in table AMs.
+ */
+extern bool XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot);
+
+/* Support for catalog timetravel for logical decoding */
+struct HTAB;
+extern struct HTAB *HistoricSnapshotGetTupleCids(void);
+extern void SetupHistoricSnapshot(Snapshot snapshot_now, struct HTAB *tuplecids);
+extern void TeardownHistoricSnapshot(bool is_error);
+extern bool HistoricSnapshotActive(void);
+
+extern Size EstimateSnapshotSpace(Snapshot snapshot);
+extern void SerializeSnapshot(Snapshot snapshot, char *start_address);
+extern Snapshot RestoreSnapshot(char *start_address);
+extern void RestoreTransactionSnapshot(Snapshot snapshot, void *master_pgproc);
+
+#endif							/* SNAPMGR_H */