time_mapping.c

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/*-------------------------------------------------------------------------
 *
 * time_mapping.c
 *    time to XID mapping information
 *
 * Copyright (c) 2020-2021, 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);
}