Expose oldSnapshotControl definition via new header.

This makes it possible for code outside snapmgr.c to examine the
contents of this data structure. This commit does not add any code
which actually does so; a subsequent commit will make that change.

Patch by me, reviewed by Thomas Munro, Dilip Kumar, Hamid Akhtar.

Discussion: http://postgr.es/m/CA+TgmoY=aqf0zjTD+3dUWYkgMiNDegDLFjo+6ze=Wtpik+3XqA@mail.gmail.com

src/backend/utils/time/snapmgr.c

@@ -64,6 +64,7 @@
 #include "storage/spin.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
+#include "utils/old_snapshot.h"
 #include "utils/rel.h"
 #include "utils/resowner_private.h"
 #include "utils/snapmgr.h"
@@ -76,59 +77,7 @@
  */
 int			old_snapshot_threshold; /* number of minutes, -1 disables */
 
-/*
+volatile OldSnapshotControlData *oldSnapshotControl;
- * Structure for dealing with old_snapshot_threshold implementation.
- */
-typedef struct OldSnapshotControlData
-{
-	/*
-	 * Variables for old snapshot handling are shared among processes and are
-	 * only allowed to move forward.
-	 */
-	slock_t		mutex_current;	/* protect current_timestamp */
-	TimestampTz current_timestamp;	/* latest snapshot timestamp */
-	slock_t		mutex_latest_xmin;	/* protect latest_xmin and next_map_update */
-	TransactionId latest_xmin;	/* latest snapshot xmin */
-	TimestampTz next_map_update;	/* latest snapshot valid up to */
-	slock_t		mutex_threshold;	/* protect threshold fields */
-	TimestampTz threshold_timestamp;	/* earlier snapshot is old */
-	TransactionId threshold_xid;	/* earlier xid may be gone */
-
-	/*
-	 * Keep one xid per minute for old snapshot error handling.
-	 *
-	 * Use a circular buffer with a head offset, a count of entries currently
-	 * used, and a timestamp corresponding to the xid at the head offset.  A
-	 * count_used value of zero means that there are no times stored; a
-	 * count_used value of OLD_SNAPSHOT_TIME_MAP_ENTRIES means that the buffer
-	 * is full and the head must be advanced to add new entries.  Use
-	 * timestamps aligned to minute boundaries, since that seems less
-	 * surprising than aligning based on the first usage timestamp.  The
-	 * latest bucket is effectively stored within latest_xmin.  The circular
-	 * buffer is updated when we get a new xmin value that doesn't fall into
-	 * the same interval.
-	 *
-	 * It is OK if the xid for a given time slot is from earlier than
-	 * calculated by adding the number of minutes corresponding to the
-	 * (possibly wrapped) distance from the head offset to the time of the
-	 * head entry, since that just results in the vacuuming of old tuples
-	 * being slightly less aggressive.  It would not be OK for it to be off in
-	 * the other direction, since it might result in vacuuming tuples that are
-	 * still expected to be there.
-	 *
-	 * Use of an SLRU was considered but not chosen because it is more
-	 * heavyweight than is needed for this, and would probably not be any less
-	 * code to implement.
-	 *
-	 * Persistence is not needed.
-	 */
-	int			head_offset;	/* subscript of oldest tracked time */
-	TimestampTz head_timestamp; /* time corresponding to head xid */
-	int			count_used;		/* how many slots are in use */
-	TransactionId xid_by_minute[FLEXIBLE_ARRAY_MEMBER];
-} OldSnapshotControlData;
-
-static volatile OldSnapshotControlData *oldSnapshotControl;
 
 
 /*

src/include/utils/old_snapshot.h

@@ -0,0 +1,75 @@
+/*-------------------------------------------------------------------------
+ *
+ * old_snapshot.h
+ *		Data structures for 'snapshot too old'
+ *
+ * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/include/utils/old_snapshot.h
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef OLD_SNAPSHOT_H
+#define OLD_SNAPSHOT_H
+
+#include "datatype/timestamp.h"
+#include "storage/s_lock.h"
+
+/*
+ * Structure for dealing with old_snapshot_threshold implementation.
+ */
+typedef struct OldSnapshotControlData
+{
+	/*
+	 * Variables for old snapshot handling are shared among processes and are
+	 * only allowed to move forward.
+	 */
+	slock_t		mutex_current;	/* protect current_timestamp */
+	TimestampTz current_timestamp;	/* latest snapshot timestamp */
+	slock_t		mutex_latest_xmin;	/* protect latest_xmin and next_map_update */
+	TransactionId latest_xmin;	/* latest snapshot xmin */
+	TimestampTz next_map_update;	/* latest snapshot valid up to */
+	slock_t		mutex_threshold;	/* protect threshold fields */
+	TimestampTz threshold_timestamp;	/* earlier snapshot is old */
+	TransactionId threshold_xid;	/* earlier xid may be gone */
+
+	/*
+	 * Keep one xid per minute for old snapshot error handling.
+	 *
+	 * Use a circular buffer with a head offset, a count of entries currently
+	 * used, and a timestamp corresponding to the xid at the head offset.  A
+	 * count_used value of zero means that there are no times stored; a
+	 * count_used value of OLD_SNAPSHOT_TIME_MAP_ENTRIES means that the buffer
+	 * is full and the head must be advanced to add new entries.  Use
+	 * timestamps aligned to minute boundaries, since that seems less
+	 * surprising than aligning based on the first usage timestamp.  The
+	 * latest bucket is effectively stored within latest_xmin.  The circular
+	 * buffer is updated when we get a new xmin value that doesn't fall into
+	 * the same interval.
+	 *
+	 * It is OK if the xid for a given time slot is from earlier than
+	 * calculated by adding the number of minutes corresponding to the
+	 * (possibly wrapped) distance from the head offset to the time of the
+	 * head entry, since that just results in the vacuuming of old tuples
+	 * being slightly less aggressive.  It would not be OK for it to be off in
+	 * the other direction, since it might result in vacuuming tuples that are
+	 * still expected to be there.
+	 *
+	 * Use of an SLRU was considered but not chosen because it is more
+	 * heavyweight than is needed for this, and would probably not be any less
+	 * code to implement.
+	 *
+	 * Persistence is not needed.
+	 */
+	int			head_offset;	/* subscript of oldest tracked time */
+	TimestampTz head_timestamp; /* time corresponding to head xid */
+	int			count_used;		/* how many slots are in use */
+	TransactionId xid_by_minute[FLEXIBLE_ARRAY_MEMBER];
+} OldSnapshotControlData;
+
+extern PGDLLIMPORT volatile OldSnapshotControlData *oldSnapshotControl;
+
+#endif