txid.c

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/*-------------------------------------------------------------------------
 * txid.c
 *
 *  Export internal transaction IDs to user level.
 *
 * Note that only top-level transaction IDs are ever converted to TXID.
 * This is important because TXIDs frequently persist beyond the global
 * xmin horizon, or may even be shipped to other machines, so we cannot
 * rely on being able to correlate subtransaction IDs with their parents
 * via functions such as SubTransGetTopmostTransaction().
 *
 *
 *  Copyright (c) 2003-2013, PostgreSQL Global Development Group
 *  Author: Jan Wieck, Afilias USA INC.
 *  64-bit txids: Marko Kreen, Skype Technologies
 *
 *  src/backend/utils/adt/txid.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/transam.h"
#include "access/xact.h"
#include "funcapi.h"
#include "miscadmin.h"
#include "libpq/pqformat.h"
#include "utils/builtins.h"
#include "utils/snapmgr.h"


/* txid will be signed int8 in database, so must limit to 63 bits */
#define MAX_TXID   UINT64CONST(0x7FFFFFFFFFFFFFFF)

/* Use unsigned variant internally */
typedef uint64 txid;

/* sprintf format code for uint64 */
#define TXID_FMT UINT64_FORMAT

/*
 * If defined, use bsearch() function for searching for txids in snapshots
 * that have more than the specified number of values.
 */
#define USE_BSEARCH_IF_NXIP_GREATER 30


/*
 * Snapshot containing 8byte txids.
 */
typedef struct
{
    /*
     * 4-byte length hdr, should not be touched directly.
     *
     * Explicit embedding is ok as we want always correct alignment anyway.
     */
    int32       __varsz;

    uint32      nxip;           /* number of txids in xip array */
    txid        xmin;
    txid        xmax;
    txid        xip[1];         /* in-progress txids, xmin <= xip[i] < xmax */
} TxidSnapshot;

#define TXID_SNAPSHOT_SIZE(nxip) \
    (offsetof(TxidSnapshot, xip) + sizeof(txid) * (nxip))

/*
 * Epoch values from xact.c
 */
typedef struct
{
    TransactionId last_xid;
    uint32      epoch;
} TxidEpoch;


/*
 * Fetch epoch data from xact.c.
 */
static void
load_xid_epoch(TxidEpoch *state)
{
    GetNextXidAndEpoch(&state->last_xid, &state->epoch);
}

/*
 * do a TransactionId -> txid conversion for an XID near the given epoch
 */
static txid
convert_xid(TransactionId xid, const TxidEpoch *state)
{
    uint64      epoch;

    /* return special xid's as-is */
    if (!TransactionIdIsNormal(xid))
        return (txid) xid;

    /* xid can be on either side when near wrap-around */
    epoch = (uint64) state->epoch;
    if (xid > state->last_xid &&
        TransactionIdPrecedes(xid, state->last_xid))
        epoch--;
    else if (xid < state->last_xid &&
             TransactionIdFollows(xid, state->last_xid))
        epoch++;

    return (epoch << 32) | xid;
}

/*
 * txid comparator for qsort/bsearch
 */
static int
cmp_txid(const void *aa, const void *bb)
{
    txid        a = *(const txid *) aa;
    txid        b = *(const txid *) bb;

    if (a < b)
        return -1;
    if (a > b)
        return 1;
    return 0;
}

/*
 * sort a snapshot's txids, so we can use bsearch() later.
 *
 * For consistency of on-disk representation, we always sort even if bsearch
 * will not be used.
 */
static void
sort_snapshot(TxidSnapshot *snap)
{
    if (snap->nxip > 1)
        qsort(snap->xip, snap->nxip, sizeof(txid), cmp_txid);
}

/*
 * check txid visibility.
 */
static bool
is_visible_txid(txid value, const TxidSnapshot *snap)
{
    if (value < snap->xmin)
        return true;
    else if (value >= snap->xmax)
        return false;
#ifdef USE_BSEARCH_IF_NXIP_GREATER
    else if (snap->nxip > USE_BSEARCH_IF_NXIP_GREATER)
    {
        void       *res;

        res = bsearch(&value, snap->xip, snap->nxip, sizeof(txid), cmp_txid);
        /* if found, transaction is still in progress */
        return (res) ? false : true;
    }
#endif
    else
    {
        uint32      i;

        for (i = 0; i < snap->nxip; i++)
        {
            if (value == snap->xip[i])
                return false;
        }
        return true;
    }
}

/*
 * helper functions to use StringInfo for TxidSnapshot creation.
 */

static StringInfo
buf_init(txid xmin, txid xmax)
{
    TxidSnapshot snap;
    StringInfo  buf;

    snap.xmin = xmin;
    snap.xmax = xmax;
    snap.nxip = 0;

    buf = makeStringInfo();
    appendBinaryStringInfo(buf, (char *) &snap, TXID_SNAPSHOT_SIZE(0));
    return buf;
}

static void
buf_add_txid(StringInfo buf, txid xid)
{
    TxidSnapshot *snap = (TxidSnapshot *) buf->data;

    /* do this before possible realloc */
    snap->nxip++;

    appendBinaryStringInfo(buf, (char *) &xid, sizeof(xid));
}

static TxidSnapshot *
buf_finalize(StringInfo buf)
{
    TxidSnapshot *snap = (TxidSnapshot *) buf->data;

    SET_VARSIZE(snap, buf->len);

    /* buf is not needed anymore */
    buf->data = NULL;
    pfree(buf);

    return snap;
}

/*
 * simple number parser.
 *
 * We return 0 on error, which is invalid value for txid.
 */
static txid
str2txid(const char *s, const char **endp)
{
    txid        val = 0;
    txid        cutoff = MAX_TXID / 10;
    txid        cutlim = MAX_TXID % 10;

    for (; *s; s++)
    {
        unsigned    d;

        if (*s < '0' || *s > '9')
            break;
        d = *s - '0';

        /*
         * check for overflow
         */
        if (val > cutoff || (val == cutoff && d > cutlim))
        {
            val = 0;
            break;
        }

        val = val * 10 + d;
    }
    if (endp)
        *endp = s;
    return val;
}

/*
 * parse snapshot from cstring
 */
static TxidSnapshot *
parse_snapshot(const char *str)
{
    txid        xmin;
    txid        xmax;
    txid        last_val = 0,
                val;
    const char *str_start = str;
    const char *endp;
    StringInfo  buf;

    xmin = str2txid(str, &endp);
    if (*endp != ':')
        goto bad_format;
    str = endp + 1;

    xmax = str2txid(str, &endp);
    if (*endp != ':')
        goto bad_format;
    str = endp + 1;

    /* it should look sane */
    if (xmin == 0 || xmax == 0 || xmin > xmax)
        goto bad_format;

    /* allocate buffer */
    buf = buf_init(xmin, xmax);

    /* loop over values */
    while (*str != '\0')
    {
        /* read next value */
        val = str2txid(str, &endp);
        str = endp;

        /* require the input to be in order */
        if (val < xmin || val >= xmax || val <= last_val)
            goto bad_format;

        buf_add_txid(buf, val);
        last_val = val;

        if (*str == ',')
            str++;
        else if (*str != '\0')
            goto bad_format;
    }

    return buf_finalize(buf);

bad_format:
    elog(ERROR, "invalid input for txid_snapshot: \"%s\"", str_start);
    return NULL;
}

/*
 * Public functions.
 *
 * txid_current() and txid_current_snapshot() are the only ones that
 * communicate with core xid machinery.  All the others work on data
 * returned by them.
 */

/*
 * txid_current() returns int8
 *
 *  Return the current toplevel transaction ID as TXID
 *  If the current transaction does not have one, one is assigned.
 */
Datum
txid_current(PG_FUNCTION_ARGS)
{
    txid        val;
    TxidEpoch   state;

    /*
     * Must prevent during recovery because if an xid is not assigned we try
     * to assign one, which would fail. Programs already rely on this function
     * to always return a valid current xid, so we should not change this to
     * return NULL or similar invalid xid.
     */
    PreventCommandDuringRecovery("txid_current()");

    load_xid_epoch(&state);

    val = convert_xid(GetTopTransactionId(), &state);

    PG_RETURN_INT64(val);
}

/*
 * txid_current_snapshot() returns txid_snapshot
 *
 *      Return current snapshot in TXID format
 *
 * Note that only top-transaction XIDs are included in the snapshot.
 */
Datum
txid_current_snapshot(PG_FUNCTION_ARGS)
{
    TxidSnapshot *snap;
    uint32      nxip,
                i,
                size;
    TxidEpoch   state;
    Snapshot    cur;

    cur = GetActiveSnapshot();
    if (cur == NULL)
        elog(ERROR, "no active snapshot set");

    load_xid_epoch(&state);

    /* allocate */
    nxip = cur->xcnt;
    size = TXID_SNAPSHOT_SIZE(nxip);
    snap = palloc(size);
    SET_VARSIZE(snap, size);

    /* fill */
    snap->xmin = convert_xid(cur->xmin, &state);
    snap->xmax = convert_xid(cur->xmax, &state);
    snap->nxip = nxip;
    for (i = 0; i < nxip; i++)
        snap->xip[i] = convert_xid(cur->xip[i], &state);

    /* we want them guaranteed to be in ascending order */
    sort_snapshot(snap);

    PG_RETURN_POINTER(snap);
}

/*
 * txid_snapshot_in(cstring) returns txid_snapshot
 *
 *      input function for type txid_snapshot
 */
Datum
txid_snapshot_in(PG_FUNCTION_ARGS)
{
    char       *str = PG_GETARG_CSTRING(0);
    TxidSnapshot *snap;

    snap = parse_snapshot(str);

    PG_RETURN_POINTER(snap);
}

/*
 * txid_snapshot_out(txid_snapshot) returns cstring
 *
 *      output function for type txid_snapshot
 */
Datum
txid_snapshot_out(PG_FUNCTION_ARGS)
{
    TxidSnapshot *snap = (TxidSnapshot *) PG_GETARG_VARLENA_P(0);
    StringInfoData str;
    uint32      i;

    initStringInfo(&str);

    appendStringInfo(&str, TXID_FMT ":", snap->xmin);
    appendStringInfo(&str, TXID_FMT ":", snap->xmax);

    for (i = 0; i < snap->nxip; i++)
    {
        if (i > 0)
            appendStringInfoChar(&str, ',');
        appendStringInfo(&str, TXID_FMT, snap->xip[i]);
    }

    PG_RETURN_CSTRING(str.data);
}

/*
 * txid_snapshot_recv(internal) returns txid_snapshot
 *
 *      binary input function for type txid_snapshot
 *
 *      format: int4 nxip, int8 xmin, int8 xmax, int8 xip
 */
Datum
txid_snapshot_recv(PG_FUNCTION_ARGS)
{
    StringInfo  buf = (StringInfo) PG_GETARG_POINTER(0);
    TxidSnapshot *snap;
    txid        last = 0;
    int         nxip;
    int         i;
    int         avail;
    int         expect;
    txid        xmin,
                xmax;

    /*
     * load nxip and check for nonsense.
     *
     * (nxip > avail) check is against int overflows in 'expect'.
     */
    nxip = pq_getmsgint(buf, 4);
    avail = buf->len - buf->cursor;
    expect = 8 + 8 + nxip * 8;
    if (nxip < 0 || nxip > avail || expect > avail)
        goto bad_format;

    xmin = pq_getmsgint64(buf);
    xmax = pq_getmsgint64(buf);
    if (xmin == 0 || xmax == 0 || xmin > xmax || xmax > MAX_TXID)
        goto bad_format;

    snap = palloc(TXID_SNAPSHOT_SIZE(nxip));
    snap->xmin = xmin;
    snap->xmax = xmax;
    snap->nxip = nxip;
    SET_VARSIZE(snap, TXID_SNAPSHOT_SIZE(nxip));

    for (i = 0; i < nxip; i++)
    {
        txid        cur = pq_getmsgint64(buf);

        if (cur <= last || cur < xmin || cur >= xmax)
            goto bad_format;
        snap->xip[i] = cur;
        last = cur;
    }
    PG_RETURN_POINTER(snap);

bad_format:
    elog(ERROR, "invalid snapshot data");
    return (Datum) NULL;
}

/*
 * txid_snapshot_send(txid_snapshot) returns bytea
 *
 *      binary output function for type txid_snapshot
 *
 *      format: int4 nxip, int8 xmin, int8 xmax, int8 xip
 */
Datum
txid_snapshot_send(PG_FUNCTION_ARGS)
{
    TxidSnapshot *snap = (TxidSnapshot *) PG_GETARG_VARLENA_P(0);
    StringInfoData buf;
    uint32      i;

    pq_begintypsend(&buf);
    pq_sendint(&buf, snap->nxip, 4);
    pq_sendint64(&buf, snap->xmin);
    pq_sendint64(&buf, snap->xmax);
    for (i = 0; i < snap->nxip; i++)
        pq_sendint64(&buf, snap->xip[i]);
    PG_RETURN_BYTEA_P(pq_endtypsend(&buf));
}

/*
 * txid_visible_in_snapshot(int8, txid_snapshot) returns bool
 *
 *      is txid visible in snapshot ?
 */
Datum
txid_visible_in_snapshot(PG_FUNCTION_ARGS)
{
    txid        value = PG_GETARG_INT64(0);
    TxidSnapshot *snap = (TxidSnapshot *) PG_GETARG_VARLENA_P(1);

    PG_RETURN_BOOL(is_visible_txid(value, snap));
}

/*
 * txid_snapshot_xmin(txid_snapshot) returns int8
 *
 *      return snapshot's xmin
 */
Datum
txid_snapshot_xmin(PG_FUNCTION_ARGS)
{
    TxidSnapshot *snap = (TxidSnapshot *) PG_GETARG_VARLENA_P(0);

    PG_RETURN_INT64(snap->xmin);
}

/*
 * txid_snapshot_xmax(txid_snapshot) returns int8
 *
 *      return snapshot's xmax
 */
Datum
txid_snapshot_xmax(PG_FUNCTION_ARGS)
{
    TxidSnapshot *snap = (TxidSnapshot *) PG_GETARG_VARLENA_P(0);

    PG_RETURN_INT64(snap->xmax);
}

/*
 * txid_snapshot_xip(txid_snapshot) returns setof int8
 *
 *      return in-progress TXIDs in snapshot.
 */
Datum
txid_snapshot_xip(PG_FUNCTION_ARGS)
{
    FuncCallContext *fctx;
    TxidSnapshot *snap;
    txid        value;

    /* on first call initialize snap_state and get copy of snapshot */
    if (SRF_IS_FIRSTCALL())
    {
        TxidSnapshot *arg = (TxidSnapshot *) PG_GETARG_VARLENA_P(0);

        fctx = SRF_FIRSTCALL_INIT();

        /* make a copy of user snapshot */
        snap = MemoryContextAlloc(fctx->multi_call_memory_ctx, VARSIZE(arg));
        memcpy(snap, arg, VARSIZE(arg));

        fctx->user_fctx = snap;
    }

    /* return values one-by-one */
    fctx = SRF_PERCALL_SETUP();
    snap = fctx->user_fctx;
    if (fctx->call_cntr < snap->nxip)
    {
        value = snap->xip[fctx->call_cntr];
        SRF_RETURN_NEXT(fctx, Int64GetDatum(value));
    }
    else
    {
        SRF_RETURN_DONE(fctx);
    }
}