libpqwalreceiver.c

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/*-------------------------------------------------------------------------
 *
 * libpqwalreceiver.c
 *
 * This file contains the libpq-specific parts of walreceiver. It's
 * loaded as a dynamic module to avoid linking the main server binary with
 * libpq.
 *
 * Portions Copyright (c) 2010-2020, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *    src/backend/replication/libpqwalreceiver/libpqwalreceiver.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <unistd.h>
#include <sys/time.h>

#include "access/xlog.h"
#include "catalog/pg_type.h"
#include "funcapi.h"
#include "libpq-fe.h"
#include "mb/pg_wchar.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "pqexpbuffer.h"
#include "replication/walreceiver.h"
#include "utils/builtins.h"
#include "utils/memutils.h"
#include "utils/pg_lsn.h"
#include "utils/tuplestore.h"

PG_MODULE_MAGIC;

void        _PG_init(void);

struct WalReceiverConn
{
    /* Current connection to the primary, if any */
    PGconn     *streamConn;
    /* Used to remember if the connection is logical or physical */
    bool        logical;
    /* Buffer for currently read records */
    char       *recvBuf;
};

/* Prototypes for interface functions */
static WalReceiverConn *libpqrcv_connect(const char *conninfo,
                                         bool logical, const char *appname,
                                         char **err);
static void libpqrcv_check_conninfo(const char *conninfo);
static char *libpqrcv_get_conninfo(WalReceiverConn *conn);
static void libpqrcv_get_senderinfo(WalReceiverConn *conn,
                                    char **sender_host, int *sender_port);
static char *libpqrcv_identify_system(WalReceiverConn *conn,
                                      TimeLineID *primary_tli);
static int  libpqrcv_server_version(WalReceiverConn *conn);
static void libpqrcv_readtimelinehistoryfile(WalReceiverConn *conn,
                                             TimeLineID tli, char **filename,
                                             char **content, int *len);
static bool libpqrcv_startstreaming(WalReceiverConn *conn,
                                    const WalRcvStreamOptions *options);
static void libpqrcv_endstreaming(WalReceiverConn *conn,
                                  TimeLineID *next_tli);
static int  libpqrcv_receive(WalReceiverConn *conn, char **buffer,
                             pgsocket *wait_fd);
static void libpqrcv_send(WalReceiverConn *conn, const char *buffer,
                          int nbytes);
static char *libpqrcv_create_slot(WalReceiverConn *conn,
                                  const char *slotname,
                                  bool temporary,
                                  CRSSnapshotAction snapshot_action,
                                  XLogRecPtr *lsn);
static pid_t libpqrcv_get_backend_pid(WalReceiverConn *conn);
static WalRcvExecResult *libpqrcv_exec(WalReceiverConn *conn,
                                       const char *query,
                                       const int nRetTypes,
                                       const Oid *retTypes);
static void libpqrcv_disconnect(WalReceiverConn *conn);

static WalReceiverFunctionsType PQWalReceiverFunctions = {
    libpqrcv_connect,
    libpqrcv_check_conninfo,
    libpqrcv_get_conninfo,
    libpqrcv_get_senderinfo,
    libpqrcv_identify_system,
    libpqrcv_server_version,
    libpqrcv_readtimelinehistoryfile,
    libpqrcv_startstreaming,
    libpqrcv_endstreaming,
    libpqrcv_receive,
    libpqrcv_send,
    libpqrcv_create_slot,
    libpqrcv_get_backend_pid,
    libpqrcv_exec,
    libpqrcv_disconnect
};

/* Prototypes for private functions */
static PGresult *libpqrcv_PQexec(PGconn *streamConn, const char *query);
static PGresult *libpqrcv_PQgetResult(PGconn *streamConn);
static char *stringlist_to_identifierstr(PGconn *conn, List *strings);

/*
 * Module initialization function
 */
void
_PG_init(void)
{
    if (WalReceiverFunctions != NULL)
        elog(ERROR, "libpqwalreceiver already loaded");
    WalReceiverFunctions = &PQWalReceiverFunctions;
}

/*
 * Establish the connection to the primary server for XLOG streaming
 *
 * Returns NULL on error and fills the err with palloc'ed error message.
 */
static WalReceiverConn *
libpqrcv_connect(const char *conninfo, bool logical, const char *appname,
                 char **err)
{
    WalReceiverConn *conn;
    PostgresPollingStatusType status;
    const char *keys[5];
    const char *vals[5];
    int         i = 0;

    /*
     * We use the expand_dbname parameter to process the connection string (or
     * URI), and pass some extra options.
     */
    keys[i] = "dbname";
    vals[i] = conninfo;
    keys[++i] = "replication";
    vals[i] = logical ? "database" : "true";
    if (!logical)
    {
        /*
         * The database name is ignored by the server in replication mode, but
         * specify "replication" for .pgpass lookup.
         */
        keys[++i] = "dbname";
        vals[i] = "replication";
    }
    keys[++i] = "fallback_application_name";
    vals[i] = appname;
    if (logical)
    {
        keys[++i] = "client_encoding";
        vals[i] = GetDatabaseEncodingName();
    }
    keys[++i] = NULL;
    vals[i] = NULL;

    Assert(i < sizeof(keys));

    conn = palloc0(sizeof(WalReceiverConn));
    conn->streamConn = PQconnectStartParams(keys, vals,
                                             /* expand_dbname = */ true);
    if (PQstatus(conn->streamConn) == CONNECTION_BAD)
    {
        *err = pchomp(PQerrorMessage(conn->streamConn));
        return NULL;
    }

    /*
     * Poll connection until we have OK or FAILED status.
     *
     * Per spec for PQconnectPoll, first wait till socket is write-ready.
     */
    status = PGRES_POLLING_WRITING;
    do
    {
        int         io_flag;
        int         rc;

        if (status == PGRES_POLLING_READING)
            io_flag = WL_SOCKET_READABLE;
#ifdef WIN32
        /* Windows needs a different test while waiting for connection-made */
        else if (PQstatus(conn->streamConn) == CONNECTION_STARTED)
            io_flag = WL_SOCKET_CONNECTED;
#endif
        else
            io_flag = WL_SOCKET_WRITEABLE;

        rc = WaitLatchOrSocket(MyLatch,
                               WL_EXIT_ON_PM_DEATH | WL_LATCH_SET | io_flag,
                               PQsocket(conn->streamConn),
                               0,
                               WAIT_EVENT_LIBPQWALRECEIVER_CONNECT);

        /* Interrupted? */
        if (rc & WL_LATCH_SET)
        {
            ResetLatch(MyLatch);
            ProcessWalRcvInterrupts();
        }

        /* If socket is ready, advance the libpq state machine */
        if (rc & io_flag)
            status = PQconnectPoll(conn->streamConn);
    } while (status != PGRES_POLLING_OK && status != PGRES_POLLING_FAILED);

    if (PQstatus(conn->streamConn) != CONNECTION_OK)
    {
        *err = pchomp(PQerrorMessage(conn->streamConn));
        return NULL;
    }

    conn->logical = logical;

    return conn;
}

/*
 * Validate connection info string (just try to parse it)
 */
static void
libpqrcv_check_conninfo(const char *conninfo)
{
    PQconninfoOption *opts = NULL;
    char       *err = NULL;

    opts = PQconninfoParse(conninfo, &err);
    if (opts == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_SYNTAX_ERROR),
                 errmsg("invalid connection string syntax: %s", err)));

    PQconninfoFree(opts);
}

/*
 * Return a user-displayable conninfo string.  Any security-sensitive fields
 * are obfuscated.
 */
static char *
libpqrcv_get_conninfo(WalReceiverConn *conn)
{
    PQconninfoOption *conn_opts;
    PQconninfoOption *conn_opt;
    PQExpBufferData buf;
    char       *retval;

    Assert(conn->streamConn != NULL);

    initPQExpBuffer(&buf);
    conn_opts = PQconninfo(conn->streamConn);

    if (conn_opts == NULL)
        ereport(ERROR,
                (errmsg("could not parse connection string: %s",
                        _("out of memory"))));

    /* build a clean connection string from pieces */
    for (conn_opt = conn_opts; conn_opt->keyword != NULL; conn_opt++)
    {
        bool        obfuscate;

        /* Skip debug and empty options */
        if (strchr(conn_opt->dispchar, 'D') ||
            conn_opt->val == NULL ||
            conn_opt->val[0] == '\0')
            continue;

        /* Obfuscate security-sensitive options */
        obfuscate = strchr(conn_opt->dispchar, '*') != NULL;

        appendPQExpBuffer(&buf, "%s%s=%s",
                          buf.len == 0 ? "" : " ",
                          conn_opt->keyword,
                          obfuscate ? "********" : conn_opt->val);
    }

    PQconninfoFree(conn_opts);

    retval = PQExpBufferDataBroken(buf) ? NULL : pstrdup(buf.data);
    termPQExpBuffer(&buf);
    return retval;
}

/*
 * Provides information of sender this WAL receiver is connected to.
 */
static void
libpqrcv_get_senderinfo(WalReceiverConn *conn, char **sender_host,
                        int *sender_port)
{
    char       *ret = NULL;

    *sender_host = NULL;
    *sender_port = 0;

    Assert(conn->streamConn != NULL);

    ret = PQhost(conn->streamConn);
    if (ret && strlen(ret) != 0)
        *sender_host = pstrdup(ret);

    ret = PQport(conn->streamConn);
    if (ret && strlen(ret) != 0)
        *sender_port = atoi(ret);
}

/*
 * Check that primary's system identifier matches ours, and fetch the current
 * timeline ID of the primary.
 */
static char *
libpqrcv_identify_system(WalReceiverConn *conn, TimeLineID *primary_tli)
{
    PGresult   *res;
    char       *primary_sysid;

    /*
     * Get the system identifier and timeline ID as a DataRow message from the
     * primary server.
     */
    res = libpqrcv_PQexec(conn->streamConn, "IDENTIFY_SYSTEM");
    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        PQclear(res);
        ereport(ERROR,
                (errmsg("could not receive database system identifier and timeline ID from "
                        "the primary server: %s",
                        pchomp(PQerrorMessage(conn->streamConn)))));
    }
    if (PQnfields(res) < 3 || PQntuples(res) != 1)
    {
        int         ntuples = PQntuples(res);
        int         nfields = PQnfields(res);

        PQclear(res);
        ereport(ERROR,
                (errmsg("invalid response from primary server"),
                 errdetail("Could not identify system: got %d rows and %d fields, expected %d rows and %d or more fields.",
                           ntuples, nfields, 3, 1)));
    }
    primary_sysid = pstrdup(PQgetvalue(res, 0, 0));
    *primary_tli = pg_strtoint32(PQgetvalue(res, 0, 1));
    PQclear(res);

    return primary_sysid;
}

/*
 * Thin wrapper around libpq to obtain server version.
 */
static int
libpqrcv_server_version(WalReceiverConn *conn)
{
    return PQserverVersion(conn->streamConn);
}

/*
 * Start streaming WAL data from given streaming options.
 *
 * Returns true if we switched successfully to copy-both mode. False
 * means the server received the command and executed it successfully, but
 * didn't switch to copy-mode.  That means that there was no WAL on the
 * requested timeline and starting point, because the server switched to
 * another timeline at or before the requested starting point. On failure,
 * throws an ERROR.
 */
static bool
libpqrcv_startstreaming(WalReceiverConn *conn,
                        const WalRcvStreamOptions *options)
{
    StringInfoData cmd;
    PGresult   *res;

    Assert(options->logical == conn->logical);
    Assert(options->slotname || !options->logical);

    initStringInfo(&cmd);

    /* Build the command. */
    appendStringInfoString(&cmd, "START_REPLICATION");
    if (options->slotname != NULL)
        appendStringInfo(&cmd, " SLOT \"%s\"",
                         options->slotname);

    if (options->logical)
        appendStringInfoString(&cmd, " LOGICAL");

    appendStringInfo(&cmd, " %X/%X",
                     (uint32) (options->startpoint >> 32),
                     (uint32) options->startpoint);

    /*
     * Additional options are different depending on if we are doing logical
     * or physical replication.
     */
    if (options->logical)
    {
        char       *pubnames_str;
        List       *pubnames;
        char       *pubnames_literal;

        appendStringInfoString(&cmd, " (");

        appendStringInfo(&cmd, "proto_version '%u'",
                         options->proto.logical.proto_version);

        pubnames = options->proto.logical.publication_names;
        pubnames_str = stringlist_to_identifierstr(conn->streamConn, pubnames);
        if (!pubnames_str)
            ereport(ERROR,
                    (errmsg("could not start WAL streaming: %s",
                            pchomp(PQerrorMessage(conn->streamConn)))));
        pubnames_literal = PQescapeLiteral(conn->streamConn, pubnames_str,
                                           strlen(pubnames_str));
        if (!pubnames_literal)
            ereport(ERROR,
                    (errmsg("could not start WAL streaming: %s",
                            pchomp(PQerrorMessage(conn->streamConn)))));
        appendStringInfo(&cmd, ", publication_names %s", pubnames_literal);
        PQfreemem(pubnames_literal);
        pfree(pubnames_str);

        if (options->proto.logical.binary &&
            PQserverVersion(conn->streamConn) >= 140000)
            appendStringInfoString(&cmd, ", binary 'true'");

        appendStringInfoChar(&cmd, ')');
    }
    else
        appendStringInfo(&cmd, " TIMELINE %u",
                         options->proto.physical.startpointTLI);

    /* Start streaming. */
    res = libpqrcv_PQexec(conn->streamConn, cmd.data);
    pfree(cmd.data);

    if (PQresultStatus(res) == PGRES_COMMAND_OK)
    {
        PQclear(res);
        return false;
    }
    else if (PQresultStatus(res) != PGRES_COPY_BOTH)
    {
        PQclear(res);
        ereport(ERROR,
                (errmsg("could not start WAL streaming: %s",
                        pchomp(PQerrorMessage(conn->streamConn)))));
    }
    PQclear(res);
    return true;
}

/*
 * Stop streaming WAL data. Returns the next timeline's ID in *next_tli, as
 * reported by the server, or 0 if it did not report it.
 */
static void
libpqrcv_endstreaming(WalReceiverConn *conn, TimeLineID *next_tli)
{
    PGresult   *res;

    /*
     * Send copy-end message.  As in libpqrcv_PQexec, this could theoretically
     * block, but the risk seems small.
     */
    if (PQputCopyEnd(conn->streamConn, NULL) <= 0 ||
        PQflush(conn->streamConn))
        ereport(ERROR,
                (errmsg("could not send end-of-streaming message to primary: %s",
                        pchomp(PQerrorMessage(conn->streamConn)))));

    *next_tli = 0;

    /*
     * After COPY is finished, we should receive a result set indicating the
     * next timeline's ID, or just CommandComplete if the server was shut
     * down.
     *
     * If we had not yet received CopyDone from the backend, PGRES_COPY_OUT is
     * also possible in case we aborted the copy in mid-stream.
     */
    res = libpqrcv_PQgetResult(conn->streamConn);
    if (PQresultStatus(res) == PGRES_TUPLES_OK)
    {
        /*
         * Read the next timeline's ID. The server also sends the timeline's
         * starting point, but it is ignored.
         */
        if (PQnfields(res) < 2 || PQntuples(res) != 1)
            ereport(ERROR,
                    (errmsg("unexpected result set after end-of-streaming")));
        *next_tli = pg_strtoint32(PQgetvalue(res, 0, 0));
        PQclear(res);

        /* the result set should be followed by CommandComplete */
        res = libpqrcv_PQgetResult(conn->streamConn);
    }
    else if (PQresultStatus(res) == PGRES_COPY_OUT)
    {
        PQclear(res);

        /* End the copy */
        if (PQendcopy(conn->streamConn))
            ereport(ERROR,
                    (errmsg("error while shutting down streaming COPY: %s",
                            pchomp(PQerrorMessage(conn->streamConn)))));

        /* CommandComplete should follow */
        res = libpqrcv_PQgetResult(conn->streamConn);
    }

    if (PQresultStatus(res) != PGRES_COMMAND_OK)
        ereport(ERROR,
                (errmsg("error reading result of streaming command: %s",
                        pchomp(PQerrorMessage(conn->streamConn)))));
    PQclear(res);

    /* Verify that there are no more results */
    res = libpqrcv_PQgetResult(conn->streamConn);
    if (res != NULL)
        ereport(ERROR,
                (errmsg("unexpected result after CommandComplete: %s",
                        pchomp(PQerrorMessage(conn->streamConn)))));
}

/*
 * Fetch the timeline history file for 'tli' from primary.
 */
static void
libpqrcv_readtimelinehistoryfile(WalReceiverConn *conn,
                                 TimeLineID tli, char **filename,
                                 char **content, int *len)
{
    PGresult   *res;
    char        cmd[64];

    Assert(!conn->logical);

    /*
     * Request the primary to send over the history file for given timeline.
     */
    snprintf(cmd, sizeof(cmd), "TIMELINE_HISTORY %u", tli);
    res = libpqrcv_PQexec(conn->streamConn, cmd);
    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        PQclear(res);
        ereport(ERROR,
                (errmsg("could not receive timeline history file from "
                        "the primary server: %s",
                        pchomp(PQerrorMessage(conn->streamConn)))));
    }
    if (PQnfields(res) != 2 || PQntuples(res) != 1)
    {
        int         ntuples = PQntuples(res);
        int         nfields = PQnfields(res);

        PQclear(res);
        ereport(ERROR,
                (errmsg("invalid response from primary server"),
                 errdetail("Expected 1 tuple with 2 fields, got %d tuples with %d fields.",
                           ntuples, nfields)));
    }
    *filename = pstrdup(PQgetvalue(res, 0, 0));

    *len = PQgetlength(res, 0, 1);
    *content = palloc(*len);
    memcpy(*content, PQgetvalue(res, 0, 1), *len);
    PQclear(res);
}

/*
 * Send a query and wait for the results by using the asynchronous libpq
 * functions and socket readiness events.
 *
 * We must not use the regular blocking libpq functions like PQexec()
 * since they are uninterruptible by signals on some platforms, such as
 * Windows.
 *
 * The function is modeled on PQexec() in libpq, but only implements
 * those parts that are in use in the walreceiver api.
 *
 * May return NULL, rather than an error result, on failure.
 */
static PGresult *
libpqrcv_PQexec(PGconn *streamConn, const char *query)
{
    PGresult   *lastResult = NULL;

    /*
     * PQexec() silently discards any prior query results on the connection.
     * This is not required for this function as it's expected that the caller
     * (which is this library in all cases) will behave correctly and we don't
     * have to be backwards compatible with old libpq.
     */

    /*
     * Submit the query.  Since we don't use non-blocking mode, this could
     * theoretically block.  In practice, since we don't send very long query
     * strings, the risk seems negligible.
     */
    if (!PQsendQuery(streamConn, query))
        return NULL;

    for (;;)
    {
        /* Wait for, and collect, the next PGresult. */
        PGresult   *result;

        result = libpqrcv_PQgetResult(streamConn);
        if (result == NULL)
            break;              /* query is complete, or failure */

        /*
         * Emulate PQexec()'s behavior of returning the last result when there
         * are many.  We are fine with returning just last error message.
         */
        PQclear(lastResult);
        lastResult = result;

        if (PQresultStatus(lastResult) == PGRES_COPY_IN ||
            PQresultStatus(lastResult) == PGRES_COPY_OUT ||
            PQresultStatus(lastResult) == PGRES_COPY_BOTH ||
            PQstatus(streamConn) == CONNECTION_BAD)
            break;
    }

    return lastResult;
}

/*
 * Perform the equivalent of PQgetResult(), but watch for interrupts.
 */
static PGresult *
libpqrcv_PQgetResult(PGconn *streamConn)
{
    /*
     * Collect data until PQgetResult is ready to get the result without
     * blocking.
     */
    while (PQisBusy(streamConn))
    {
        int         rc;

        /*
         * We don't need to break down the sleep into smaller increments,
         * since we'll get interrupted by signals and can handle any
         * interrupts here.
         */
        rc = WaitLatchOrSocket(MyLatch,
                               WL_EXIT_ON_PM_DEATH | WL_SOCKET_READABLE |
                               WL_LATCH_SET,
                               PQsocket(streamConn),
                               0,
                               WAIT_EVENT_LIBPQWALRECEIVER_RECEIVE);

        /* Interrupted? */
        if (rc & WL_LATCH_SET)
        {
            ResetLatch(MyLatch);
            ProcessWalRcvInterrupts();
        }

        /* Consume whatever data is available from the socket */
        if (PQconsumeInput(streamConn) == 0)
        {
            /* trouble; return NULL */
            return NULL;
        }
    }

    /* Now we can collect and return the next PGresult */
    return PQgetResult(streamConn);
}

/*
 * Disconnect connection to primary, if any.
 */
static void
libpqrcv_disconnect(WalReceiverConn *conn)
{
    PQfinish(conn->streamConn);
    if (conn->recvBuf != NULL)
        PQfreemem(conn->recvBuf);
    pfree(conn);
}

/*
 * Receive a message available from XLOG stream.
 *
 * Returns:
 *
 *   If data was received, returns the length of the data. *buffer is set to
 *   point to a buffer holding the received message. The buffer is only valid
 *   until the next libpqrcv_* call.
 *
 *   If no data was available immediately, returns 0, and *wait_fd is set to a
 *   socket descriptor which can be waited on before trying again.
 *
 *   -1 if the server ended the COPY.
 *
 * ereports on error.
 */
static int
libpqrcv_receive(WalReceiverConn *conn, char **buffer,
                 pgsocket *wait_fd)
{
    int         rawlen;

    if (conn->recvBuf != NULL)
        PQfreemem(conn->recvBuf);
    conn->recvBuf = NULL;

    /* Try to receive a CopyData message */
    rawlen = PQgetCopyData(conn->streamConn, &conn->recvBuf, 1);
    if (rawlen == 0)
    {
        /* Try consuming some data. */
        if (PQconsumeInput(conn->streamConn) == 0)
            ereport(ERROR,
                    (errmsg("could not receive data from WAL stream: %s",
                            pchomp(PQerrorMessage(conn->streamConn)))));

        /* Now that we've consumed some input, try again */
        rawlen = PQgetCopyData(conn->streamConn, &conn->recvBuf, 1);
        if (rawlen == 0)
        {
            /* Tell caller to try again when our socket is ready. */
            *wait_fd = PQsocket(conn->streamConn);
            return 0;
        }
    }
    if (rawlen == -1)           /* end-of-streaming or error */
    {
        PGresult   *res;

        res = libpqrcv_PQgetResult(conn->streamConn);
        if (PQresultStatus(res) == PGRES_COMMAND_OK)
        {
            PQclear(res);

            /* Verify that there are no more results. */
            res = libpqrcv_PQgetResult(conn->streamConn);
            if (res != NULL)
            {
                PQclear(res);

                /*
                 * If the other side closed the connection orderly (otherwise
                 * we'd seen an error, or PGRES_COPY_IN) don't report an error
                 * here, but let callers deal with it.
                 */
                if (PQstatus(conn->streamConn) == CONNECTION_BAD)
                    return -1;

                ereport(ERROR,
                        (errmsg("unexpected result after CommandComplete: %s",
                                PQerrorMessage(conn->streamConn))));
            }

            return -1;
        }
        else if (PQresultStatus(res) == PGRES_COPY_IN)
        {
            PQclear(res);
            return -1;
        }
        else
        {
            PQclear(res);
            ereport(ERROR,
                    (errmsg("could not receive data from WAL stream: %s",
                            pchomp(PQerrorMessage(conn->streamConn)))));
        }
    }
    if (rawlen < -1)
        ereport(ERROR,
                (errmsg("could not receive data from WAL stream: %s",
                        pchomp(PQerrorMessage(conn->streamConn)))));

    /* Return received messages to caller */
    *buffer = conn->recvBuf;
    return rawlen;
}

/*
 * Send a message to XLOG stream.
 *
 * ereports on error.
 */
static void
libpqrcv_send(WalReceiverConn *conn, const char *buffer, int nbytes)
{
    if (PQputCopyData(conn->streamConn, buffer, nbytes) <= 0 ||
        PQflush(conn->streamConn))
        ereport(ERROR,
                (errmsg("could not send data to WAL stream: %s",
                        pchomp(PQerrorMessage(conn->streamConn)))));
}

/*
 * Create new replication slot.
 * Returns the name of the exported snapshot for logical slot or NULL for
 * physical slot.
 */
static char *
libpqrcv_create_slot(WalReceiverConn *conn, const char *slotname,
                     bool temporary, CRSSnapshotAction snapshot_action,
                     XLogRecPtr *lsn)
{
    PGresult   *res;
    StringInfoData cmd;
    char       *snapshot;

    initStringInfo(&cmd);

    appendStringInfo(&cmd, "CREATE_REPLICATION_SLOT \"%s\"", slotname);

    if (temporary)
        appendStringInfoString(&cmd, " TEMPORARY");

    if (conn->logical)
    {
        appendStringInfoString(&cmd, " LOGICAL pgoutput");
        switch (snapshot_action)
        {
            case CRS_EXPORT_SNAPSHOT:
                appendStringInfoString(&cmd, " EXPORT_SNAPSHOT");
                break;
            case CRS_NOEXPORT_SNAPSHOT:
                appendStringInfoString(&cmd, " NOEXPORT_SNAPSHOT");
                break;
            case CRS_USE_SNAPSHOT:
                appendStringInfoString(&cmd, " USE_SNAPSHOT");
                break;
        }
    }
    else
    {
        appendStringInfoString(&cmd, " PHYSICAL RESERVE_WAL");
    }

    res = libpqrcv_PQexec(conn->streamConn, cmd.data);
    pfree(cmd.data);

    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        PQclear(res);
        ereport(ERROR,
                (errmsg("could not create replication slot \"%s\": %s",
                        slotname, pchomp(PQerrorMessage(conn->streamConn)))));
    }

    if (lsn)
        *lsn = DatumGetLSN(DirectFunctionCall1Coll(pg_lsn_in, InvalidOid,
                                                   CStringGetDatum(PQgetvalue(res, 0, 1))));

    if (!PQgetisnull(res, 0, 2))
        snapshot = pstrdup(PQgetvalue(res, 0, 2));
    else
        snapshot = NULL;

    PQclear(res);

    return snapshot;
}

/*
 * Return PID of remote backend process.
 */
static pid_t
libpqrcv_get_backend_pid(WalReceiverConn *conn)
{
    return PQbackendPID(conn->streamConn);
}

/*
 * Convert tuple query result to tuplestore.
 */
static void
libpqrcv_processTuples(PGresult *pgres, WalRcvExecResult *walres,
                       const int nRetTypes, const Oid *retTypes)
{
    int         tupn;
    int         coln;
    int         nfields = PQnfields(pgres);
    HeapTuple   tuple;
    AttInMetadata *attinmeta;
    MemoryContext rowcontext;
    MemoryContext oldcontext;

    /* Make sure we got expected number of fields. */
    if (nfields != nRetTypes)
        ereport(ERROR,
                (errmsg("invalid query response"),
                 errdetail("Expected %d fields, got %d fields.",
                           nRetTypes, nfields)));

    walres->tuplestore = tuplestore_begin_heap(true, false, work_mem);

    /* Create tuple descriptor corresponding to expected result. */
    walres->tupledesc = CreateTemplateTupleDesc(nRetTypes);
    for (coln = 0; coln < nRetTypes; coln++)
        TupleDescInitEntry(walres->tupledesc, (AttrNumber) coln + 1,
                           PQfname(pgres, coln), retTypes[coln], -1, 0);
    attinmeta = TupleDescGetAttInMetadata(walres->tupledesc);

    /* No point in doing more here if there were no tuples returned. */
    if (PQntuples(pgres) == 0)
        return;

    /* Create temporary context for local allocations. */
    rowcontext = AllocSetContextCreate(CurrentMemoryContext,
                                       "libpqrcv query result context",
                                       ALLOCSET_DEFAULT_SIZES);

    /* Process returned rows. */
    for (tupn = 0; tupn < PQntuples(pgres); tupn++)
    {
        char       *cstrs[MaxTupleAttributeNumber];

        ProcessWalRcvInterrupts();

        /* Do the allocations in temporary context. */
        oldcontext = MemoryContextSwitchTo(rowcontext);

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

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

        /* Clean up */
        MemoryContextSwitchTo(oldcontext);
        MemoryContextReset(rowcontext);
    }

    MemoryContextDelete(rowcontext);
}

/*
 * Public interface for sending generic queries (and commands).
 *
 * This can only be called from process connected to database.
 */
static WalRcvExecResult *
libpqrcv_exec(WalReceiverConn *conn, const char *query,
              const int nRetTypes, const Oid *retTypes)
{
    PGresult   *pgres = NULL;
    WalRcvExecResult *walres = palloc0(sizeof(WalRcvExecResult));

    if (MyDatabaseId == InvalidOid)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("the query interface requires a database connection")));

    pgres = libpqrcv_PQexec(conn->streamConn, query);

    switch (PQresultStatus(pgres))
    {
        case PGRES_SINGLE_TUPLE:
        case PGRES_TUPLES_OK:
            walres->status = WALRCV_OK_TUPLES;
            libpqrcv_processTuples(pgres, walres, nRetTypes, retTypes);
            break;

        case PGRES_COPY_IN:
            walres->status = WALRCV_OK_COPY_IN;
            break;

        case PGRES_COPY_OUT:
            walres->status = WALRCV_OK_COPY_OUT;
            break;

        case PGRES_COPY_BOTH:
            walres->status = WALRCV_OK_COPY_BOTH;
            break;

        case PGRES_COMMAND_OK:
            walres->status = WALRCV_OK_COMMAND;
            break;

            /* Empty query is considered error. */
        case PGRES_EMPTY_QUERY:
            walres->status = WALRCV_ERROR;
            walres->err = _("empty query");
            break;

        case PGRES_NONFATAL_ERROR:
        case PGRES_FATAL_ERROR:
        case PGRES_BAD_RESPONSE:
            walres->status = WALRCV_ERROR;
            walres->err = pchomp(PQerrorMessage(conn->streamConn));
            break;
    }

    PQclear(pgres);

    return walres;
}

/*
 * Given a List of strings, return it as single comma separated
 * string, quoting identifiers as needed.
 *
 * This is essentially the reverse of SplitIdentifierString.
 *
 * The caller should free the result.
 */
static char *
stringlist_to_identifierstr(PGconn *conn, List *strings)
{
    ListCell   *lc;
    StringInfoData res;
    bool        first = true;

    initStringInfo(&res);

    foreach(lc, strings)
    {
        char       *val = strVal(lfirst(lc));
        char       *val_escaped;

        if (first)
            first = false;
        else
            appendStringInfoChar(&res, ',');

        val_escaped = PQescapeIdentifier(conn, val, strlen(val));
        if (!val_escaped)
        {
            free(res.data);
            return NULL;
        }
        appendStringInfoString(&res, val_escaped);
        PQfreemem(val_escaped);
    }

    return res.data;
}