init

db_include/utils/old_snapshot.h

@@ -0,0 +1,75 @@
+/*-------------------------------------------------------------------------
+ *
+ * old_snapshot.h
+ *		Data structures for 'snapshot too old'
+ *
+ * Portions Copyright (c) 1996-2021, 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