proto.c

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/*-------------------------------------------------------------------------
 *
 * proto.c
 *      logical replication protocol functions
 *
 * Copyright (c) 2015-2021, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *      src/backend/replication/logical/proto.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/sysattr.h"
#include "catalog/pg_namespace.h"
#include "catalog/pg_type.h"
#include "libpq/pqformat.h"
#include "replication/logicalproto.h"
#include "utils/lsyscache.h"
#include "utils/syscache.h"

/*
 * Protocol message flags.
 */
#define LOGICALREP_IS_REPLICA_IDENTITY 1

#define TRUNCATE_CASCADE        (1<<0)
#define TRUNCATE_RESTART_SEQS   (1<<1)

static void logicalrep_write_attrs(StringInfo out, Relation rel);
static void logicalrep_write_tuple(StringInfo out, Relation rel,
                                   HeapTuple tuple, bool binary);

static void logicalrep_read_attrs(StringInfo in, LogicalRepRelation *rel);
static void logicalrep_read_tuple(StringInfo in, LogicalRepTupleData *tuple);

static void logicalrep_write_namespace(StringInfo out, Oid nspid);
static const char *logicalrep_read_namespace(StringInfo in);

/*
 * Write BEGIN to the output stream.
 */
void
logicalrep_write_begin(StringInfo out, ReorderBufferTXN *txn)
{
    pq_sendbyte(out, LOGICAL_REP_MSG_BEGIN);

    /* fixed fields */
    pq_sendint64(out, txn->final_lsn);
    pq_sendint64(out, txn->commit_time);
    pq_sendint32(out, txn->xid);
}

/*
 * Read transaction BEGIN from the stream.
 */
void
logicalrep_read_begin(StringInfo in, LogicalRepBeginData *begin_data)
{
    /* read fields */
    begin_data->final_lsn = pq_getmsgint64(in);
    if (begin_data->final_lsn == InvalidXLogRecPtr)
        elog(ERROR, "final_lsn not set in begin message");
    begin_data->committime = pq_getmsgint64(in);
    begin_data->xid = pq_getmsgint(in, 4);
}


/*
 * Write COMMIT to the output stream.
 */
void
logicalrep_write_commit(StringInfo out, ReorderBufferTXN *txn,
                        XLogRecPtr commit_lsn)
{
    uint8       flags = 0;

    pq_sendbyte(out, LOGICAL_REP_MSG_COMMIT);

    /* send the flags field (unused for now) */
    pq_sendbyte(out, flags);

    /* send fields */
    pq_sendint64(out, commit_lsn);
    pq_sendint64(out, txn->end_lsn);
    pq_sendint64(out, txn->commit_time);
}

/*
 * Read transaction COMMIT from the stream.
 */
void
logicalrep_read_commit(StringInfo in, LogicalRepCommitData *commit_data)
{
    /* read flags (unused for now) */
    uint8       flags = pq_getmsgbyte(in);

    if (flags != 0)
        elog(ERROR, "unrecognized flags %u in commit message", flags);

    /* read fields */
    commit_data->commit_lsn = pq_getmsgint64(in);
    commit_data->end_lsn = pq_getmsgint64(in);
    commit_data->committime = pq_getmsgint64(in);
}

/*
 * Write ORIGIN to the output stream.
 */
void
logicalrep_write_origin(StringInfo out, const char *origin,
                        XLogRecPtr origin_lsn)
{
    pq_sendbyte(out, LOGICAL_REP_MSG_ORIGIN);

    /* fixed fields */
    pq_sendint64(out, origin_lsn);

    /* origin string */
    pq_sendstring(out, origin);
}

/*
 * Read ORIGIN from the output stream.
 */
char *
logicalrep_read_origin(StringInfo in, XLogRecPtr *origin_lsn)
{
    /* fixed fields */
    *origin_lsn = pq_getmsgint64(in);

    /* return origin */
    return pstrdup(pq_getmsgstring(in));
}

/*
 * Write INSERT to the output stream.
 */
void
logicalrep_write_insert(StringInfo out, TransactionId xid, Relation rel,
                        HeapTuple newtuple, bool binary)
{
    pq_sendbyte(out, LOGICAL_REP_MSG_INSERT);

    /* transaction ID (if not valid, we're not streaming) */
    if (TransactionIdIsValid(xid))
        pq_sendint32(out, xid);

    /* use Oid as relation identifier */
    pq_sendint32(out, RelationGetRelid(rel));

    pq_sendbyte(out, 'N');      /* new tuple follows */
    logicalrep_write_tuple(out, rel, newtuple, binary);
}

/*
 * Read INSERT from stream.
 *
 * Fills the new tuple.
 */
LogicalRepRelId
logicalrep_read_insert(StringInfo in, LogicalRepTupleData *newtup)
{
    char        action;
    LogicalRepRelId relid;

    /* read the relation id */
    relid = pq_getmsgint(in, 4);

    action = pq_getmsgbyte(in);
    if (action != 'N')
        elog(ERROR, "expected new tuple but got %d",
             action);

    logicalrep_read_tuple(in, newtup);

    return relid;
}

/*
 * Write UPDATE to the output stream.
 */
void
logicalrep_write_update(StringInfo out, TransactionId xid, Relation rel,
                        HeapTuple oldtuple, HeapTuple newtuple, bool binary)
{
    pq_sendbyte(out, LOGICAL_REP_MSG_UPDATE);

    Assert(rel->rd_rel->relreplident == REPLICA_IDENTITY_DEFAULT ||
           rel->rd_rel->relreplident == REPLICA_IDENTITY_FULL ||
           rel->rd_rel->relreplident == REPLICA_IDENTITY_INDEX);

    /* transaction ID (if not valid, we're not streaming) */
    if (TransactionIdIsValid(xid))
        pq_sendint32(out, xid);

    /* use Oid as relation identifier */
    pq_sendint32(out, RelationGetRelid(rel));

    if (oldtuple != NULL)
    {
        if (rel->rd_rel->relreplident == REPLICA_IDENTITY_FULL)
            pq_sendbyte(out, 'O');  /* old tuple follows */
        else
            pq_sendbyte(out, 'K');  /* old key follows */
        logicalrep_write_tuple(out, rel, oldtuple, binary);
    }

    pq_sendbyte(out, 'N');      /* new tuple follows */
    logicalrep_write_tuple(out, rel, newtuple, binary);
}

/*
 * Read UPDATE from stream.
 */
LogicalRepRelId
logicalrep_read_update(StringInfo in, bool *has_oldtuple,
                       LogicalRepTupleData *oldtup,
                       LogicalRepTupleData *newtup)
{
    char        action;
    LogicalRepRelId relid;

    /* read the relation id */
    relid = pq_getmsgint(in, 4);

    /* read and verify action */
    action = pq_getmsgbyte(in);
    if (action != 'K' && action != 'O' && action != 'N')
        elog(ERROR, "expected action 'N', 'O' or 'K', got %c",
             action);

    /* check for old tuple */
    if (action == 'K' || action == 'O')
    {
        logicalrep_read_tuple(in, oldtup);
        *has_oldtuple = true;

        action = pq_getmsgbyte(in);
    }
    else
        *has_oldtuple = false;

    /* check for new  tuple */
    if (action != 'N')
        elog(ERROR, "expected action 'N', got %c",
             action);

    logicalrep_read_tuple(in, newtup);

    return relid;
}

/*
 * Write DELETE to the output stream.
 */
void
logicalrep_write_delete(StringInfo out, TransactionId xid, Relation rel,
                        HeapTuple oldtuple, bool binary)
{
    Assert(rel->rd_rel->relreplident == REPLICA_IDENTITY_DEFAULT ||
           rel->rd_rel->relreplident == REPLICA_IDENTITY_FULL ||
           rel->rd_rel->relreplident == REPLICA_IDENTITY_INDEX);

    pq_sendbyte(out, LOGICAL_REP_MSG_DELETE);

    /* transaction ID (if not valid, we're not streaming) */
    if (TransactionIdIsValid(xid))
        pq_sendint32(out, xid);

    /* use Oid as relation identifier */
    pq_sendint32(out, RelationGetRelid(rel));

    if (rel->rd_rel->relreplident == REPLICA_IDENTITY_FULL)
        pq_sendbyte(out, 'O');  /* old tuple follows */
    else
        pq_sendbyte(out, 'K');  /* old key follows */

    logicalrep_write_tuple(out, rel, oldtuple, binary);
}

/*
 * Read DELETE from stream.
 *
 * Fills the old tuple.
 */
LogicalRepRelId
logicalrep_read_delete(StringInfo in, LogicalRepTupleData *oldtup)
{
    char        action;
    LogicalRepRelId relid;

    /* read the relation id */
    relid = pq_getmsgint(in, 4);

    /* read and verify action */
    action = pq_getmsgbyte(in);
    if (action != 'K' && action != 'O')
        elog(ERROR, "expected action 'O' or 'K', got %c", action);

    logicalrep_read_tuple(in, oldtup);

    return relid;
}

/*
 * Write TRUNCATE to the output stream.
 */
void
logicalrep_write_truncate(StringInfo out,
                          TransactionId xid,
                          int nrelids,
                          Oid relids[],
                          bool cascade, bool restart_seqs)
{
    int         i;
    uint8       flags = 0;

    pq_sendbyte(out, LOGICAL_REP_MSG_TRUNCATE);

    /* transaction ID (if not valid, we're not streaming) */
    if (TransactionIdIsValid(xid))
        pq_sendint32(out, xid);

    pq_sendint32(out, nrelids);

    /* encode and send truncate flags */
    if (cascade)
        flags |= TRUNCATE_CASCADE;
    if (restart_seqs)
        flags |= TRUNCATE_RESTART_SEQS;
    pq_sendint8(out, flags);

    for (i = 0; i < nrelids; i++)
        pq_sendint32(out, relids[i]);
}

/*
 * Read TRUNCATE from stream.
 */
List *
logicalrep_read_truncate(StringInfo in,
                         bool *cascade, bool *restart_seqs)
{
    int         i;
    int         nrelids;
    List       *relids = NIL;
    uint8       flags;

    nrelids = pq_getmsgint(in, 4);

    /* read and decode truncate flags */
    flags = pq_getmsgint(in, 1);
    *cascade = (flags & TRUNCATE_CASCADE) > 0;
    *restart_seqs = (flags & TRUNCATE_RESTART_SEQS) > 0;

    for (i = 0; i < nrelids; i++)
        relids = lappend_oid(relids, pq_getmsgint(in, 4));

    return relids;
}

/*
 * Write relation description to the output stream.
 */
void
logicalrep_write_rel(StringInfo out, TransactionId xid, Relation rel)
{
    char       *relname;

    pq_sendbyte(out, LOGICAL_REP_MSG_RELATION);

    /* transaction ID (if not valid, we're not streaming) */
    if (TransactionIdIsValid(xid))
        pq_sendint32(out, xid);

    /* use Oid as relation identifier */
    pq_sendint32(out, RelationGetRelid(rel));

    /* send qualified relation name */
    logicalrep_write_namespace(out, RelationGetNamespace(rel));
    relname = RelationGetRelationName(rel);
    pq_sendstring(out, relname);

    /* send replica identity */
    pq_sendbyte(out, rel->rd_rel->relreplident);

    /* send the attribute info */
    logicalrep_write_attrs(out, rel);
}

/*
 * Read the relation info from stream and return as LogicalRepRelation.
 */
LogicalRepRelation *
logicalrep_read_rel(StringInfo in)
{
    LogicalRepRelation *rel = palloc(sizeof(LogicalRepRelation));

    rel->remoteid = pq_getmsgint(in, 4);

    /* Read relation name from stream */
    rel->nspname = pstrdup(logicalrep_read_namespace(in));
    rel->relname = pstrdup(pq_getmsgstring(in));

    /* Read the replica identity. */
    rel->replident = pq_getmsgbyte(in);

    /* Get attribute description */
    logicalrep_read_attrs(in, rel);

    return rel;
}

/*
 * Write type info to the output stream.
 *
 * This function will always write base type info.
 */
void
logicalrep_write_typ(StringInfo out, TransactionId xid, Oid typoid)
{
    Oid         basetypoid = getBaseType(typoid);
    HeapTuple   tup;
    Form_pg_type typtup;

    pq_sendbyte(out, LOGICAL_REP_MSG_TYPE);

    /* transaction ID (if not valid, we're not streaming) */
    if (TransactionIdIsValid(xid))
        pq_sendint32(out, xid);

    tup = SearchSysCache1(TYPEOID, ObjectIdGetDatum(basetypoid));
    if (!HeapTupleIsValid(tup))
        elog(ERROR, "cache lookup failed for type %u", basetypoid);
    typtup = (Form_pg_type) GETSTRUCT(tup);

    /* use Oid as relation identifier */
    pq_sendint32(out, typoid);

    /* send qualified type name */
    logicalrep_write_namespace(out, typtup->typnamespace);
    pq_sendstring(out, NameStr(typtup->typname));

    ReleaseSysCache(tup);
}

/*
 * Read type info from the output stream.
 */
void
logicalrep_read_typ(StringInfo in, LogicalRepTyp *ltyp)
{
    ltyp->remoteid = pq_getmsgint(in, 4);

    /* Read type name from stream */
    ltyp->nspname = pstrdup(logicalrep_read_namespace(in));
    ltyp->typname = pstrdup(pq_getmsgstring(in));
}

/*
 * Write a tuple to the outputstream, in the most efficient format possible.
 */
static void
logicalrep_write_tuple(StringInfo out, Relation rel, HeapTuple tuple, bool binary)
{
    TupleDesc   desc;
    Datum       values[MaxTupleAttributeNumber];
    bool        isnull[MaxTupleAttributeNumber];
    int         i;
    uint16      nliveatts = 0;

    desc = RelationGetDescr(rel);

    for (i = 0; i < desc->natts; i++)
    {
        if (TupleDescAttr(desc, i)->attisdropped || TupleDescAttr(desc, i)->attgenerated)
            continue;
        nliveatts++;
    }
    pq_sendint16(out, nliveatts);

    /* try to allocate enough memory from the get-go */
    enlargeStringInfo(out, tuple->t_len +
                      nliveatts * (1 + 4));

    heap_deform_tuple(tuple, desc, values, isnull);

    /* Write the values */
    for (i = 0; i < desc->natts; i++)
    {
        HeapTuple   typtup;
        Form_pg_type typclass;
        Form_pg_attribute att = TupleDescAttr(desc, i);

        if (att->attisdropped || att->attgenerated)
            continue;

        if (isnull[i])
        {
            pq_sendbyte(out, LOGICALREP_COLUMN_NULL);
            continue;
        }

        if (att->attlen == -1 && VARATT_IS_EXTERNAL_ONDISK(values[i]))
        {
            /*
             * Unchanged toasted datum.  (Note that we don't promise to detect
             * unchanged data in general; this is just a cheap check to avoid
             * sending large values unnecessarily.)
             */
            pq_sendbyte(out, LOGICALREP_COLUMN_UNCHANGED);
            continue;
        }

        typtup = SearchSysCache1(TYPEOID, ObjectIdGetDatum(att->atttypid));
        if (!HeapTupleIsValid(typtup))
            elog(ERROR, "cache lookup failed for type %u", att->atttypid);
        typclass = (Form_pg_type) GETSTRUCT(typtup);

        /*
         * Send in binary if requested and type has suitable send function.
         */
        if (binary && OidIsValid(typclass->typsend))
        {
            bytea      *outputbytes;
            int         len;

            pq_sendbyte(out, LOGICALREP_COLUMN_BINARY);
            outputbytes = OidSendFunctionCall(typclass->typsend, values[i]);
            len = VARSIZE(outputbytes) - VARHDRSZ;
            pq_sendint(out, len, 4);    /* length */
            pq_sendbytes(out, VARDATA(outputbytes), len);   /* data */
            pfree(outputbytes);
        }
        else
        {
            char       *outputstr;

            pq_sendbyte(out, LOGICALREP_COLUMN_TEXT);
            outputstr = OidOutputFunctionCall(typclass->typoutput, values[i]);
            pq_sendcountedtext(out, outputstr, strlen(outputstr), false);
            pfree(outputstr);
        }

        ReleaseSysCache(typtup);
    }
}

/*
 * Read tuple in logical replication format from stream.
 */
static void
logicalrep_read_tuple(StringInfo in, LogicalRepTupleData *tuple)
{
    int         i;
    int         natts;

    /* Get number of attributes */
    natts = pq_getmsgint(in, 2);

    /* Allocate space for per-column values; zero out unused StringInfoDatas */
    tuple->colvalues = (StringInfoData *) palloc0(natts * sizeof(StringInfoData));
    tuple->colstatus = (char *) palloc(natts * sizeof(char));
    tuple->ncols = natts;

    /* Read the data */
    for (i = 0; i < natts; i++)
    {
        char        kind;
        int         len;
        StringInfo  value = &tuple->colvalues[i];

        kind = pq_getmsgbyte(in);
        tuple->colstatus[i] = kind;

        switch (kind)
        {
            case LOGICALREP_COLUMN_NULL:
                /* nothing more to do */
                break;
            case LOGICALREP_COLUMN_UNCHANGED:
                /* we don't receive the value of an unchanged column */
                break;
            case LOGICALREP_COLUMN_TEXT:
                len = pq_getmsgint(in, 4);  /* read length */

                /* and data */
                value->data = palloc(len + 1);
                pq_copymsgbytes(in, value->data, len);
                value->data[len] = '\0';
                /* make StringInfo fully valid */
                value->len = len;
                value->cursor = 0;
                value->maxlen = len;
                break;
            case LOGICALREP_COLUMN_BINARY:
                len = pq_getmsgint(in, 4);  /* read length */

                /* and data */
                value->data = palloc(len + 1);
                pq_copymsgbytes(in, value->data, len);
                /* not strictly necessary but per StringInfo practice */
                value->data[len] = '\0';
                /* make StringInfo fully valid */
                value->len = len;
                value->cursor = 0;
                value->maxlen = len;
                break;
            default:
                elog(ERROR, "unrecognized data representation type '%c'", kind);
        }
    }
}

/*
 * Write relation attribute metadata to the stream.
 */
static void
logicalrep_write_attrs(StringInfo out, Relation rel)
{
    TupleDesc   desc;
    int         i;
    uint16      nliveatts = 0;
    Bitmapset  *idattrs = NULL;
    bool        replidentfull;

    desc = RelationGetDescr(rel);

    /* send number of live attributes */
    for (i = 0; i < desc->natts; i++)
    {
        if (TupleDescAttr(desc, i)->attisdropped || TupleDescAttr(desc, i)->attgenerated)
            continue;
        nliveatts++;
    }
    pq_sendint16(out, nliveatts);

    /* fetch bitmap of REPLICATION IDENTITY attributes */
    replidentfull = (rel->rd_rel->relreplident == REPLICA_IDENTITY_FULL);
    if (!replidentfull)
        idattrs = RelationGetIndexAttrBitmap(rel,
                                             INDEX_ATTR_BITMAP_IDENTITY_KEY);

    /* send the attributes */
    for (i = 0; i < desc->natts; i++)
    {
        Form_pg_attribute att = TupleDescAttr(desc, i);
        uint8       flags = 0;

        if (att->attisdropped || att->attgenerated)
            continue;

        /* REPLICA IDENTITY FULL means all columns are sent as part of key. */
        if (replidentfull ||
            bms_is_member(att->attnum - FirstLowInvalidHeapAttributeNumber,
                          idattrs))
            flags |= LOGICALREP_IS_REPLICA_IDENTITY;

        pq_sendbyte(out, flags);

        /* attribute name */
        pq_sendstring(out, NameStr(att->attname));

        /* attribute type id */
        pq_sendint32(out, (int) att->atttypid);

        /* attribute mode */
        pq_sendint32(out, att->atttypmod);
    }

    bms_free(idattrs);
}

/*
 * Read relation attribute metadata from the stream.
 */
static void
logicalrep_read_attrs(StringInfo in, LogicalRepRelation *rel)
{
    int         i;
    int         natts;
    char      **attnames;
    Oid        *atttyps;
    Bitmapset  *attkeys = NULL;

    natts = pq_getmsgint(in, 2);
    attnames = palloc(natts * sizeof(char *));
    atttyps = palloc(natts * sizeof(Oid));

    /* read the attributes */
    for (i = 0; i < natts; i++)
    {
        uint8       flags;

        /* Check for replica identity column */
        flags = pq_getmsgbyte(in);
        if (flags & LOGICALREP_IS_REPLICA_IDENTITY)
            attkeys = bms_add_member(attkeys, i);

        /* attribute name */
        attnames[i] = pstrdup(pq_getmsgstring(in));

        /* attribute type id */
        atttyps[i] = (Oid) pq_getmsgint(in, 4);

        /* we ignore attribute mode for now */
        (void) pq_getmsgint(in, 4);
    }

    rel->attnames = attnames;
    rel->atttyps = atttyps;
    rel->attkeys = attkeys;
    rel->natts = natts;
}

/*
 * Write the namespace name or empty string for pg_catalog (to save space).
 */
static void
logicalrep_write_namespace(StringInfo out, Oid nspid)
{
    if (nspid == PG_CATALOG_NAMESPACE)
        pq_sendbyte(out, '\0');
    else
    {
        char       *nspname = get_namespace_name(nspid);

        if (nspname == NULL)
            elog(ERROR, "cache lookup failed for namespace %u",
                 nspid);

        pq_sendstring(out, nspname);
    }
}

/*
 * Read the namespace name while treating empty string as pg_catalog.
 */
static const char *
logicalrep_read_namespace(StringInfo in)
{
    const char *nspname = pq_getmsgstring(in);

    if (nspname[0] == '\0')
        nspname = "pg_catalog";

    return nspname;
}

/*
 * Write the information for the start stream message to the output stream.
 */
void
logicalrep_write_stream_start(StringInfo out,
                              TransactionId xid, bool first_segment)
{
    pq_sendbyte(out, LOGICAL_REP_MSG_STREAM_START);

    Assert(TransactionIdIsValid(xid));

    /* transaction ID (we're starting to stream, so must be valid) */
    pq_sendint32(out, xid);

    /* 1 if this is the first streaming segment for this xid */
    pq_sendbyte(out, first_segment ? 1 : 0);
}

/*
 * Read the information about the start stream message from output stream.
 */
TransactionId
logicalrep_read_stream_start(StringInfo in, bool *first_segment)
{
    TransactionId xid;

    Assert(first_segment);

    xid = pq_getmsgint(in, 4);
    *first_segment = (pq_getmsgbyte(in) == 1);

    return xid;
}

/*
 * Write the stop stream message to the output stream.
 */
void
logicalrep_write_stream_stop(StringInfo out)
{
    pq_sendbyte(out, LOGICAL_REP_MSG_STREAM_END);
}

/*
 * Write STREAM COMMIT to the output stream.
 */
void
logicalrep_write_stream_commit(StringInfo out, ReorderBufferTXN *txn,
                               XLogRecPtr commit_lsn)
{
    uint8       flags = 0;

    pq_sendbyte(out, LOGICAL_REP_MSG_STREAM_COMMIT);

    Assert(TransactionIdIsValid(txn->xid));

    /* transaction ID */
    pq_sendint32(out, txn->xid);

    /* send the flags field (unused for now) */
    pq_sendbyte(out, flags);

    /* send fields */
    pq_sendint64(out, commit_lsn);
    pq_sendint64(out, txn->end_lsn);
    pq_sendint64(out, txn->commit_time);
}

/*
 * Read STREAM COMMIT from the output stream.
 */
TransactionId
logicalrep_read_stream_commit(StringInfo in, LogicalRepCommitData *commit_data)
{
    TransactionId xid;
    uint8       flags;

    xid = pq_getmsgint(in, 4);

    /* read flags (unused for now) */
    flags = pq_getmsgbyte(in);

    if (flags != 0)
        elog(ERROR, "unrecognized flags %u in commit message", flags);

    /* read fields */
    commit_data->commit_lsn = pq_getmsgint64(in);
    commit_data->end_lsn = pq_getmsgint64(in);
    commit_data->committime = pq_getmsgint64(in);

    return xid;
}

/*
 * Write STREAM ABORT to the output stream. Note that xid and subxid will be
 * same for the top-level transaction abort.
 */
void
logicalrep_write_stream_abort(StringInfo out, TransactionId xid,
                              TransactionId subxid)
{
    pq_sendbyte(out, LOGICAL_REP_MSG_STREAM_ABORT);

    Assert(TransactionIdIsValid(xid) && TransactionIdIsValid(subxid));

    /* transaction ID */
    pq_sendint32(out, xid);
    pq_sendint32(out, subxid);
}

/*
 * Read STREAM ABORT from the output stream.
 */
void
logicalrep_read_stream_abort(StringInfo in, TransactionId *xid,
                             TransactionId *subxid)
{
    Assert(xid && subxid);

    *xid = pq_getmsgint(in, 4);
    *subxid = pq_getmsgint(in, 4);
}