dblink.c

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/*
 * dblink.c
 *
 * Functions returning results from a remote database
 *
 * Joe Conway <mail@joeconway.com>
 * And contributors:
 * Darko Prenosil <Darko.Prenosil@finteh.hr>
 * Shridhar Daithankar <shridhar_daithankar@persistent.co.in>
 *
 * contrib/dblink/dblink.c
 * Copyright (c) 2001-2020, PostgreSQL Global Development Group
 * ALL RIGHTS RESERVED;
 *
 * Permission to use, copy, modify, and distribute this software and its
 * documentation for any purpose, without fee, and without a written agreement
 * is hereby granted, provided that the above copyright notice and this
 * paragraph and the following two paragraphs appear in all copies.
 *
 * IN NO EVENT SHALL THE AUTHOR OR DISTRIBUTORS BE LIABLE TO ANY PARTY FOR
 * DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, INCLUDING
 * LOST PROFITS, ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS
 * DOCUMENTATION, EVEN IF THE AUTHOR OR DISTRIBUTORS HAVE BEEN ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 *
 * THE AUTHOR AND DISTRIBUTORS SPECIFICALLY DISCLAIMS ANY WARRANTIES,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS FOR A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS
 * ON AN "AS IS" BASIS, AND THE AUTHOR AND DISTRIBUTORS HAS NO OBLIGATIONS TO
 * PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
 *
 */
#include "postgres.h"

#include <limits.h>

#include "access/htup_details.h"
#include "access/relation.h"
#include "access/reloptions.h"
#include "access/table.h"
#include "catalog/indexing.h"
#include "catalog/namespace.h"
#include "catalog/pg_foreign_data_wrapper.h"
#include "catalog/pg_foreign_server.h"
#include "catalog/pg_type.h"
#include "catalog/pg_user_mapping.h"
#include "executor/spi.h"
#include "foreign/foreign.h"
#include "funcapi.h"
#include "lib/stringinfo.h"
#include "libpq-fe.h"
#include "mb/pg_wchar.h"
#include "miscadmin.h"
#include "parser/scansup.h"
#include "utils/acl.h"
#include "utils/builtins.h"
#include "utils/fmgroids.h"
#include "utils/guc.h"
#include "utils/lsyscache.h"
#include "utils/memutils.h"
#include "utils/rel.h"
#include "utils/varlena.h"

PG_MODULE_MAGIC;

typedef struct remoteConn
{
    PGconn     *conn;           /* Hold the remote connection */
    int         openCursorCount;    /* The number of open cursors */
    bool        newXactForCursor;   /* Opened a transaction for a cursor */
} remoteConn;

typedef struct storeInfo
{
    FunctionCallInfo fcinfo;
    Tuplestorestate *tuplestore;
    AttInMetadata *attinmeta;
    MemoryContext tmpcontext;
    char      **cstrs;
    /* temp storage for results to avoid leaks on exception */
    PGresult   *last_res;
    PGresult   *cur_res;
} storeInfo;

/*
 * Internal declarations
 */
static Datum dblink_record_internal(FunctionCallInfo fcinfo, bool is_async);
static void prepTuplestoreResult(FunctionCallInfo fcinfo);
static void materializeResult(FunctionCallInfo fcinfo, PGconn *conn,
                              PGresult *res);
static void materializeQueryResult(FunctionCallInfo fcinfo,
                                   PGconn *conn,
                                   const char *conname,
                                   const char *sql,
                                   bool fail);
static PGresult *storeQueryResult(volatile storeInfo *sinfo, PGconn *conn, const char *sql);
static void storeRow(volatile storeInfo *sinfo, PGresult *res, bool first);
static remoteConn *getConnectionByName(const char *name);
static HTAB *createConnHash(void);
static void createNewConnection(const char *name, remoteConn *rconn);
static void deleteConnection(const char *name);
static char **get_pkey_attnames(Relation rel, int16 *indnkeyatts);
static char **get_text_array_contents(ArrayType *array, int *numitems);
static char *get_sql_insert(Relation rel, int *pkattnums, int pknumatts, char **src_pkattvals, char **tgt_pkattvals);
static char *get_sql_delete(Relation rel, int *pkattnums, int pknumatts, char **tgt_pkattvals);
static char *get_sql_update(Relation rel, int *pkattnums, int pknumatts, char **src_pkattvals, char **tgt_pkattvals);
static char *quote_ident_cstr(char *rawstr);
static int  get_attnum_pk_pos(int *pkattnums, int pknumatts, int key);
static HeapTuple get_tuple_of_interest(Relation rel, int *pkattnums, int pknumatts, char **src_pkattvals);
static Relation get_rel_from_relname(text *relname_text, LOCKMODE lockmode, AclMode aclmode);
static char *generate_relation_name(Relation rel);
static void dblink_connstr_check(const char *connstr);
static void dblink_security_check(PGconn *conn, remoteConn *rconn);
static void dblink_res_error(PGconn *conn, const char *conname, PGresult *res,
                             bool fail, const char *fmt,...) pg_attribute_printf(5, 6);
static char *get_connect_string(const char *servername);
static char *escape_param_str(const char *from);
static void validate_pkattnums(Relation rel,
                               int2vector *pkattnums_arg, int32 pknumatts_arg,
                               int **pkattnums, int *pknumatts);
static bool is_valid_dblink_option(const PQconninfoOption *options,
                                   const char *option, Oid context);
static int  applyRemoteGucs(PGconn *conn);
static void restoreLocalGucs(int nestlevel);

/* Global */
static remoteConn *pconn = NULL;
static HTAB *remoteConnHash = NULL;

/*
 *  Following is list that holds multiple remote connections.
 *  Calling convention of each dblink function changes to accept
 *  connection name as the first parameter. The connection list is
 *  much like ecpg e.g. a mapping between a name and a PGconn object.
 */

typedef struct remoteConnHashEnt
{
    char        name[NAMEDATALEN];
    remoteConn *rconn;
} remoteConnHashEnt;

/* initial number of connection hashes */
#define NUMCONN 16

static char *
xpstrdup(const char *in)
{
    if (in == NULL)
        return NULL;
    return pstrdup(in);
}

static void
pg_attribute_noreturn()
dblink_res_internalerror(PGconn *conn, PGresult *res, const char *p2)
{
    char       *msg = pchomp(PQerrorMessage(conn));

    if (res)
        PQclear(res);
    elog(ERROR, "%s: %s", p2, msg);
}

static void
pg_attribute_noreturn()
dblink_conn_not_avail(const char *conname)
{
    if (conname)
        ereport(ERROR,
                (errcode(ERRCODE_CONNECTION_DOES_NOT_EXIST),
                 errmsg("connection \"%s\" not available", conname)));
    else
        ereport(ERROR,
                (errcode(ERRCODE_CONNECTION_DOES_NOT_EXIST),
                 errmsg("connection not available")));
}

static void
dblink_get_conn(char *conname_or_str,
                PGconn *volatile *conn_p, char **conname_p, volatile bool *freeconn_p)
{
    remoteConn *rconn = getConnectionByName(conname_or_str);
    PGconn     *conn;
    char       *conname;
    bool        freeconn;

    if (rconn)
    {
        conn = rconn->conn;
        conname = conname_or_str;
        freeconn = false;
    }
    else
    {
        const char *connstr;

        connstr = get_connect_string(conname_or_str);
        if (connstr == NULL)
            connstr = conname_or_str;
        dblink_connstr_check(connstr);
        conn = PQconnectdb(connstr);
        if (PQstatus(conn) == CONNECTION_BAD)
        {
            char       *msg = pchomp(PQerrorMessage(conn));

            PQfinish(conn);
            ereport(ERROR,
                    (errcode(ERRCODE_SQLCLIENT_UNABLE_TO_ESTABLISH_SQLCONNECTION),
                     errmsg("could not establish connection"),
                     errdetail_internal("%s", msg)));
        }
        dblink_security_check(conn, rconn);
        if (PQclientEncoding(conn) != GetDatabaseEncoding())
            PQsetClientEncoding(conn, GetDatabaseEncodingName());
        freeconn = true;
        conname = NULL;
    }

    *conn_p = conn;
    *conname_p = conname;
    *freeconn_p = freeconn;
}

static PGconn *
dblink_get_named_conn(const char *conname)
{
    remoteConn *rconn = getConnectionByName(conname);

    if (rconn)
        return rconn->conn;

    dblink_conn_not_avail(conname);
    return NULL;                /* keep compiler quiet */
}

static void
dblink_init(void)
{
    if (!pconn)
    {
        pconn = (remoteConn *) MemoryContextAlloc(TopMemoryContext, sizeof(remoteConn));
        pconn->conn = NULL;
        pconn->openCursorCount = 0;
        pconn->newXactForCursor = false;
    }
}

/*
 * Create a persistent connection to another database
 */
PG_FUNCTION_INFO_V1(dblink_connect);
Datum
dblink_connect(PG_FUNCTION_ARGS)
{
    char       *conname_or_str = NULL;
    char       *connstr = NULL;
    char       *connname = NULL;
    char       *msg;
    PGconn     *conn = NULL;
    remoteConn *rconn = NULL;

    dblink_init();

    if (PG_NARGS() == 2)
    {
        conname_or_str = text_to_cstring(PG_GETARG_TEXT_PP(1));
        connname = text_to_cstring(PG_GETARG_TEXT_PP(0));
    }
    else if (PG_NARGS() == 1)
        conname_or_str = text_to_cstring(PG_GETARG_TEXT_PP(0));

    if (connname)
        rconn = (remoteConn *) MemoryContextAlloc(TopMemoryContext,
                                                  sizeof(remoteConn));

    /* first check for valid foreign data server */
    connstr = get_connect_string(conname_or_str);
    if (connstr == NULL)
        connstr = conname_or_str;

    /* check password in connection string if not superuser */
    dblink_connstr_check(connstr);
    conn = PQconnectdb(connstr);

    if (PQstatus(conn) == CONNECTION_BAD)
    {
        msg = pchomp(PQerrorMessage(conn));
        PQfinish(conn);
        if (rconn)
            pfree(rconn);

        ereport(ERROR,
                (errcode(ERRCODE_SQLCLIENT_UNABLE_TO_ESTABLISH_SQLCONNECTION),
                 errmsg("could not establish connection"),
                 errdetail_internal("%s", msg)));
    }

    /* check password actually used if not superuser */
    dblink_security_check(conn, rconn);

    /* attempt to set client encoding to match server encoding, if needed */
    if (PQclientEncoding(conn) != GetDatabaseEncoding())
        PQsetClientEncoding(conn, GetDatabaseEncodingName());

    if (connname)
    {
        rconn->conn = conn;
        createNewConnection(connname, rconn);
    }
    else
    {
        if (pconn->conn)
            PQfinish(pconn->conn);
        pconn->conn = conn;
    }

    PG_RETURN_TEXT_P(cstring_to_text("OK"));
}

/*
 * Clear a persistent connection to another database
 */
PG_FUNCTION_INFO_V1(dblink_disconnect);
Datum
dblink_disconnect(PG_FUNCTION_ARGS)
{
    char       *conname = NULL;
    remoteConn *rconn = NULL;
    PGconn     *conn = NULL;

    dblink_init();

    if (PG_NARGS() == 1)
    {
        conname = text_to_cstring(PG_GETARG_TEXT_PP(0));
        rconn = getConnectionByName(conname);
        if (rconn)
            conn = rconn->conn;
    }
    else
        conn = pconn->conn;

    if (!conn)
        dblink_conn_not_avail(conname);

    PQfinish(conn);
    if (rconn)
    {
        deleteConnection(conname);
        pfree(rconn);
    }
    else
        pconn->conn = NULL;

    PG_RETURN_TEXT_P(cstring_to_text("OK"));
}

/*
 * opens a cursor using a persistent connection
 */
PG_FUNCTION_INFO_V1(dblink_open);
Datum
dblink_open(PG_FUNCTION_ARGS)
{
    PGresult   *res = NULL;
    PGconn     *conn;
    char       *curname = NULL;
    char       *sql = NULL;
    char       *conname = NULL;
    StringInfoData buf;
    remoteConn *rconn = NULL;
    bool        fail = true;    /* default to backward compatible behavior */

    dblink_init();
    initStringInfo(&buf);

    if (PG_NARGS() == 2)
    {
        /* text,text */
        curname = text_to_cstring(PG_GETARG_TEXT_PP(0));
        sql = text_to_cstring(PG_GETARG_TEXT_PP(1));
        rconn = pconn;
    }
    else if (PG_NARGS() == 3)
    {
        /* might be text,text,text or text,text,bool */
        if (get_fn_expr_argtype(fcinfo->flinfo, 2) == BOOLOID)
        {
            curname = text_to_cstring(PG_GETARG_TEXT_PP(0));
            sql = text_to_cstring(PG_GETARG_TEXT_PP(1));
            fail = PG_GETARG_BOOL(2);
            rconn = pconn;
        }
        else
        {
            conname = text_to_cstring(PG_GETARG_TEXT_PP(0));
            curname = text_to_cstring(PG_GETARG_TEXT_PP(1));
            sql = text_to_cstring(PG_GETARG_TEXT_PP(2));
            rconn = getConnectionByName(conname);
        }
    }
    else if (PG_NARGS() == 4)
    {
        /* text,text,text,bool */
        conname = text_to_cstring(PG_GETARG_TEXT_PP(0));
        curname = text_to_cstring(PG_GETARG_TEXT_PP(1));
        sql = text_to_cstring(PG_GETARG_TEXT_PP(2));
        fail = PG_GETARG_BOOL(3);
        rconn = getConnectionByName(conname);
    }

    if (!rconn || !rconn->conn)
        dblink_conn_not_avail(conname);

    conn = rconn->conn;

    /* If we are not in a transaction, start one */
    if (PQtransactionStatus(conn) == PQTRANS_IDLE)
    {
        res = PQexec(conn, "BEGIN");
        if (PQresultStatus(res) != PGRES_COMMAND_OK)
            dblink_res_internalerror(conn, res, "begin error");
        PQclear(res);
        rconn->newXactForCursor = true;

        /*
         * Since transaction state was IDLE, we force cursor count to
         * initially be 0. This is needed as a previous ABORT might have wiped
         * out our transaction without maintaining the cursor count for us.
         */
        rconn->openCursorCount = 0;
    }

    /* if we started a transaction, increment cursor count */
    if (rconn->newXactForCursor)
        (rconn->openCursorCount)++;

    appendStringInfo(&buf, "DECLARE %s CURSOR FOR %s", curname, sql);
    res = PQexec(conn, buf.data);
    if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        dblink_res_error(conn, conname, res, fail,
                         "while opening cursor \"%s\"", curname);
        PG_RETURN_TEXT_P(cstring_to_text("ERROR"));
    }

    PQclear(res);
    PG_RETURN_TEXT_P(cstring_to_text("OK"));
}

/*
 * closes a cursor
 */
PG_FUNCTION_INFO_V1(dblink_close);
Datum
dblink_close(PG_FUNCTION_ARGS)
{
    PGconn     *conn;
    PGresult   *res = NULL;
    char       *curname = NULL;
    char       *conname = NULL;
    StringInfoData buf;
    remoteConn *rconn = NULL;
    bool        fail = true;    /* default to backward compatible behavior */

    dblink_init();
    initStringInfo(&buf);

    if (PG_NARGS() == 1)
    {
        /* text */
        curname = text_to_cstring(PG_GETARG_TEXT_PP(0));
        rconn = pconn;
    }
    else if (PG_NARGS() == 2)
    {
        /* might be text,text or text,bool */
        if (get_fn_expr_argtype(fcinfo->flinfo, 1) == BOOLOID)
        {
            curname = text_to_cstring(PG_GETARG_TEXT_PP(0));
            fail = PG_GETARG_BOOL(1);
            rconn = pconn;
        }
        else
        {
            conname = text_to_cstring(PG_GETARG_TEXT_PP(0));
            curname = text_to_cstring(PG_GETARG_TEXT_PP(1));
            rconn = getConnectionByName(conname);
        }
    }
    if (PG_NARGS() == 3)
    {
        /* text,text,bool */
        conname = text_to_cstring(PG_GETARG_TEXT_PP(0));
        curname = text_to_cstring(PG_GETARG_TEXT_PP(1));
        fail = PG_GETARG_BOOL(2);
        rconn = getConnectionByName(conname);
    }

    if (!rconn || !rconn->conn)
        dblink_conn_not_avail(conname);

    conn = rconn->conn;

    appendStringInfo(&buf, "CLOSE %s", curname);

    /* close the cursor */
    res = PQexec(conn, buf.data);
    if (!res || PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        dblink_res_error(conn, conname, res, fail,
                         "while closing cursor \"%s\"", curname);
        PG_RETURN_TEXT_P(cstring_to_text("ERROR"));
    }

    PQclear(res);

    /* if we started a transaction, decrement cursor count */
    if (rconn->newXactForCursor)
    {
        (rconn->openCursorCount)--;

        /* if count is zero, commit the transaction */
        if (rconn->openCursorCount == 0)
        {
            rconn->newXactForCursor = false;

            res = PQexec(conn, "COMMIT");
            if (PQresultStatus(res) != PGRES_COMMAND_OK)
                dblink_res_internalerror(conn, res, "commit error");
            PQclear(res);
        }
    }

    PG_RETURN_TEXT_P(cstring_to_text("OK"));
}

/*
 * Fetch results from an open cursor
 */
PG_FUNCTION_INFO_V1(dblink_fetch);
Datum
dblink_fetch(PG_FUNCTION_ARGS)
{
    PGresult   *res = NULL;
    char       *conname = NULL;
    remoteConn *rconn = NULL;
    PGconn     *conn = NULL;
    StringInfoData buf;
    char       *curname = NULL;
    int         howmany = 0;
    bool        fail = true;    /* default to backward compatible */

    prepTuplestoreResult(fcinfo);

    dblink_init();

    if (PG_NARGS() == 4)
    {
        /* text,text,int,bool */
        conname = text_to_cstring(PG_GETARG_TEXT_PP(0));
        curname = text_to_cstring(PG_GETARG_TEXT_PP(1));
        howmany = PG_GETARG_INT32(2);
        fail = PG_GETARG_BOOL(3);

        rconn = getConnectionByName(conname);
        if (rconn)
            conn = rconn->conn;
    }
    else if (PG_NARGS() == 3)
    {
        /* text,text,int or text,int,bool */
        if (get_fn_expr_argtype(fcinfo->flinfo, 2) == BOOLOID)
        {
            curname = text_to_cstring(PG_GETARG_TEXT_PP(0));
            howmany = PG_GETARG_INT32(1);
            fail = PG_GETARG_BOOL(2);
            conn = pconn->conn;
        }
        else
        {
            conname = text_to_cstring(PG_GETARG_TEXT_PP(0));
            curname = text_to_cstring(PG_GETARG_TEXT_PP(1));
            howmany = PG_GETARG_INT32(2);

            rconn = getConnectionByName(conname);
            if (rconn)
                conn = rconn->conn;
        }
    }
    else if (PG_NARGS() == 2)
    {
        /* text,int */
        curname = text_to_cstring(PG_GETARG_TEXT_PP(0));
        howmany = PG_GETARG_INT32(1);
        conn = pconn->conn;
    }

    if (!conn)
        dblink_conn_not_avail(conname);

    initStringInfo(&buf);
    appendStringInfo(&buf, "FETCH %d FROM %s", howmany, curname);

    /*
     * Try to execute the query.  Note that since libpq uses malloc, the
     * PGresult will be long-lived even though we are still in a short-lived
     * memory context.
     */
    res = PQexec(conn, buf.data);
    if (!res ||
        (PQresultStatus(res) != PGRES_COMMAND_OK &&
         PQresultStatus(res) != PGRES_TUPLES_OK))
    {
        dblink_res_error(conn, conname, res, fail,
                         "while fetching from cursor \"%s\"", curname);
        return (Datum) 0;
    }
    else if (PQresultStatus(res) == PGRES_COMMAND_OK)
    {
        /* cursor does not exist - closed already or bad name */
        PQclear(res);
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_CURSOR_NAME),
                 errmsg("cursor \"%s\" does not exist", curname)));
    }

    materializeResult(fcinfo, conn, res);
    return (Datum) 0;
}

/*
 * Note: this is the new preferred version of dblink
 */
PG_FUNCTION_INFO_V1(dblink_record);
Datum
dblink_record(PG_FUNCTION_ARGS)
{
    return dblink_record_internal(fcinfo, false);
}

PG_FUNCTION_INFO_V1(dblink_send_query);
Datum
dblink_send_query(PG_FUNCTION_ARGS)
{
    PGconn     *conn;
    char       *sql;
    int         retval;

    if (PG_NARGS() == 2)
    {
        conn = dblink_get_named_conn(text_to_cstring(PG_GETARG_TEXT_PP(0)));
        sql = text_to_cstring(PG_GETARG_TEXT_PP(1));
    }
    else
        /* shouldn't happen */
        elog(ERROR, "wrong number of arguments");

    /* async query send */
    retval = PQsendQuery(conn, sql);
    if (retval != 1)
        elog(NOTICE, "could not send query: %s", pchomp(PQerrorMessage(conn)));

    PG_RETURN_INT32(retval);
}

PG_FUNCTION_INFO_V1(dblink_get_result);
Datum
dblink_get_result(PG_FUNCTION_ARGS)
{
    return dblink_record_internal(fcinfo, true);
}

static Datum
dblink_record_internal(FunctionCallInfo fcinfo, bool is_async)
{
    PGconn     *volatile conn = NULL;
    volatile bool freeconn = false;

    prepTuplestoreResult(fcinfo);

    dblink_init();

    PG_TRY();
    {
        char       *sql = NULL;
        char       *conname = NULL;
        bool        fail = true;    /* default to backward compatible */

        if (!is_async)
        {
            if (PG_NARGS() == 3)
            {
                /* text,text,bool */
                conname = text_to_cstring(PG_GETARG_TEXT_PP(0));
                sql = text_to_cstring(PG_GETARG_TEXT_PP(1));
                fail = PG_GETARG_BOOL(2);
                dblink_get_conn(conname, &conn, &conname, &freeconn);
            }
            else if (PG_NARGS() == 2)
            {
                /* text,text or text,bool */
                if (get_fn_expr_argtype(fcinfo->flinfo, 1) == BOOLOID)
                {
                    sql = text_to_cstring(PG_GETARG_TEXT_PP(0));
                    fail = PG_GETARG_BOOL(1);
                    conn = pconn->conn;
                }
                else
                {
                    conname = text_to_cstring(PG_GETARG_TEXT_PP(0));
                    sql = text_to_cstring(PG_GETARG_TEXT_PP(1));
                    dblink_get_conn(conname, &conn, &conname, &freeconn);
                }
            }
            else if (PG_NARGS() == 1)
            {
                /* text */
                conn = pconn->conn;
                sql = text_to_cstring(PG_GETARG_TEXT_PP(0));
            }
            else
                /* shouldn't happen */
                elog(ERROR, "wrong number of arguments");
        }
        else                    /* is_async */
        {
            /* get async result */
            conname = text_to_cstring(PG_GETARG_TEXT_PP(0));

            if (PG_NARGS() == 2)
            {
                /* text,bool */
                fail = PG_GETARG_BOOL(1);
                conn = dblink_get_named_conn(conname);
            }
            else if (PG_NARGS() == 1)
            {
                /* text */
                conn = dblink_get_named_conn(conname);
            }
            else
                /* shouldn't happen */
                elog(ERROR, "wrong number of arguments");
        }

        if (!conn)
            dblink_conn_not_avail(conname);

        if (!is_async)
        {
            /* synchronous query, use efficient tuple collection method */
            materializeQueryResult(fcinfo, conn, conname, sql, fail);
        }
        else
        {
            /* async result retrieval, do it the old way */
            PGresult   *res = PQgetResult(conn);

            /* NULL means we're all done with the async results */
            if (res)
            {
                if (PQresultStatus(res) != PGRES_COMMAND_OK &&
                    PQresultStatus(res) != PGRES_TUPLES_OK)
                {
                    dblink_res_error(conn, conname, res, fail,
                                     "while executing query");
                    /* if fail isn't set, we'll return an empty query result */
                }
                else
                {
                    materializeResult(fcinfo, conn, res);
                }
            }
        }
    }
    PG_FINALLY();
    {
        /* if needed, close the connection to the database */
        if (freeconn)
            PQfinish(conn);
    }
    PG_END_TRY();

    return (Datum) 0;
}

/*
 * Verify function caller can handle a tuplestore result, and set up for that.
 *
 * Note: if the caller returns without actually creating a tuplestore, the
 * executor will treat the function result as an empty set.
 */
static void
prepTuplestoreResult(FunctionCallInfo fcinfo)
{
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;

    /* check to see if query supports us returning a tuplestore */
    if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("set-valued function called in context that cannot accept a set")));
    if (!(rsinfo->allowedModes & SFRM_Materialize))
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("materialize mode required, but it is not allowed in this context")));

    /* let the executor know we're sending back a tuplestore */
    rsinfo->returnMode = SFRM_Materialize;

    /* caller must fill these to return a non-empty result */
    rsinfo->setResult = NULL;
    rsinfo->setDesc = NULL;
}

/*
 * Copy the contents of the PGresult into a tuplestore to be returned
 * as the result of the current function.
 * The PGresult will be released in this function.
 */
static void
materializeResult(FunctionCallInfo fcinfo, PGconn *conn, PGresult *res)
{
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;

    /* prepTuplestoreResult must have been called previously */
    Assert(rsinfo->returnMode == SFRM_Materialize);

    PG_TRY();
    {
        TupleDesc   tupdesc;
        bool        is_sql_cmd;
        int         ntuples;
        int         nfields;

        if (PQresultStatus(res) == PGRES_COMMAND_OK)
        {
            is_sql_cmd = true;

            /*
             * need a tuple descriptor representing one TEXT column to return
             * the command status string as our result tuple
             */
            tupdesc = CreateTemplateTupleDesc(1);
            TupleDescInitEntry(tupdesc, (AttrNumber) 1, "status",
                               TEXTOID, -1, 0);
            ntuples = 1;
            nfields = 1;
        }
        else
        {
            Assert(PQresultStatus(res) == PGRES_TUPLES_OK);

            is_sql_cmd = false;

            /* get a tuple descriptor for our result type */
            switch (get_call_result_type(fcinfo, NULL, &tupdesc))
            {
                case TYPEFUNC_COMPOSITE:
                    /* success */
                    break;
                case TYPEFUNC_RECORD:
                    /* failed to determine actual type of RECORD */
                    ereport(ERROR,
                            (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                             errmsg("function returning record called in context "
                                    "that cannot accept type record")));
                    break;
                default:
                    /* result type isn't composite */
                    elog(ERROR, "return type must be a row type");
                    break;
            }

            /* make sure we have a persistent copy of the tupdesc */
            tupdesc = CreateTupleDescCopy(tupdesc);
            ntuples = PQntuples(res);
            nfields = PQnfields(res);
        }

        /*
         * check result and tuple descriptor have the same number of columns
         */
        if (nfields != tupdesc->natts)
            ereport(ERROR,
                    (errcode(ERRCODE_DATATYPE_MISMATCH),
                     errmsg("remote query result rowtype does not match "
                            "the specified FROM clause rowtype")));

        if (ntuples > 0)
        {
            AttInMetadata *attinmeta;
            int         nestlevel = -1;
            Tuplestorestate *tupstore;
            MemoryContext oldcontext;
            int         row;
            char      **values;

            attinmeta = TupleDescGetAttInMetadata(tupdesc);

            /* Set GUCs to ensure we read GUC-sensitive data types correctly */
            if (!is_sql_cmd)
                nestlevel = applyRemoteGucs(conn);

            oldcontext = MemoryContextSwitchTo(
                                               rsinfo->econtext->ecxt_per_query_memory);
            tupstore = tuplestore_begin_heap(true, false, work_mem);
            rsinfo->setResult = tupstore;
            rsinfo->setDesc = tupdesc;
            MemoryContextSwitchTo(oldcontext);

            values = (char **) palloc(nfields * sizeof(char *));

            /* put all tuples into the tuplestore */
            for (row = 0; row < ntuples; row++)
            {
                HeapTuple   tuple;

                if (!is_sql_cmd)
                {
                    int         i;

                    for (i = 0; i < nfields; i++)
                    {
                        if (PQgetisnull(res, row, i))
                            values[i] = NULL;
                        else
                            values[i] = PQgetvalue(res, row, i);
                    }
                }
                else
                {
                    values[0] = PQcmdStatus(res);
                }

                /* build the tuple and put it into the tuplestore. */
                tuple = BuildTupleFromCStrings(attinmeta, values);
                tuplestore_puttuple(tupstore, tuple);
            }

            /* clean up GUC settings, if we changed any */
            restoreLocalGucs(nestlevel);

            /* clean up and return the tuplestore */
            tuplestore_donestoring(tupstore);
        }
    }
    PG_FINALLY();
    {
        /* be sure to release the libpq result */
        PQclear(res);
    }
    PG_END_TRY();
}

/*
 * Execute the given SQL command and store its results into a tuplestore
 * to be returned as the result of the current function.
 *
 * This is equivalent to PQexec followed by materializeResult, but we make
 * use of libpq's single-row mode to avoid accumulating the whole result
 * inside libpq before it gets transferred to the tuplestore.
 */
static void
materializeQueryResult(FunctionCallInfo fcinfo,
                       PGconn *conn,
                       const char *conname,
                       const char *sql,
                       bool fail)
{
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    PGresult   *volatile res = NULL;
    volatile storeInfo sinfo = {0};

    /* prepTuplestoreResult must have been called previously */
    Assert(rsinfo->returnMode == SFRM_Materialize);

    sinfo.fcinfo = fcinfo;

    PG_TRY();
    {
        /* Create short-lived memory context for data conversions */
        sinfo.tmpcontext = AllocSetContextCreate(CurrentMemoryContext,
                                                 "dblink temporary context",
                                                 ALLOCSET_DEFAULT_SIZES);

        /* execute query, collecting any tuples into the tuplestore */
        res = storeQueryResult(&sinfo, conn, sql);

        if (!res ||
            (PQresultStatus(res) != PGRES_COMMAND_OK &&
             PQresultStatus(res) != PGRES_TUPLES_OK))
        {
            /*
             * dblink_res_error will clear the passed PGresult, so we need
             * this ugly dance to avoid doing so twice during error exit
             */
            PGresult   *res1 = res;

            res = NULL;
            dblink_res_error(conn, conname, res1, fail,
                             "while executing query");
            /* if fail isn't set, we'll return an empty query result */
        }
        else if (PQresultStatus(res) == PGRES_COMMAND_OK)
        {
            /*
             * storeRow didn't get called, so we need to convert the command
             * status string to a tuple manually
             */
            TupleDesc   tupdesc;
            AttInMetadata *attinmeta;
            Tuplestorestate *tupstore;
            HeapTuple   tuple;
            char       *values[1];
            MemoryContext oldcontext;

            /*
             * need a tuple descriptor representing one TEXT column to return
             * the command status string as our result tuple
             */
            tupdesc = CreateTemplateTupleDesc(1);
            TupleDescInitEntry(tupdesc, (AttrNumber) 1, "status",
                               TEXTOID, -1, 0);
            attinmeta = TupleDescGetAttInMetadata(tupdesc);

            oldcontext = MemoryContextSwitchTo(
                                               rsinfo->econtext->ecxt_per_query_memory);
            tupstore = tuplestore_begin_heap(true, false, work_mem);
            rsinfo->setResult = tupstore;
            rsinfo->setDesc = tupdesc;
            MemoryContextSwitchTo(oldcontext);

            values[0] = PQcmdStatus(res);

            /* build the tuple and put it into the tuplestore. */
            tuple = BuildTupleFromCStrings(attinmeta, values);
            tuplestore_puttuple(tupstore, tuple);

            PQclear(res);
            res = NULL;
        }
        else
        {
            Assert(PQresultStatus(res) == PGRES_TUPLES_OK);
            /* storeRow should have created a tuplestore */
            Assert(rsinfo->setResult != NULL);

            PQclear(res);
            res = NULL;
        }

        /* clean up data conversion short-lived memory context */
        if (sinfo.tmpcontext != NULL)
            MemoryContextDelete(sinfo.tmpcontext);
        sinfo.tmpcontext = NULL;

        PQclear(sinfo.last_res);
        sinfo.last_res = NULL;
        PQclear(sinfo.cur_res);
        sinfo.cur_res = NULL;
    }
    PG_CATCH();
    {
        /* be sure to release any libpq result we collected */
        PQclear(res);
        PQclear(sinfo.last_res);
        PQclear(sinfo.cur_res);
        /* and clear out any pending data in libpq */
        while ((res = PQgetResult(conn)) != NULL)
            PQclear(res);
        PG_RE_THROW();
    }
    PG_END_TRY();
}

/*
 * Execute query, and send any result rows to sinfo->tuplestore.
 */
static PGresult *
storeQueryResult(volatile storeInfo *sinfo, PGconn *conn, const char *sql)
{
    bool        first = true;
    int         nestlevel = -1;
    PGresult   *res;

    if (!PQsendQuery(conn, sql))
        elog(ERROR, "could not send query: %s", pchomp(PQerrorMessage(conn)));

    if (!PQsetSingleRowMode(conn))  /* shouldn't fail */
        elog(ERROR, "failed to set single-row mode for dblink query");

    for (;;)
    {
        CHECK_FOR_INTERRUPTS();

        sinfo->cur_res = PQgetResult(conn);
        if (!sinfo->cur_res)
            break;

        if (PQresultStatus(sinfo->cur_res) == PGRES_SINGLE_TUPLE)
        {
            /* got one row from possibly-bigger resultset */

            /*
             * Set GUCs to ensure we read GUC-sensitive data types correctly.
             * We shouldn't do this until we have a row in hand, to ensure
             * libpq has seen any earlier ParameterStatus protocol messages.
             */
            if (first && nestlevel < 0)
                nestlevel = applyRemoteGucs(conn);

            storeRow(sinfo, sinfo->cur_res, first);

            PQclear(sinfo->cur_res);
            sinfo->cur_res = NULL;
            first = false;
        }
        else
        {
            /* if empty resultset, fill tuplestore header */
            if (first && PQresultStatus(sinfo->cur_res) == PGRES_TUPLES_OK)
                storeRow(sinfo, sinfo->cur_res, first);

            /* store completed result at last_res */
            PQclear(sinfo->last_res);
            sinfo->last_res = sinfo->cur_res;
            sinfo->cur_res = NULL;
            first = true;
        }
    }

    /* clean up GUC settings, if we changed any */
    restoreLocalGucs(nestlevel);

    /* return last_res */
    res = sinfo->last_res;
    sinfo->last_res = NULL;
    return res;
}

/*
 * Send single row to sinfo->tuplestore.
 *
 * If "first" is true, create the tuplestore using PGresult's metadata
 * (in this case the PGresult might contain either zero or one row).
 */
static void
storeRow(volatile storeInfo *sinfo, PGresult *res, bool first)
{
    int         nfields = PQnfields(res);
    HeapTuple   tuple;
    int         i;
    MemoryContext oldcontext;

    if (first)
    {
        /* Prepare for new result set */
        ReturnSetInfo *rsinfo = (ReturnSetInfo *) sinfo->fcinfo->resultinfo;
        TupleDesc   tupdesc;

        /*
         * It's possible to get more than one result set if the query string
         * contained multiple SQL commands.  In that case, we follow PQexec's
         * traditional behavior of throwing away all but the last result.
         */
        if (sinfo->tuplestore)
            tuplestore_end(sinfo->tuplestore);
        sinfo->tuplestore = NULL;

        /* get a tuple descriptor for our result type */
        switch (get_call_result_type(sinfo->fcinfo, NULL, &tupdesc))
        {
            case TYPEFUNC_COMPOSITE:
                /* success */
                break;
            case TYPEFUNC_RECORD:
                /* failed to determine actual type of RECORD */
                ereport(ERROR,
                        (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                         errmsg("function returning record called in context "
                                "that cannot accept type record")));
                break;
            default:
                /* result type isn't composite */
                elog(ERROR, "return type must be a row type");
                break;
        }

        /* make sure we have a persistent copy of the tupdesc */
        tupdesc = CreateTupleDescCopy(tupdesc);

        /* check result and tuple descriptor have the same number of columns */
        if (nfields != tupdesc->natts)
            ereport(ERROR,
                    (errcode(ERRCODE_DATATYPE_MISMATCH),
                     errmsg("remote query result rowtype does not match "
                            "the specified FROM clause rowtype")));

        /* Prepare attinmeta for later data conversions */
        sinfo->attinmeta = TupleDescGetAttInMetadata(tupdesc);

        /* Create a new, empty tuplestore */
        oldcontext = MemoryContextSwitchTo(rsinfo->econtext->ecxt_per_query_memory);
        sinfo->tuplestore = tuplestore_begin_heap(true, false, work_mem);
        rsinfo->setResult = sinfo->tuplestore;
        rsinfo->setDesc = tupdesc;
        MemoryContextSwitchTo(oldcontext);

        /* Done if empty resultset */
        if (PQntuples(res) == 0)
            return;

        /*
         * Set up sufficiently-wide string pointers array; this won't change
         * in size so it's easy to preallocate.
         */
        if (sinfo->cstrs)
            pfree(sinfo->cstrs);
        sinfo->cstrs = (char **) palloc(nfields * sizeof(char *));
    }

    /* Should have a single-row result if we get here */
    Assert(PQntuples(res) == 1);

    /*
     * Do the following work in a temp context that we reset after each tuple.
     * This cleans up not only the data we have direct access to, but any
     * cruft the I/O functions might leak.
     */
    oldcontext = MemoryContextSwitchTo(sinfo->tmpcontext);

    /*
     * Fill cstrs with null-terminated strings of column values.
     */
    for (i = 0; i < nfields; i++)
    {
        if (PQgetisnull(res, 0, i))
            sinfo->cstrs[i] = NULL;
        else
            sinfo->cstrs[i] = PQgetvalue(res, 0, i);
    }

    /* Convert row to a tuple, and add it to the tuplestore */
    tuple = BuildTupleFromCStrings(sinfo->attinmeta, sinfo->cstrs);

    tuplestore_puttuple(sinfo->tuplestore, tuple);

    /* Clean up */
    MemoryContextSwitchTo(oldcontext);
    MemoryContextReset(sinfo->tmpcontext);
}

/*
 * List all open dblink connections by name.
 * Returns an array of all connection names.
 * Takes no params
 */
PG_FUNCTION_INFO_V1(dblink_get_connections);
Datum
dblink_get_connections(PG_FUNCTION_ARGS)
{
    HASH_SEQ_STATUS status;
    remoteConnHashEnt *hentry;
    ArrayBuildState *astate = NULL;

    if (remoteConnHash)
    {
        hash_seq_init(&status, remoteConnHash);
        while ((hentry = (remoteConnHashEnt *) hash_seq_search(&status)) != NULL)
        {
            /* stash away current value */
            astate = accumArrayResult(astate,
                                      CStringGetTextDatum(hentry->name),
                                      false, TEXTOID, CurrentMemoryContext);
        }
    }

    if (astate)
        PG_RETURN_ARRAYTYPE_P(makeArrayResult(astate,
                                              CurrentMemoryContext));
    else
        PG_RETURN_NULL();
}

/*
 * Checks if a given remote connection is busy
 *
 * Returns 1 if the connection is busy, 0 otherwise
 * Params:
 *  text connection_name - name of the connection to check
 *
 */
PG_FUNCTION_INFO_V1(dblink_is_busy);
Datum
dblink_is_busy(PG_FUNCTION_ARGS)
{
    PGconn     *conn;

    dblink_init();
    conn = dblink_get_named_conn(text_to_cstring(PG_GETARG_TEXT_PP(0)));

    PQconsumeInput(conn);
    PG_RETURN_INT32(PQisBusy(conn));
}

/*
 * Cancels a running request on a connection
 *
 * Returns text:
 *  "OK" if the cancel request has been sent correctly,
 *      an error message otherwise
 *
 * Params:
 *  text connection_name - name of the connection to check
 *
 */
PG_FUNCTION_INFO_V1(dblink_cancel_query);
Datum
dblink_cancel_query(PG_FUNCTION_ARGS)
{
    int         res;
    PGconn     *conn;
    PGcancel   *cancel;
    char        errbuf[256];

    dblink_init();
    conn = dblink_get_named_conn(text_to_cstring(PG_GETARG_TEXT_PP(0)));
    cancel = PQgetCancel(conn);

    res = PQcancel(cancel, errbuf, 256);
    PQfreeCancel(cancel);

    if (res == 1)
        PG_RETURN_TEXT_P(cstring_to_text("OK"));
    else
        PG_RETURN_TEXT_P(cstring_to_text(errbuf));
}


/*
 * Get error message from a connection
 *
 * Returns text:
 *  "OK" if no error, an error message otherwise
 *
 * Params:
 *  text connection_name - name of the connection to check
 *
 */
PG_FUNCTION_INFO_V1(dblink_error_message);
Datum
dblink_error_message(PG_FUNCTION_ARGS)
{
    char       *msg;
    PGconn     *conn;

    dblink_init();
    conn = dblink_get_named_conn(text_to_cstring(PG_GETARG_TEXT_PP(0)));

    msg = PQerrorMessage(conn);
    if (msg == NULL || msg[0] == '\0')
        PG_RETURN_TEXT_P(cstring_to_text("OK"));
    else
        PG_RETURN_TEXT_P(cstring_to_text(pchomp(msg)));
}

/*
 * Execute an SQL non-SELECT command
 */
PG_FUNCTION_INFO_V1(dblink_exec);
Datum
dblink_exec(PG_FUNCTION_ARGS)
{
    text       *volatile sql_cmd_status = NULL;
    PGconn     *volatile conn = NULL;
    volatile bool freeconn = false;

    dblink_init();

    PG_TRY();
    {
        PGresult   *res = NULL;
        char       *sql = NULL;
        char       *conname = NULL;
        bool        fail = true;    /* default to backward compatible behavior */

        if (PG_NARGS() == 3)
        {
            /* must be text,text,bool */
            conname = text_to_cstring(PG_GETARG_TEXT_PP(0));
            sql = text_to_cstring(PG_GETARG_TEXT_PP(1));
            fail = PG_GETARG_BOOL(2);
            dblink_get_conn(conname, &conn, &conname, &freeconn);
        }
        else if (PG_NARGS() == 2)
        {
            /* might be text,text or text,bool */
            if (get_fn_expr_argtype(fcinfo->flinfo, 1) == BOOLOID)
            {
                sql = text_to_cstring(PG_GETARG_TEXT_PP(0));
                fail = PG_GETARG_BOOL(1);
                conn = pconn->conn;
            }
            else
            {
                conname = text_to_cstring(PG_GETARG_TEXT_PP(0));
                sql = text_to_cstring(PG_GETARG_TEXT_PP(1));
                dblink_get_conn(conname, &conn, &conname, &freeconn);
            }
        }
        else if (PG_NARGS() == 1)
        {
            /* must be single text argument */
            conn = pconn->conn;
            sql = text_to_cstring(PG_GETARG_TEXT_PP(0));
        }
        else
            /* shouldn't happen */
            elog(ERROR, "wrong number of arguments");

        if (!conn)
            dblink_conn_not_avail(conname);

        res = PQexec(conn, sql);
        if (!res ||
            (PQresultStatus(res) != PGRES_COMMAND_OK &&
             PQresultStatus(res) != PGRES_TUPLES_OK))
        {
            dblink_res_error(conn, conname, res, fail,
                             "while executing command");

            /*
             * and save a copy of the command status string to return as our
             * result tuple
             */
            sql_cmd_status = cstring_to_text("ERROR");
        }
        else if (PQresultStatus(res) == PGRES_COMMAND_OK)
        {
            /*
             * and save a copy of the command status string to return as our
             * result tuple
             */
            sql_cmd_status = cstring_to_text(PQcmdStatus(res));
            PQclear(res);
        }
        else
        {
            PQclear(res);
            ereport(ERROR,
                    (errcode(ERRCODE_S_R_E_PROHIBITED_SQL_STATEMENT_ATTEMPTED),
                     errmsg("statement returning results not allowed")));
        }
    }
    PG_FINALLY();
    {
        /* if needed, close the connection to the database */
        if (freeconn)
            PQfinish(conn);
    }
    PG_END_TRY();

    PG_RETURN_TEXT_P(sql_cmd_status);
}


/*
 * dblink_get_pkey
 *
 * Return list of primary key fields for the supplied relation,
 * or NULL if none exists.
 */
PG_FUNCTION_INFO_V1(dblink_get_pkey);
Datum
dblink_get_pkey(PG_FUNCTION_ARGS)
{
    int16       indnkeyatts;
    char      **results;
    FuncCallContext *funcctx;
    int32       call_cntr;
    int32       max_calls;
    AttInMetadata *attinmeta;
    MemoryContext oldcontext;

    /* stuff done only on the first call of the function */
    if (SRF_IS_FIRSTCALL())
    {
        Relation    rel;
        TupleDesc   tupdesc;

        /* create a function context for cross-call persistence */
        funcctx = SRF_FIRSTCALL_INIT();

        /*
         * switch to memory context appropriate for multiple function calls
         */
        oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);

        /* open target relation */
        rel = get_rel_from_relname(PG_GETARG_TEXT_PP(0), AccessShareLock, ACL_SELECT);

        /* get the array of attnums */
        results = get_pkey_attnames(rel, &indnkeyatts);

        relation_close(rel, AccessShareLock);

        /*
         * need a tuple descriptor representing one INT and one TEXT column
         */
        tupdesc = CreateTemplateTupleDesc(2);
        TupleDescInitEntry(tupdesc, (AttrNumber) 1, "position",
                           INT4OID, -1, 0);
        TupleDescInitEntry(tupdesc, (AttrNumber) 2, "colname",
                           TEXTOID, -1, 0);

        /*
         * Generate attribute metadata needed later to produce tuples from raw
         * C strings
         */
        attinmeta = TupleDescGetAttInMetadata(tupdesc);
        funcctx->attinmeta = attinmeta;

        if ((results != NULL) && (indnkeyatts > 0))
        {
            funcctx->max_calls = indnkeyatts;

            /* got results, keep track of them */
            funcctx->user_fctx = results;
        }
        else
        {
            /* fast track when no results */
            MemoryContextSwitchTo(oldcontext);
            SRF_RETURN_DONE(funcctx);
        }

        MemoryContextSwitchTo(oldcontext);
    }

    /* stuff done on every call of the function */
    funcctx = SRF_PERCALL_SETUP();

    /*
     * initialize per-call variables
     */
    call_cntr = funcctx->call_cntr;
    max_calls = funcctx->max_calls;

    results = (char **) funcctx->user_fctx;
    attinmeta = funcctx->attinmeta;

    if (call_cntr < max_calls)  /* do when there is more left to send */
    {
        char      **values;
        HeapTuple   tuple;
        Datum       result;

        values = (char **) palloc(2 * sizeof(char *));
        values[0] = psprintf("%d", call_cntr + 1);
        values[1] = results[call_cntr];

        /* build the tuple */
        tuple = BuildTupleFromCStrings(attinmeta, values);

        /* make the tuple into a datum */
        result = HeapTupleGetDatum(tuple);

        SRF_RETURN_NEXT(funcctx, result);
    }
    else
    {
        /* do when there is no more left */
        SRF_RETURN_DONE(funcctx);
    }
}


/*
 * dblink_build_sql_insert
 *
 * Used to generate an SQL insert statement
 * based on an existing tuple in a local relation.
 * This is useful for selectively replicating data
 * to another server via dblink.
 *
 * API:
 * <relname> - name of local table of interest
 * <pkattnums> - an int2vector of attnums which will be used
 * to identify the local tuple of interest
 * <pknumatts> - number of attnums in pkattnums
 * <src_pkattvals_arry> - text array of key values which will be used
 * to identify the local tuple of interest
 * <tgt_pkattvals_arry> - text array of key values which will be used
 * to build the string for execution remotely. These are substituted
 * for their counterparts in src_pkattvals_arry
 */
PG_FUNCTION_INFO_V1(dblink_build_sql_insert);
Datum
dblink_build_sql_insert(PG_FUNCTION_ARGS)
{
    text       *relname_text = PG_GETARG_TEXT_PP(0);
    int2vector *pkattnums_arg = (int2vector *) PG_GETARG_POINTER(1);
    int32       pknumatts_arg = PG_GETARG_INT32(2);
    ArrayType  *src_pkattvals_arry = PG_GETARG_ARRAYTYPE_P(3);
    ArrayType  *tgt_pkattvals_arry = PG_GETARG_ARRAYTYPE_P(4);
    Relation    rel;
    int        *pkattnums;
    int         pknumatts;
    char      **src_pkattvals;
    char      **tgt_pkattvals;
    int         src_nitems;
    int         tgt_nitems;
    char       *sql;

    /*
     * Open target relation.
     */
    rel = get_rel_from_relname(relname_text, AccessShareLock, ACL_SELECT);

    /*
     * Process pkattnums argument.
     */
    validate_pkattnums(rel, pkattnums_arg, pknumatts_arg,
                       &pkattnums, &pknumatts);

    /*
     * Source array is made up of key values that will be used to locate the
     * tuple of interest from the local system.
     */
    src_pkattvals = get_text_array_contents(src_pkattvals_arry, &src_nitems);

    /*
     * There should be one source array key value for each key attnum
     */
    if (src_nitems != pknumatts)
        ereport(ERROR,
                (errcode(ERRCODE_ARRAY_SUBSCRIPT_ERROR),
                 errmsg("source key array length must match number of key attributes")));

    /*
     * Target array is made up of key values that will be used to build the
     * SQL string for use on the remote system.
     */
    tgt_pkattvals = get_text_array_contents(tgt_pkattvals_arry, &tgt_nitems);

    /*
     * There should be one target array key value for each key attnum
     */
    if (tgt_nitems != pknumatts)
        ereport(ERROR,
                (errcode(ERRCODE_ARRAY_SUBSCRIPT_ERROR),
                 errmsg("target key array length must match number of key attributes")));

    /*
     * Prep work is finally done. Go get the SQL string.
     */
    sql = get_sql_insert(rel, pkattnums, pknumatts, src_pkattvals, tgt_pkattvals);

    /*
     * Now we can close the relation.
     */
    relation_close(rel, AccessShareLock);

    /*
     * And send it
     */
    PG_RETURN_TEXT_P(cstring_to_text(sql));
}


/*
 * dblink_build_sql_delete
 *
 * Used to generate an SQL delete statement.
 * This is useful for selectively replicating a
 * delete to another server via dblink.
 *
 * API:
 * <relname> - name of remote table of interest
 * <pkattnums> - an int2vector of attnums which will be used
 * to identify the remote tuple of interest
 * <pknumatts> - number of attnums in pkattnums
 * <tgt_pkattvals_arry> - text array of key values which will be used
 * to build the string for execution remotely.
 */
PG_FUNCTION_INFO_V1(dblink_build_sql_delete);
Datum
dblink_build_sql_delete(PG_FUNCTION_ARGS)
{
    text       *relname_text = PG_GETARG_TEXT_PP(0);
    int2vector *pkattnums_arg = (int2vector *) PG_GETARG_POINTER(1);
    int32       pknumatts_arg = PG_GETARG_INT32(2);
    ArrayType  *tgt_pkattvals_arry = PG_GETARG_ARRAYTYPE_P(3);
    Relation    rel;
    int        *pkattnums;
    int         pknumatts;
    char      **tgt_pkattvals;
    int         tgt_nitems;
    char       *sql;

    /*
     * Open target relation.
     */
    rel = get_rel_from_relname(relname_text, AccessShareLock, ACL_SELECT);

    /*
     * Process pkattnums argument.
     */
    validate_pkattnums(rel, pkattnums_arg, pknumatts_arg,
                       &pkattnums, &pknumatts);

    /*
     * Target array is made up of key values that will be used to build the
     * SQL string for use on the remote system.
     */
    tgt_pkattvals = get_text_array_contents(tgt_pkattvals_arry, &tgt_nitems);

    /*
     * There should be one target array key value for each key attnum
     */
    if (tgt_nitems != pknumatts)
        ereport(ERROR,
                (errcode(ERRCODE_ARRAY_SUBSCRIPT_ERROR),
                 errmsg("target key array length must match number of key attributes")));

    /*
     * Prep work is finally done. Go get the SQL string.
     */
    sql = get_sql_delete(rel, pkattnums, pknumatts, tgt_pkattvals);

    /*
     * Now we can close the relation.
     */
    relation_close(rel, AccessShareLock);

    /*
     * And send it
     */
    PG_RETURN_TEXT_P(cstring_to_text(sql));
}


/*
 * dblink_build_sql_update
 *
 * Used to generate an SQL update statement
 * based on an existing tuple in a local relation.
 * This is useful for selectively replicating data
 * to another server via dblink.
 *
 * API:
 * <relname> - name of local table of interest
 * <pkattnums> - an int2vector of attnums which will be used
 * to identify the local tuple of interest
 * <pknumatts> - number of attnums in pkattnums
 * <src_pkattvals_arry> - text array of key values which will be used
 * to identify the local tuple of interest
 * <tgt_pkattvals_arry> - text array of key values which will be used
 * to build the string for execution remotely. These are substituted
 * for their counterparts in src_pkattvals_arry
 */
PG_FUNCTION_INFO_V1(dblink_build_sql_update);
Datum
dblink_build_sql_update(PG_FUNCTION_ARGS)
{
    text       *relname_text = PG_GETARG_TEXT_PP(0);
    int2vector *pkattnums_arg = (int2vector *) PG_GETARG_POINTER(1);
    int32       pknumatts_arg = PG_GETARG_INT32(2);
    ArrayType  *src_pkattvals_arry = PG_GETARG_ARRAYTYPE_P(3);
    ArrayType  *tgt_pkattvals_arry = PG_GETARG_ARRAYTYPE_P(4);
    Relation    rel;
    int        *pkattnums;
    int         pknumatts;
    char      **src_pkattvals;
    char      **tgt_pkattvals;
    int         src_nitems;
    int         tgt_nitems;
    char       *sql;

    /*
     * Open target relation.
     */
    rel = get_rel_from_relname(relname_text, AccessShareLock, ACL_SELECT);

    /*
     * Process pkattnums argument.
     */
    validate_pkattnums(rel, pkattnums_arg, pknumatts_arg,
                       &pkattnums, &pknumatts);

    /*
     * Source array is made up of key values that will be used to locate the
     * tuple of interest from the local system.
     */
    src_pkattvals = get_text_array_contents(src_pkattvals_arry, &src_nitems);

    /*
     * There should be one source array key value for each key attnum
     */
    if (src_nitems != pknumatts)
        ereport(ERROR,
                (errcode(ERRCODE_ARRAY_SUBSCRIPT_ERROR),
                 errmsg("source key array length must match number of key attributes")));

    /*
     * Target array is made up of key values that will be used to build the
     * SQL string for use on the remote system.
     */
    tgt_pkattvals = get_text_array_contents(tgt_pkattvals_arry, &tgt_nitems);

    /*
     * There should be one target array key value for each key attnum
     */
    if (tgt_nitems != pknumatts)
        ereport(ERROR,
                (errcode(ERRCODE_ARRAY_SUBSCRIPT_ERROR),
                 errmsg("target key array length must match number of key attributes")));

    /*
     * Prep work is finally done. Go get the SQL string.
     */
    sql = get_sql_update(rel, pkattnums, pknumatts, src_pkattvals, tgt_pkattvals);

    /*
     * Now we can close the relation.
     */
    relation_close(rel, AccessShareLock);

    /*
     * And send it
     */
    PG_RETURN_TEXT_P(cstring_to_text(sql));
}

/*
 * dblink_current_query
 * return the current query string
 * to allow its use in (among other things)
 * rewrite rules
 */
PG_FUNCTION_INFO_V1(dblink_current_query);
Datum
dblink_current_query(PG_FUNCTION_ARGS)
{
    /* This is now just an alias for the built-in function current_query() */
    PG_RETURN_DATUM(current_query(fcinfo));
}

/*
 * Retrieve async notifications for a connection.
 *
 * Returns a setof record of notifications, or an empty set if none received.
 * Can optionally take a named connection as parameter, but uses the unnamed
 * connection per default.
 *
 */
#define DBLINK_NOTIFY_COLS      3

PG_FUNCTION_INFO_V1(dblink_get_notify);
Datum
dblink_get_notify(PG_FUNCTION_ARGS)
{
    PGconn     *conn;
    PGnotify   *notify;
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    TupleDesc   tupdesc;
    Tuplestorestate *tupstore;
    MemoryContext per_query_ctx;
    MemoryContext oldcontext;

    prepTuplestoreResult(fcinfo);

    dblink_init();
    if (PG_NARGS() == 1)
        conn = dblink_get_named_conn(text_to_cstring(PG_GETARG_TEXT_PP(0)));
    else
        conn = pconn->conn;

    /* create the tuplestore in per-query memory */
    per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;
    oldcontext = MemoryContextSwitchTo(per_query_ctx);

    tupdesc = CreateTemplateTupleDesc(DBLINK_NOTIFY_COLS);
    TupleDescInitEntry(tupdesc, (AttrNumber) 1, "notify_name",
                       TEXTOID, -1, 0);
    TupleDescInitEntry(tupdesc, (AttrNumber) 2, "be_pid",
                       INT4OID, -1, 0);
    TupleDescInitEntry(tupdesc, (AttrNumber) 3, "extra",
                       TEXTOID, -1, 0);

    tupstore = tuplestore_begin_heap(true, false, work_mem);
    rsinfo->setResult = tupstore;
    rsinfo->setDesc = tupdesc;

    MemoryContextSwitchTo(oldcontext);

    PQconsumeInput(conn);
    while ((notify = PQnotifies(conn)) != NULL)
    {
        Datum       values[DBLINK_NOTIFY_COLS];
        bool        nulls[DBLINK_NOTIFY_COLS];

        memset(values, 0, sizeof(values));
        memset(nulls, 0, sizeof(nulls));

        if (notify->relname != NULL)
            values[0] = CStringGetTextDatum(notify->relname);
        else
            nulls[0] = true;

        values[1] = Int32GetDatum(notify->be_pid);

        if (notify->extra != NULL)
            values[2] = CStringGetTextDatum(notify->extra);
        else
            nulls[2] = true;

        tuplestore_putvalues(tupstore, tupdesc, values, nulls);

        PQfreemem(notify);
        PQconsumeInput(conn);
    }

    /* clean up and return the tuplestore */
    tuplestore_donestoring(tupstore);

    return (Datum) 0;
}

/*
 * Validate the options given to a dblink foreign server or user mapping.
 * Raise an error if any option is invalid.
 *
 * We just check the names of options here, so semantic errors in options,
 * such as invalid numeric format, will be detected at the attempt to connect.
 */
PG_FUNCTION_INFO_V1(dblink_fdw_validator);
Datum
dblink_fdw_validator(PG_FUNCTION_ARGS)
{
    List       *options_list = untransformRelOptions(PG_GETARG_DATUM(0));
    Oid         context = PG_GETARG_OID(1);
    ListCell   *cell;

    static const PQconninfoOption *options = NULL;

    /*
     * Get list of valid libpq options.
     *
     * To avoid unnecessary work, we get the list once and use it throughout
     * the lifetime of this backend process.  We don't need to care about
     * memory context issues, because PQconndefaults allocates with malloc.
     */
    if (!options)
    {
        options = PQconndefaults();
        if (!options)           /* assume reason for failure is OOM */
            ereport(ERROR,
                    (errcode(ERRCODE_FDW_OUT_OF_MEMORY),
                     errmsg("out of memory"),
                     errdetail("Could not get libpq's default connection options.")));
    }

    /* Validate each supplied option. */
    foreach(cell, options_list)
    {
        DefElem    *def = (DefElem *) lfirst(cell);

        if (!is_valid_dblink_option(options, def->defname, context))
        {
            /*
             * Unknown option, or invalid option for the context specified, so
             * complain about it.  Provide a hint with list of valid options
             * for the context.
             */
            StringInfoData buf;
            const PQconninfoOption *opt;

            initStringInfo(&buf);
            for (opt = options; opt->keyword; opt++)
            {
                if (is_valid_dblink_option(options, opt->keyword, context))
                    appendStringInfo(&buf, "%s%s",
                                     (buf.len > 0) ? ", " : "",
                                     opt->keyword);
            }
            ereport(ERROR,
                    (errcode(ERRCODE_FDW_OPTION_NAME_NOT_FOUND),
                     errmsg("invalid option \"%s\"", def->defname),
                     errhint("Valid options in this context are: %s",
                             buf.data)));
        }
    }

    PG_RETURN_VOID();
}


/*************************************************************
 * internal functions
 */


/*
 * get_pkey_attnames
 *
 * Get the primary key attnames for the given relation.
 * Return NULL, and set indnkeyatts = 0, if no primary key exists.
 */
static char **
get_pkey_attnames(Relation rel, int16 *indnkeyatts)
{
    Relation    indexRelation;
    ScanKeyData skey;
    SysScanDesc scan;
    HeapTuple   indexTuple;
    int         i;
    char      **result = NULL;
    TupleDesc   tupdesc;

    /* initialize indnkeyatts to 0 in case no primary key exists */
    *indnkeyatts = 0;

    tupdesc = rel->rd_att;

    /* Prepare to scan pg_index for entries having indrelid = this rel. */
    indexRelation = table_open(IndexRelationId, AccessShareLock);
    ScanKeyInit(&skey,
                Anum_pg_index_indrelid,
                BTEqualStrategyNumber, F_OIDEQ,
                ObjectIdGetDatum(RelationGetRelid(rel)));

    scan = systable_beginscan(indexRelation, IndexIndrelidIndexId, true,
                              NULL, 1, &skey);

    while (HeapTupleIsValid(indexTuple = systable_getnext(scan)))
    {
        Form_pg_index index = (Form_pg_index) GETSTRUCT(indexTuple);

        /* we're only interested if it is the primary key */
        if (index->indisprimary)
        {
            *indnkeyatts = index->indnkeyatts;
            if (*indnkeyatts > 0)
            {
                result = (char **) palloc(*indnkeyatts * sizeof(char *));

                for (i = 0; i < *indnkeyatts; i++)
                    result[i] = SPI_fname(tupdesc, index->indkey.values[i]);
            }
            break;
        }
    }

    systable_endscan(scan);
    table_close(indexRelation, AccessShareLock);

    return result;
}

/*
 * Deconstruct a text[] into C-strings (note any NULL elements will be
 * returned as NULL pointers)
 */
static char **
get_text_array_contents(ArrayType *array, int *numitems)
{
    int         ndim = ARR_NDIM(array);
    int        *dims = ARR_DIMS(array);
    int         nitems;
    int16       typlen;
    bool        typbyval;
    char        typalign;
    char      **values;
    char       *ptr;
    bits8      *bitmap;
    int         bitmask;
    int         i;

    Assert(ARR_ELEMTYPE(array) == TEXTOID);

    *numitems = nitems = ArrayGetNItems(ndim, dims);

    get_typlenbyvalalign(ARR_ELEMTYPE(array),
                         &typlen, &typbyval, &typalign);

    values = (char **) palloc(nitems * sizeof(char *));

    ptr = ARR_DATA_PTR(array);
    bitmap = ARR_NULLBITMAP(array);
    bitmask = 1;

    for (i = 0; i < nitems; i++)
    {
        if (bitmap && (*bitmap & bitmask) == 0)
        {
            values[i] = NULL;
        }
        else
        {
            values[i] = TextDatumGetCString(PointerGetDatum(ptr));
            ptr = att_addlength_pointer(ptr, typlen, ptr);
            ptr = (char *) att_align_nominal(ptr, typalign);
        }

        /* advance bitmap pointer if any */
        if (bitmap)
        {
            bitmask <<= 1;
            if (bitmask == 0x100)
            {
                bitmap++;
                bitmask = 1;
            }
        }
    }

    return values;
}

static char *
get_sql_insert(Relation rel, int *pkattnums, int pknumatts, char **src_pkattvals, char **tgt_pkattvals)
{
    char       *relname;
    HeapTuple   tuple;
    TupleDesc   tupdesc;
    int         natts;
    StringInfoData buf;
    char       *val;
    int         key;
    int         i;
    bool        needComma;

    initStringInfo(&buf);

    /* get relation name including any needed schema prefix and quoting */
    relname = generate_relation_name(rel);

    tupdesc = rel->rd_att;
    natts = tupdesc->natts;

    tuple = get_tuple_of_interest(rel, pkattnums, pknumatts, src_pkattvals);
    if (!tuple)
        ereport(ERROR,
                (errcode(ERRCODE_CARDINALITY_VIOLATION),
                 errmsg("source row not found")));

    appendStringInfo(&buf, "INSERT INTO %s(", relname);

    needComma = false;
    for (i = 0; i < natts; i++)
    {
        Form_pg_attribute att = TupleDescAttr(tupdesc, i);

        if (att->attisdropped)
            continue;

        if (needComma)
            appendStringInfoChar(&buf, ',');

        appendStringInfoString(&buf,
                               quote_ident_cstr(NameStr(att->attname)));
        needComma = true;
    }

    appendStringInfoString(&buf, ") VALUES(");

    /*
     * Note: i is physical column number (counting from 0).
     */
    needComma = false;
    for (i = 0; i < natts; i++)
    {
        if (TupleDescAttr(tupdesc, i)->attisdropped)
            continue;

        if (needComma)
            appendStringInfoChar(&buf, ',');

        key = get_attnum_pk_pos(pkattnums, pknumatts, i);

        if (key >= 0)
            val = tgt_pkattvals[key] ? pstrdup(tgt_pkattvals[key]) : NULL;
        else
            val = SPI_getvalue(tuple, tupdesc, i + 1);

        if (val != NULL)
        {
            appendStringInfoString(&buf, quote_literal_cstr(val));
            pfree(val);
        }
        else
            appendStringInfoString(&buf, "NULL");
        needComma = true;
    }
    appendStringInfoChar(&buf, ')');

    return buf.data;
}

static char *
get_sql_delete(Relation rel, int *pkattnums, int pknumatts, char **tgt_pkattvals)
{
    char       *relname;
    TupleDesc   tupdesc;
    StringInfoData buf;
    int         i;

    initStringInfo(&buf);

    /* get relation name including any needed schema prefix and quoting */
    relname = generate_relation_name(rel);

    tupdesc = rel->rd_att;

    appendStringInfo(&buf, "DELETE FROM %s WHERE ", relname);
    for (i = 0; i < pknumatts; i++)
    {
        int         pkattnum = pkattnums[i];
        Form_pg_attribute attr = TupleDescAttr(tupdesc, pkattnum);

        if (i > 0)
            appendStringInfoString(&buf, " AND ");

        appendStringInfoString(&buf,
                               quote_ident_cstr(NameStr(attr->attname)));

        if (tgt_pkattvals[i] != NULL)
            appendStringInfo(&buf, " = %s",
                             quote_literal_cstr(tgt_pkattvals[i]));
        else
            appendStringInfoString(&buf, " IS NULL");
    }

    return buf.data;
}

static char *
get_sql_update(Relation rel, int *pkattnums, int pknumatts, char **src_pkattvals, char **tgt_pkattvals)
{
    char       *relname;
    HeapTuple   tuple;
    TupleDesc   tupdesc;
    int         natts;
    StringInfoData buf;
    char       *val;
    int         key;
    int         i;
    bool        needComma;

    initStringInfo(&buf);

    /* get relation name including any needed schema prefix and quoting */
    relname = generate_relation_name(rel);

    tupdesc = rel->rd_att;
    natts = tupdesc->natts;

    tuple = get_tuple_of_interest(rel, pkattnums, pknumatts, src_pkattvals);
    if (!tuple)
        ereport(ERROR,
                (errcode(ERRCODE_CARDINALITY_VIOLATION),
                 errmsg("source row not found")));

    appendStringInfo(&buf, "UPDATE %s SET ", relname);

    /*
     * Note: i is physical column number (counting from 0).
     */
    needComma = false;
    for (i = 0; i < natts; i++)
    {
        Form_pg_attribute attr = TupleDescAttr(tupdesc, i);

        if (attr->attisdropped)
            continue;

        if (needComma)
            appendStringInfoString(&buf, ", ");

        appendStringInfo(&buf, "%s = ",
                         quote_ident_cstr(NameStr(attr->attname)));

        key = get_attnum_pk_pos(pkattnums, pknumatts, i);

        if (key >= 0)
            val = tgt_pkattvals[key] ? pstrdup(tgt_pkattvals[key]) : NULL;
        else
            val = SPI_getvalue(tuple, tupdesc, i + 1);

        if (val != NULL)
        {
            appendStringInfoString(&buf, quote_literal_cstr(val));
            pfree(val);
        }
        else
            appendStringInfoString(&buf, "NULL");
        needComma = true;
    }

    appendStringInfoString(&buf, " WHERE ");

    for (i = 0; i < pknumatts; i++)
    {
        int         pkattnum = pkattnums[i];
        Form_pg_attribute attr = TupleDescAttr(tupdesc, pkattnum);

        if (i > 0)
            appendStringInfoString(&buf, " AND ");

        appendStringInfoString(&buf,
                               quote_ident_cstr(NameStr(attr->attname)));

        val = tgt_pkattvals[i];

        if (val != NULL)
            appendStringInfo(&buf, " = %s", quote_literal_cstr(val));
        else
            appendStringInfoString(&buf, " IS NULL");
    }

    return buf.data;
}

/*
 * Return a properly quoted identifier.
 * Uses quote_ident in quote.c
 */
static char *
quote_ident_cstr(char *rawstr)
{
    text       *rawstr_text;
    text       *result_text;
    char       *result;

    rawstr_text = cstring_to_text(rawstr);
    result_text = DatumGetTextPP(DirectFunctionCall1(quote_ident,
                                                     PointerGetDatum(rawstr_text)));
    result = text_to_cstring(result_text);

    return result;
}

static int
get_attnum_pk_pos(int *pkattnums, int pknumatts, int key)
{
    int         i;

    /*
     * Not likely a long list anyway, so just scan for the value
     */
    for (i = 0; i < pknumatts; i++)
        if (key == pkattnums[i])
            return i;

    return -1;
}

static HeapTuple
get_tuple_of_interest(Relation rel, int *pkattnums, int pknumatts, char **src_pkattvals)
{
    char       *relname;
    TupleDesc   tupdesc;
    int         natts;
    StringInfoData buf;
    int         ret;
    HeapTuple   tuple;
    int         i;

    /*
     * Connect to SPI manager
     */
    if ((ret = SPI_connect()) < 0)
        /* internal error */
        elog(ERROR, "SPI connect failure - returned %d", ret);

    initStringInfo(&buf);

    /* get relation name including any needed schema prefix and quoting */
    relname = generate_relation_name(rel);

    tupdesc = rel->rd_att;
    natts = tupdesc->natts;

    /*
     * Build sql statement to look up tuple of interest, ie, the one matching
     * src_pkattvals.  We used to use "SELECT *" here, but it's simpler to
     * generate a result tuple that matches the table's physical structure,
     * with NULLs for any dropped columns.  Otherwise we have to deal with two
     * different tupdescs and everything's very confusing.
     */
    appendStringInfoString(&buf, "SELECT ");

    for (i = 0; i < natts; i++)
    {
        Form_pg_attribute attr = TupleDescAttr(tupdesc, i);

        if (i > 0)
            appendStringInfoString(&buf, ", ");

        if (attr->attisdropped)
            appendStringInfoString(&buf, "NULL");
        else
            appendStringInfoString(&buf,
                                   quote_ident_cstr(NameStr(attr->attname)));
    }

    appendStringInfo(&buf, " FROM %s WHERE ", relname);

    for (i = 0; i < pknumatts; i++)
    {
        int         pkattnum = pkattnums[i];
        Form_pg_attribute attr = TupleDescAttr(tupdesc, pkattnum);

        if (i > 0)
            appendStringInfoString(&buf, " AND ");

        appendStringInfoString(&buf,
                               quote_ident_cstr(NameStr(attr->attname)));

        if (src_pkattvals[i] != NULL)
            appendStringInfo(&buf, " = %s",
                             quote_literal_cstr(src_pkattvals[i]));
        else
            appendStringInfoString(&buf, " IS NULL");
    }

    /*
     * Retrieve the desired tuple
     */
    ret = SPI_exec(buf.data, 0);
    pfree(buf.data);

    /*
     * Only allow one qualifying tuple
     */
    if ((ret == SPI_OK_SELECT) && (SPI_processed > 1))
        ereport(ERROR,
                (errcode(ERRCODE_CARDINALITY_VIOLATION),
                 errmsg("source criteria matched more than one record")));

    else if (ret == SPI_OK_SELECT && SPI_processed == 1)
    {
        SPITupleTable *tuptable = SPI_tuptable;

        tuple = SPI_copytuple(tuptable->vals[0]);
        SPI_finish();

        return tuple;
    }
    else
    {
        /*
         * no qualifying tuples
         */
        SPI_finish();

        return NULL;
    }

    /*
     * never reached, but keep compiler quiet
     */
    return NULL;
}

/*
 * Open the relation named by relname_text, acquire specified type of lock,
 * verify we have specified permissions.
 * Caller must close rel when done with it.
 */
static Relation
get_rel_from_relname(text *relname_text, LOCKMODE lockmode, AclMode aclmode)
{
    RangeVar   *relvar;
    Relation    rel;
    AclResult   aclresult;

    relvar = makeRangeVarFromNameList(textToQualifiedNameList(relname_text));
    rel = table_openrv(relvar, lockmode);

    aclresult = pg_class_aclcheck(RelationGetRelid(rel), GetUserId(),
                                  aclmode);
    if (aclresult != ACLCHECK_OK)
        aclcheck_error(aclresult, get_relkind_objtype(rel->rd_rel->relkind),
                       RelationGetRelationName(rel));

    return rel;
}

/*
 * generate_relation_name - copied from ruleutils.c
 *      Compute the name to display for a relation
 *
 * The result includes all necessary quoting and schema-prefixing.
 */
static char *
generate_relation_name(Relation rel)
{
    char       *nspname;
    char       *result;

    /* Qualify the name if not visible in search path */
    if (RelationIsVisible(RelationGetRelid(rel)))
        nspname = NULL;
    else
        nspname = get_namespace_name(rel->rd_rel->relnamespace);

    result = quote_qualified_identifier(nspname, RelationGetRelationName(rel));

    return result;
}


static remoteConn *
getConnectionByName(const char *name)
{
    remoteConnHashEnt *hentry;
    char       *key;

    if (!remoteConnHash)
        remoteConnHash = createConnHash();

    key = pstrdup(name);
    truncate_identifier(key, strlen(key), false);
    hentry = (remoteConnHashEnt *) hash_search(remoteConnHash,
                                               key, HASH_FIND, NULL);

    if (hentry)
        return hentry->rconn;

    return NULL;
}

static HTAB *
createConnHash(void)
{
    HASHCTL     ctl;

    ctl.keysize = NAMEDATALEN;
    ctl.entrysize = sizeof(remoteConnHashEnt);

    return hash_create("Remote Con hash", NUMCONN, &ctl, HASH_ELEM);
}

static void
createNewConnection(const char *name, remoteConn *rconn)
{
    remoteConnHashEnt *hentry;
    bool        found;
    char       *key;

    if (!remoteConnHash)
        remoteConnHash = createConnHash();

    key = pstrdup(name);
    truncate_identifier(key, strlen(key), true);
    hentry = (remoteConnHashEnt *) hash_search(remoteConnHash, key,
                                               HASH_ENTER, &found);

    if (found)
    {
        PQfinish(rconn->conn);
        pfree(rconn);

        ereport(ERROR,
                (errcode(ERRCODE_DUPLICATE_OBJECT),
                 errmsg("duplicate connection name")));
    }

    hentry->rconn = rconn;
    strlcpy(hentry->name, name, sizeof(hentry->name));
}

static void
deleteConnection(const char *name)
{
    remoteConnHashEnt *hentry;
    bool        found;
    char       *key;

    if (!remoteConnHash)
        remoteConnHash = createConnHash();

    key = pstrdup(name);
    truncate_identifier(key, strlen(key), false);
    hentry = (remoteConnHashEnt *) hash_search(remoteConnHash,
                                               key, HASH_REMOVE, &found);

    if (!hentry)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_OBJECT),
                 errmsg("undefined connection name")));

}

static void
dblink_security_check(PGconn *conn, remoteConn *rconn)
{
    if (!superuser())
    {
        if (!PQconnectionUsedPassword(conn))
        {
            PQfinish(conn);
            if (rconn)
                pfree(rconn);

            ereport(ERROR,
                    (errcode(ERRCODE_S_R_E_PROHIBITED_SQL_STATEMENT_ATTEMPTED),
                     errmsg("password is required"),
                     errdetail("Non-superuser cannot connect if the server does not request a password."),
                     errhint("Target server's authentication method must be changed.")));
        }
    }
}

/*
 * For non-superusers, insist that the connstr specify a password.  This
 * prevents a password from being picked up from .pgpass, a service file,
 * the environment, etc.  We don't want the postgres user's passwords
 * to be accessible to non-superusers.
 */
static void
dblink_connstr_check(const char *connstr)
{
    if (!superuser())
    {
        PQconninfoOption *options;
        PQconninfoOption *option;
        bool        connstr_gives_password = false;

        options = PQconninfoParse(connstr, NULL);
        if (options)
        {
            for (option = options; option->keyword != NULL; option++)
            {
                if (strcmp(option->keyword, "password") == 0)
                {
                    if (option->val != NULL && option->val[0] != '\0')
                    {
                        connstr_gives_password = true;
                        break;
                    }
                }
            }
            PQconninfoFree(options);
        }

        if (!connstr_gives_password)
            ereport(ERROR,
                    (errcode(ERRCODE_S_R_E_PROHIBITED_SQL_STATEMENT_ATTEMPTED),
                     errmsg("password is required"),
                     errdetail("Non-superusers must provide a password in the connection string.")));
    }
}

/*
 * Report an error received from the remote server
 *
 * res: the received error result (will be freed)
 * fail: true for ERROR ereport, false for NOTICE
 * fmt and following args: sprintf-style format and values for errcontext;
 * the resulting string should be worded like "while <some action>"
 */
static void
dblink_res_error(PGconn *conn, const char *conname, PGresult *res,
                 bool fail, const char *fmt,...)
{
    int         level;
    char       *pg_diag_sqlstate = PQresultErrorField(res, PG_DIAG_SQLSTATE);
    char       *pg_diag_message_primary = PQresultErrorField(res, PG_DIAG_MESSAGE_PRIMARY);
    char       *pg_diag_message_detail = PQresultErrorField(res, PG_DIAG_MESSAGE_DETAIL);
    char       *pg_diag_message_hint = PQresultErrorField(res, PG_DIAG_MESSAGE_HINT);
    char       *pg_diag_context = PQresultErrorField(res, PG_DIAG_CONTEXT);
    int         sqlstate;
    char       *message_primary;
    char       *message_detail;
    char       *message_hint;
    char       *message_context;
    va_list     ap;
    char        dblink_context_msg[512];

    if (fail)
        level = ERROR;
    else
        level = NOTICE;

    if (pg_diag_sqlstate)
        sqlstate = MAKE_SQLSTATE(pg_diag_sqlstate[0],
                                 pg_diag_sqlstate[1],
                                 pg_diag_sqlstate[2],
                                 pg_diag_sqlstate[3],
                                 pg_diag_sqlstate[4]);
    else
        sqlstate = ERRCODE_CONNECTION_FAILURE;

    message_primary = xpstrdup(pg_diag_message_primary);
    message_detail = xpstrdup(pg_diag_message_detail);
    message_hint = xpstrdup(pg_diag_message_hint);
    message_context = xpstrdup(pg_diag_context);

    /*
     * If we don't get a message from the PGresult, try the PGconn.  This is
     * needed because for connection-level failures, PQexec may just return
     * NULL, not a PGresult at all.
     */
    if (message_primary == NULL)
        message_primary = pchomp(PQerrorMessage(conn));

    /*
     * Now that we've copied all the data we need out of the PGresult, it's
     * safe to free it.  We must do this to avoid PGresult leakage.  We're
     * leaking all the strings too, but those are in palloc'd memory that will
     * get cleaned up eventually.
     */
    if (res)
        PQclear(res);

    /*
     * Format the basic errcontext string.  Below, we'll add on something
     * about the connection name.  That's a violation of the translatability
     * guidelines about constructing error messages out of parts, but since
     * there's no translation support for dblink, there's no need to worry
     * about that (yet).
     */
    va_start(ap, fmt);
    vsnprintf(dblink_context_msg, sizeof(dblink_context_msg), fmt, ap);
    va_end(ap);

    ereport(level,
            (errcode(sqlstate),
             message_primary ? errmsg_internal("%s", message_primary) :
             errmsg("could not obtain message string for remote error"),
             message_detail ? errdetail_internal("%s", message_detail) : 0,
             message_hint ? errhint("%s", message_hint) : 0,
             message_context ? (errcontext("%s", message_context)) : 0,
             conname ?
             (errcontext("%s on dblink connection named \"%s\"",
                         dblink_context_msg, conname)) :
             (errcontext("%s on unnamed dblink connection",
                         dblink_context_msg))));
}

/*
 * Obtain connection string for a foreign server
 */
static char *
get_connect_string(const char *servername)
{
    ForeignServer *foreign_server = NULL;
    UserMapping *user_mapping;
    ListCell   *cell;
    StringInfoData buf;
    ForeignDataWrapper *fdw;
    AclResult   aclresult;
    char       *srvname;

    static const PQconninfoOption *options = NULL;

    initStringInfo(&buf);

    /*
     * Get list of valid libpq options.
     *
     * To avoid unnecessary work, we get the list once and use it throughout
     * the lifetime of this backend process.  We don't need to care about
     * memory context issues, because PQconndefaults allocates with malloc.
     */
    if (!options)
    {
        options = PQconndefaults();
        if (!options)           /* assume reason for failure is OOM */
            ereport(ERROR,
                    (errcode(ERRCODE_FDW_OUT_OF_MEMORY),
                     errmsg("out of memory"),
                     errdetail("Could not get libpq's default connection options.")));
    }

    /* first gather the server connstr options */
    srvname = pstrdup(servername);
    truncate_identifier(srvname, strlen(srvname), false);
    foreign_server = GetForeignServerByName(srvname, true);

    if (foreign_server)
    {
        Oid         serverid = foreign_server->serverid;
        Oid         fdwid = foreign_server->fdwid;
        Oid         userid = GetUserId();

        user_mapping = GetUserMapping(userid, serverid);
        fdw = GetForeignDataWrapper(fdwid);

        /* Check permissions, user must have usage on the server. */
        aclresult = pg_foreign_server_aclcheck(serverid, userid, ACL_USAGE);
        if (aclresult != ACLCHECK_OK)
            aclcheck_error(aclresult, OBJECT_FOREIGN_SERVER, foreign_server->servername);

        foreach(cell, fdw->options)
        {
            DefElem    *def = lfirst(cell);

            if (is_valid_dblink_option(options, def->defname, ForeignDataWrapperRelationId))
                appendStringInfo(&buf, "%s='%s' ", def->defname,
                                 escape_param_str(strVal(def->arg)));
        }

        foreach(cell, foreign_server->options)
        {
            DefElem    *def = lfirst(cell);

            if (is_valid_dblink_option(options, def->defname, ForeignServerRelationId))
                appendStringInfo(&buf, "%s='%s' ", def->defname,
                                 escape_param_str(strVal(def->arg)));
        }

        foreach(cell, user_mapping->options)
        {

            DefElem    *def = lfirst(cell);

            if (is_valid_dblink_option(options, def->defname, UserMappingRelationId))
                appendStringInfo(&buf, "%s='%s' ", def->defname,
                                 escape_param_str(strVal(def->arg)));
        }

        return buf.data;
    }
    else
        return NULL;
}

/*
 * Escaping libpq connect parameter strings.
 *
 * Replaces "'" with "\'" and "\" with "\\".
 */
static char *
escape_param_str(const char *str)
{
    const char *cp;
    StringInfoData buf;

    initStringInfo(&buf);

    for (cp = str; *cp; cp++)
    {
        if (*cp == '\\' || *cp == '\'')
            appendStringInfoChar(&buf, '\\');
        appendStringInfoChar(&buf, *cp);
    }

    return buf.data;
}

/*
 * Validate the PK-attnums argument for dblink_build_sql_insert() and related
 * functions, and translate to the internal representation.
 *
 * The user supplies an int2vector of 1-based logical attnums, plus a count
 * argument (the need for the separate count argument is historical, but we
 * still check it).  We check that each attnum corresponds to a valid,
 * non-dropped attribute of the rel.  We do *not* prevent attnums from being
 * listed twice, though the actual use-case for such things is dubious.
 * Note that before Postgres 9.0, the user's attnums were interpreted as
 * physical not logical column numbers; this was changed for future-proofing.
 *
 * The internal representation is a palloc'd int array of 0-based physical
 * attnums.
 */
static void
validate_pkattnums(Relation rel,
                   int2vector *pkattnums_arg, int32 pknumatts_arg,
                   int **pkattnums, int *pknumatts)
{
    TupleDesc   tupdesc = rel->rd_att;
    int         natts = tupdesc->natts;
    int         i;

    /* Don't take more array elements than there are */
    pknumatts_arg = Min(pknumatts_arg, pkattnums_arg->dim1);

    /* Must have at least one pk attnum selected */
    if (pknumatts_arg <= 0)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("number of key attributes must be > 0")));

    /* Allocate output array */
    *pkattnums = (int *) palloc(pknumatts_arg * sizeof(int));
    *pknumatts = pknumatts_arg;

    /* Validate attnums and convert to internal form */
    for (i = 0; i < pknumatts_arg; i++)
    {
        int         pkattnum = pkattnums_arg->values[i];
        int         lnum;
        int         j;

        /* Can throw error immediately if out of range */
        if (pkattnum <= 0 || pkattnum > natts)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("invalid attribute number %d", pkattnum)));

        /* Identify which physical column has this logical number */
        lnum = 0;
        for (j = 0; j < natts; j++)
        {
            /* dropped columns don't count */
            if (TupleDescAttr(tupdesc, j)->attisdropped)
                continue;

            if (++lnum == pkattnum)
                break;
        }

        if (j < natts)
            (*pkattnums)[i] = j;
        else
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("invalid attribute number %d", pkattnum)));
    }
}

/*
 * Check if the specified connection option is valid.
 *
 * We basically allow whatever libpq thinks is an option, with these
 * restrictions:
 *      debug options: disallowed
 *      "client_encoding": disallowed
 *      "user": valid only in USER MAPPING options
 *      secure options (eg password): valid only in USER MAPPING options
 *      others: valid only in FOREIGN SERVER options
 *
 * We disallow client_encoding because it would be overridden anyway via
 * PQclientEncoding; allowing it to be specified would merely promote
 * confusion.
 */
static bool
is_valid_dblink_option(const PQconninfoOption *options, const char *option,
                       Oid context)
{
    const PQconninfoOption *opt;

    /* Look up the option in libpq result */
    for (opt = options; opt->keyword; opt++)
    {
        if (strcmp(opt->keyword, option) == 0)
            break;
    }
    if (opt->keyword == NULL)
        return false;

    /* Disallow debug options (particularly "replication") */
    if (strchr(opt->dispchar, 'D'))
        return false;

    /* Disallow "client_encoding" */
    if (strcmp(opt->keyword, "client_encoding") == 0)
        return false;

    /*
     * If the option is "user" or marked secure, it should be specified only
     * in USER MAPPING.  Others should be specified only in SERVER.
     */
    if (strcmp(opt->keyword, "user") == 0 || strchr(opt->dispchar, '*'))
    {
        if (context != UserMappingRelationId)
            return false;
    }
    else
    {
        if (context != ForeignServerRelationId)
            return false;
    }

    return true;
}

/*
 * Copy the remote session's values of GUCs that affect datatype I/O
 * and apply them locally in a new GUC nesting level.  Returns the new
 * nestlevel (which is needed by restoreLocalGucs to undo the settings),
 * or -1 if no new nestlevel was needed.
 *
 * We use the equivalent of a function SET option to allow the settings to
 * persist only until the caller calls restoreLocalGucs.  If an error is
 * thrown in between, guc.c will take care of undoing the settings.
 */
static int
applyRemoteGucs(PGconn *conn)
{
    static const char *const GUCsAffectingIO[] = {
        "DateStyle",
        "IntervalStyle"
    };

    int         nestlevel = -1;
    int         i;

    for (i = 0; i < lengthof(GUCsAffectingIO); i++)
    {
        const char *gucName = GUCsAffectingIO[i];
        const char *remoteVal = PQparameterStatus(conn, gucName);
        const char *localVal;

        /*
         * If the remote server is pre-8.4, it won't have IntervalStyle, but
         * that's okay because its output format won't be ambiguous.  So just
         * skip the GUC if we don't get a value for it.  (We might eventually
         * need more complicated logic with remote-version checks here.)
         */
        if (remoteVal == NULL)
            continue;

        /*
         * Avoid GUC-setting overhead if the remote and local GUCs already
         * have the same value.
         */
        localVal = GetConfigOption(gucName, false, false);
        Assert(localVal != NULL);

        if (strcmp(remoteVal, localVal) == 0)
            continue;

        /* Create new GUC nest level if we didn't already */
        if (nestlevel < 0)
            nestlevel = NewGUCNestLevel();

        /* Apply the option (this will throw error on failure) */
        (void) set_config_option(gucName, remoteVal,
                                 PGC_USERSET, PGC_S_SESSION,
                                 GUC_ACTION_SAVE, true, 0, false);
    }

    return nestlevel;
}

/*
 * Restore local GUCs after they have been overlaid with remote settings.
 */
static void
restoreLocalGucs(int nestlevel)
{
    /* Do nothing if no new nestlevel was created */
    if (nestlevel > 0)
        AtEOXact_GUC(true, nestlevel);
}