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/*-------------------------------------------------------------------------
*
* time_mapping.c
* time to XID mapping information
*
* Copyright (c) 2020-2022, PostgreSQL Global Development Group
*
* contrib/old_snapshot/time_mapping.c
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "funcapi.h"
#include "storage/lwlock.h"
#include "utils/old_snapshot.h"
#include "utils/snapmgr.h"
#include "utils/timestamp.h"
/*
* Backend-private copy of the information from oldSnapshotControl which relates
* to the time to XID mapping, plus an index so that we can iterate.
*
* Note that the length of the xid_by_minute array is given by
* OLD_SNAPSHOT_TIME_MAP_ENTRIES (which is not a compile-time constant).
*/
typedef struct
{
int current_index;
int head_offset;
TimestampTz head_timestamp;
int count_used;
TransactionId xid_by_minute[FLEXIBLE_ARRAY_MEMBER];
} OldSnapshotTimeMapping;
#define NUM_TIME_MAPPING_COLUMNS 3
PG_MODULE_MAGIC;
PG_FUNCTION_INFO_V1(pg_old_snapshot_time_mapping);
static OldSnapshotTimeMapping *GetOldSnapshotTimeMapping(void);
static TupleDesc MakeOldSnapshotTimeMappingTupleDesc(void);
static HeapTuple MakeOldSnapshotTimeMappingTuple(TupleDesc tupdesc,
OldSnapshotTimeMapping *mapping);
/*
* SQL-callable set-returning function.
*/
Datum
pg_old_snapshot_time_mapping(PG_FUNCTION_ARGS)
{
FuncCallContext *funcctx;
OldSnapshotTimeMapping *mapping;
if (SRF_IS_FIRSTCALL())
{
MemoryContext oldcontext;
funcctx = SRF_FIRSTCALL_INIT();
oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
mapping = GetOldSnapshotTimeMapping();
funcctx->user_fctx = mapping;
funcctx->tuple_desc = MakeOldSnapshotTimeMappingTupleDesc();
MemoryContextSwitchTo(oldcontext);
}
funcctx = SRF_PERCALL_SETUP();
mapping = (OldSnapshotTimeMapping *) funcctx->user_fctx;
while (mapping->current_index < mapping->count_used)
{
HeapTuple tuple;
tuple = MakeOldSnapshotTimeMappingTuple(funcctx->tuple_desc, mapping);
++mapping->current_index;
SRF_RETURN_NEXT(funcctx, HeapTupleGetDatum(tuple));
}
SRF_RETURN_DONE(funcctx);
}
/*
* Get the old snapshot time mapping data from shared memory.
*/
static OldSnapshotTimeMapping *
GetOldSnapshotTimeMapping(void)
{
OldSnapshotTimeMapping *mapping;
mapping = palloc(offsetof(OldSnapshotTimeMapping, xid_by_minute)
+ sizeof(TransactionId) * OLD_SNAPSHOT_TIME_MAP_ENTRIES);
mapping->current_index = 0;
LWLockAcquire(OldSnapshotTimeMapLock, LW_SHARED);
mapping->head_offset = oldSnapshotControl->head_offset;
mapping->head_timestamp = oldSnapshotControl->head_timestamp;
mapping->count_used = oldSnapshotControl->count_used;
for (int i = 0; i < OLD_SNAPSHOT_TIME_MAP_ENTRIES; ++i)
mapping->xid_by_minute[i] = oldSnapshotControl->xid_by_minute[i];
LWLockRelease(OldSnapshotTimeMapLock);
return mapping;
}
/*
* Build a tuple descriptor for the pg_old_snapshot_time_mapping() SRF.
*/
static TupleDesc
MakeOldSnapshotTimeMappingTupleDesc(void)
{
TupleDesc tupdesc;
tupdesc = CreateTemplateTupleDesc(NUM_TIME_MAPPING_COLUMNS);
TupleDescInitEntry(tupdesc, (AttrNumber) 1, "array_offset",
INT4OID, -1, 0);
TupleDescInitEntry(tupdesc, (AttrNumber) 2, "end_timestamp",
TIMESTAMPTZOID, -1, 0);
TupleDescInitEntry(tupdesc, (AttrNumber) 3, "newest_xmin",
XIDOID, -1, 0);
return BlessTupleDesc(tupdesc);
}
/*
* Convert one entry from the old snapshot time mapping to a HeapTuple.
*/
static HeapTuple
MakeOldSnapshotTimeMappingTuple(TupleDesc tupdesc, OldSnapshotTimeMapping *mapping)
{
Datum values[NUM_TIME_MAPPING_COLUMNS];
bool nulls[NUM_TIME_MAPPING_COLUMNS];
int array_position;
TimestampTz timestamp;
/*
* Figure out the array position corresponding to the current index.
*
* Index 0 means the oldest entry in the mapping, which is stored at
* mapping->head_offset. Index 1 means the next-oldest entry, which is a
* the following index, and so on. We wrap around when we reach the end of
* the array.
*/
array_position = (mapping->head_offset + mapping->current_index)
% OLD_SNAPSHOT_TIME_MAP_ENTRIES;
/*
* No explicit timestamp is stored for any entry other than the oldest
* one, but each entry corresponds to 1-minute period, so we can just add.
*/
timestamp = TimestampTzPlusMilliseconds(mapping->head_timestamp,
mapping->current_index * 60000);
/* Initialize nulls and values arrays. */
memset(nulls, 0, sizeof(nulls));
values[0] = Int32GetDatum(array_position);
values[1] = TimestampTzGetDatum(timestamp);
values[2] = TransactionIdGetDatum(mapping->xid_by_minute[array_position]);
return heap_form_tuple(tupdesc, values, nulls);
}
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