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/*-------------------------------------------------------------------------
*
* 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
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