fe-cancel.c

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/*-------------------------------------------------------------------------
 *
 * fe-cancel.c
 *    functions related to query cancellation
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/interfaces/libpq/fe-cancel.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres_fe.h"
 
#include <unistd.h>
 
#include "libpq-fe.h"
#include "libpq-int.h"
#include "port/pg_bswap.h"
 
 
/*
 * pg_cancel_conn (backing struct for PGcancelConn) is a wrapper around a
 * PGconn to send cancellations using PQcancelBlocking and PQcancelStart.
 * This isn't just a typedef because we want the compiler to complain when a
 * PGconn is passed to a function that expects a PGcancelConn, and vice versa.
 */
struct pg_cancel_conn
{
    PGconn      conn;
};
 
/*
 * pg_cancel (backing struct for PGcancel) stores all data necessary to send a
 * cancel request.
 */
struct pg_cancel
{
    SockAddr    raddr;          /* Remote address */
    int         be_pid;         /* PID of to-be-canceled backend */
    int         pgtcp_user_timeout; /* tcp user timeout */
    int         keepalives;     /* use TCP keepalives? */
    int         keepalives_idle;    /* time between TCP keepalives */
    int         keepalives_interval;    /* time between TCP keepalive
                                         * retransmits */
    int         keepalives_count;   /* maximum number of TCP keepalive
                                     * retransmits */
 
    /* Pre-constructed cancel request packet starts here */
    int32       cancel_pkt_len; /* in network byte order */
    char        cancel_req[FLEXIBLE_ARRAY_MEMBER];  /* CancelRequestPacket */
};
 
 
/*
 *      PQcancelCreate
 *
 * Create and return a PGcancelConn, which can be used to securely cancel a
 * query on the given connection.
 *
 * This requires either following the non-blocking flow through
 * PQcancelStart() and PQcancelPoll(), or the blocking PQcancelBlocking().
 */
PGcancelConn *
PQcancelCreate(PGconn *conn)
{
    PGconn     *cancelConn = pqMakeEmptyPGconn();
    pg_conn_host originalHost;
 
    if (cancelConn == NULL)
        return NULL;
 
    /* Check we have an open connection */
    if (!conn)
    {
        libpq_append_conn_error(cancelConn, "connection pointer is NULL");
        return (PGcancelConn *) cancelConn;
    }
 
    if (conn->sock == PGINVALID_SOCKET)
    {
        libpq_append_conn_error(cancelConn, "connection not open");
        return (PGcancelConn *) cancelConn;
    }
 
    /* Check that we have received a cancellation key */
    if (conn->be_cancel_key_len == 0)
    {
        libpq_append_conn_error(cancelConn, "no cancellation key received");
        return (PGcancelConn *) cancelConn;
    }
 
    /*
     * Indicate that this connection is used to send a cancellation
     */
    cancelConn->cancelRequest = true;
 
    if (!pqCopyPGconn(conn, cancelConn))
        return (PGcancelConn *) cancelConn;
 
    /*
     * Compute derived options
     */
    if (!pqConnectOptions2(cancelConn))
        return (PGcancelConn *) cancelConn;
 
    /*
     * Copy cancellation token data from the original connection
     */
    cancelConn->be_pid = conn->be_pid;
    if (conn->be_cancel_key != NULL)
    {
        cancelConn->be_cancel_key = malloc(conn->be_cancel_key_len);
        if (cancelConn->be_cancel_key == NULL)
            goto oom_error;
        memcpy(cancelConn->be_cancel_key, conn->be_cancel_key, conn->be_cancel_key_len);
    }
    cancelConn->be_cancel_key_len = conn->be_cancel_key_len;
    cancelConn->pversion = conn->pversion;
 
    /*
     * Cancel requests should not iterate over all possible hosts. The request
     * needs to be sent to the exact host and address that the original
     * connection used. So we manually create the host and address arrays with
     * a single element after freeing the host array that we generated from
     * the connection options.
     */
    pqReleaseConnHosts(cancelConn);
    cancelConn->nconnhost = 1;
    cancelConn->naddr = 1;
 
    cancelConn->connhost = calloc(cancelConn->nconnhost, sizeof(pg_conn_host));
    if (!cancelConn->connhost)
        goto oom_error;
 
    originalHost = conn->connhost[conn->whichhost];
    cancelConn->connhost[0].type = originalHost.type;
    if (originalHost.host)
    {
        cancelConn->connhost[0].host = strdup(originalHost.host);
        if (!cancelConn->connhost[0].host)
            goto oom_error;
    }
    if (originalHost.hostaddr)
    {
        cancelConn->connhost[0].hostaddr = strdup(originalHost.hostaddr);
        if (!cancelConn->connhost[0].hostaddr)
            goto oom_error;
    }
    if (originalHost.port)
    {
        cancelConn->connhost[0].port = strdup(originalHost.port);
        if (!cancelConn->connhost[0].port)
            goto oom_error;
    }
    if (originalHost.password)
    {
        cancelConn->connhost[0].password = strdup(originalHost.password);
        if (!cancelConn->connhost[0].password)
            goto oom_error;
    }
 
    cancelConn->addr = calloc(cancelConn->naddr, sizeof(AddrInfo));
    if (!cancelConn->addr)
        goto oom_error;
 
    cancelConn->addr[0].addr = conn->raddr;
    cancelConn->addr[0].family = conn->raddr.addr.ss_family;
 
    cancelConn->status = CONNECTION_ALLOCATED;
    return (PGcancelConn *) cancelConn;
 
oom_error:
    cancelConn->status = CONNECTION_BAD;
    libpq_append_conn_error(cancelConn, "out of memory");
    return (PGcancelConn *) cancelConn;
}
 
 
/*
 *      PQcancelBlocking
 *
 * Send a cancellation request in a blocking fashion.
 * Returns 1 if successful 0 if not.
 */
int
PQcancelBlocking(PGcancelConn *cancelConn)
{
    if (!PQcancelStart(cancelConn))
        return 0;
    return pqConnectDBComplete(&cancelConn->conn);
}
 
/*
 *      PQcancelStart
 *
 * Starts sending a cancellation request in a non-blocking fashion. Returns
 * 1 if successful 0 if not.
 */
int
PQcancelStart(PGcancelConn *cancelConn)
{
    if (!cancelConn || cancelConn->conn.status == CONNECTION_BAD)
        return 0;
 
    if (cancelConn->conn.status != CONNECTION_ALLOCATED)
    {
        libpq_append_conn_error(&cancelConn->conn,
                                "cancel request is already being sent on this connection");
        cancelConn->conn.status = CONNECTION_BAD;
        return 0;
    }
 
    return pqConnectDBStart(&cancelConn->conn);
}
 
/*
 *      PQcancelPoll
 *
 * Poll a cancel connection. For usage details see PQconnectPoll.
 */
PostgresPollingStatusType
PQcancelPoll(PGcancelConn *cancelConn)
{
    PGconn     *conn = &cancelConn->conn;
    int         n;
 
    /*
     * We leave most of the connection establishment to PQconnectPoll, since
     * it's very similar to normal connection establishment. But once we get
     * to the CONNECTION_AWAITING_RESPONSE we need to start doing our own
     * thing.
     */
    if (conn->status != CONNECTION_AWAITING_RESPONSE)
    {
        return PQconnectPoll(conn);
    }
 
    /*
     * At this point we are waiting on the server to close the connection,
     * which is its way of communicating that the cancel has been handled.
     */
 
    n = pqReadData(conn);
 
    if (n == 0)
        return PGRES_POLLING_READING;
 
#ifndef WIN32
 
    /*
     * If we receive an error report it, but only if errno is non-zero.
     * Otherwise we assume it's an EOF, which is what we expect from the
     * server.
     *
     * We skip this for Windows, because Windows is a bit special in its EOF
     * behaviour for TCP. Sometimes it will error with an ECONNRESET when
     * there is a clean connection closure. See these threads for details:
     * https://www.postgresql.org/message-id/flat/90b34057-4176-7bb0-0dbb-9822a5f6425b%40greiz-reinsdorf.de
     *
     * https://www.postgresql.org/message-id/flat/CA%2BhUKG%2BOeoETZQ%3DQw5Ub5h3tmwQhBmDA%3DnuNO3KG%3DzWfUypFAw%40mail.gmail.com
     *
     * PQcancel ignores such errors and reports success for the cancellation
     * anyway, so even if this is not always correct we do the same here.
     */
    if (n < 0 && errno != 0)
    {
        conn->status = CONNECTION_BAD;
        return PGRES_POLLING_FAILED;
    }
#endif
 
    /*
     * We don't expect any data, only connection closure. So if we strangely
     * do receive some data we consider that an error.
     */
    if (n > 0)
    {
        libpq_append_conn_error(conn, "unexpected response from server");
        conn->status = CONNECTION_BAD;
        return PGRES_POLLING_FAILED;
    }
 
    /*
     * Getting here means that we received an EOF, which is what we were
     * expecting -- the cancel request has completed.
     */
    cancelConn->conn.status = CONNECTION_OK;
    resetPQExpBuffer(&conn->errorMessage);
    return PGRES_POLLING_OK;
}
 
/*
 *      PQcancelStatus
 *
 * Get the status of a cancel connection.
 */
ConnStatusType
PQcancelStatus(const PGcancelConn *cancelConn)
{
    return PQstatus(&cancelConn->conn);
}
 
/*
 *      PQcancelSocket
 *
 * Get the socket of the cancel connection.
 */
int
PQcancelSocket(const PGcancelConn *cancelConn)
{
    return PQsocket(&cancelConn->conn);
}
 
/*
 *      PQcancelErrorMessage
 *
 * Returns the error message most recently generated by an operation on the
 * cancel connection.
 */
char *
PQcancelErrorMessage(const PGcancelConn *cancelConn)
{
    return PQerrorMessage(&cancelConn->conn);
}
 
/*
 *      PQcancelReset
 *
 * Resets the cancel connection, so it can be reused to send a new cancel
 * request.
 */
void
PQcancelReset(PGcancelConn *cancelConn)
{
    pqClosePGconn(&cancelConn->conn);
    cancelConn->conn.status = CONNECTION_ALLOCATED;
    cancelConn->conn.whichhost = 0;
    cancelConn->conn.whichaddr = 0;
    cancelConn->conn.try_next_host = false;
    cancelConn->conn.try_next_addr = false;
}
 
/*
 *      PQcancelFinish
 *
 * Closes and frees the cancel connection.
 */
void
PQcancelFinish(PGcancelConn *cancelConn)
{
    PQfinish(&cancelConn->conn);
}
 
/*
 * PQgetCancel: get a PGcancel structure corresponding to a connection.
 *
 * A copy is needed to be able to cancel a running query from a different
 * thread. If the same structure is used all structure members would have
 * to be individually locked (if the entire structure was locked, it would
 * be impossible to cancel a synchronous query because the structure would
 * have to stay locked for the duration of the query).
 */
PGcancel *
PQgetCancel(PGconn *conn)
{
    PGcancel   *cancel;
    int         cancel_req_len;
    CancelRequestPacket *req;
 
    if (!conn)
        return NULL;
 
    if (conn->sock == PGINVALID_SOCKET)
        return NULL;
 
    /* Check that we have received a cancellation key */
    if (conn->be_cancel_key_len == 0)
    {
        /*
         * In case there is no cancel key, return an all-zero PGcancel object.
         * Actually calling PQcancel on this will fail, but we allow creating
         * the PGcancel object anyway. Arguably it would be better return NULL
         * to indicate that cancellation is not possible, but there'd be no
         * way for the caller to distinguish "out of memory" from "server did
         * not send a cancel key". Also, this is how PGgetCancel() has always
         * behaved, and if we changed it, some clients would stop working
         * altogether with servers that don't support cancellation. (The
         * modern PQcancelCreate() function returns a failed connection object
         * instead.)
         *
         * The returned dummy object has cancel_pkt_len == 0; we check for
         * that in PQcancel() to identify it as a dummy.
         */
        return calloc(1, sizeof(PGcancel));
    }
 
    cancel_req_len = offsetof(CancelRequestPacket, cancelAuthCode) + conn->be_cancel_key_len;
    cancel = malloc(offsetof(PGcancel, cancel_req) + cancel_req_len);
    if (cancel == NULL)
        return NULL;
 
    memcpy(&cancel->raddr, &conn->raddr, sizeof(SockAddr));
 
    /* We use -1 to indicate an unset connection option */
    cancel->pgtcp_user_timeout = -1;
    cancel->keepalives = -1;
    cancel->keepalives_idle = -1;
    cancel->keepalives_interval = -1;
    cancel->keepalives_count = -1;
    if (conn->pgtcp_user_timeout != NULL)
    {
        if (!pqParseIntParam(conn->pgtcp_user_timeout,
                             &cancel->pgtcp_user_timeout,
                             conn, "tcp_user_timeout"))
            goto fail;
    }
    if (conn->keepalives != NULL)
    {
        if (!pqParseIntParam(conn->keepalives,
                             &cancel->keepalives,
                             conn, "keepalives"))
            goto fail;
    }
    if (conn->keepalives_idle != NULL)
    {
        if (!pqParseIntParam(conn->keepalives_idle,
                             &cancel->keepalives_idle,
                             conn, "keepalives_idle"))
            goto fail;
    }
    if (conn->keepalives_interval != NULL)
    {
        if (!pqParseIntParam(conn->keepalives_interval,
                             &cancel->keepalives_interval,
                             conn, "keepalives_interval"))
            goto fail;
    }
    if (conn->keepalives_count != NULL)
    {
        if (!pqParseIntParam(conn->keepalives_count,
                             &cancel->keepalives_count,
                             conn, "keepalives_count"))
            goto fail;
    }
 
    req = (CancelRequestPacket *) &cancel->cancel_req;
    req->cancelRequestCode = pg_hton32(CANCEL_REQUEST_CODE);
    req->backendPID = pg_hton32(conn->be_pid);
    memcpy(req->cancelAuthCode, conn->be_cancel_key, conn->be_cancel_key_len);
    /* include the length field itself in the length */
    cancel->cancel_pkt_len = pg_hton32(cancel_req_len + 4);
 
    return cancel;
 
fail:
    free(cancel);
    return NULL;
}
 
/*
 * PQsendCancelRequest
 *   Submit a CancelRequest message, but don't wait for it to finish
 *
 * Returns: 1 if successfully submitted
 *          0 if error (conn->errorMessage is set)
 */
int
PQsendCancelRequest(PGconn *cancelConn)
{
    CancelRequestPacket req;
 
    /* Start the message. */
    if (pqPutMsgStart(0, cancelConn))
        return STATUS_ERROR;
 
    /* Send the message body. */
    memset(&req, 0, offsetof(CancelRequestPacket, cancelAuthCode));
    req.cancelRequestCode = pg_hton32(CANCEL_REQUEST_CODE);
    req.backendPID = pg_hton32(cancelConn->be_pid);
    if (pqPutnchar(&req, offsetof(CancelRequestPacket, cancelAuthCode), cancelConn))
        return STATUS_ERROR;
    if (pqPutnchar(cancelConn->be_cancel_key, cancelConn->be_cancel_key_len, cancelConn))
        return STATUS_ERROR;
 
    /* Finish the message. */
    if (pqPutMsgEnd(cancelConn))
        return STATUS_ERROR;
 
    /* Flush to ensure backend gets it. */
    if (pqFlush(cancelConn))
        return STATUS_ERROR;
 
    return STATUS_OK;
}
 
/* PQfreeCancel: free a cancel structure */
void
PQfreeCancel(PGcancel *cancel)
{
    free(cancel);
}
 
 
/*
 * Sets an integer socket option on a TCP socket, if the provided value is
 * not negative.  Returns false if setsockopt fails for some reason.
 *
 * CAUTION: This needs to be signal safe, since it's used by PQcancel.
 */
#if defined(TCP_USER_TIMEOUT) || !defined(WIN32)
static bool
optional_setsockopt(int fd, int protoid, int optid, int value)
{
    if (value < 0)
        return true;
    if (setsockopt(fd, protoid, optid, (char *) &value, sizeof(value)) < 0)
        return false;
    return true;
}
#endif
 
 
/*
 * PQcancel: old, non-encrypted, but signal-safe way of requesting query cancel
 *
 * The return value is true if the cancel request was successfully
 * dispatched, false if not (in which case an error message is available).
 * Note: successful dispatch is no guarantee that there will be any effect at
 * the backend.  The application must read the operation result as usual.
 *
 * On failure, an error message is stored in *errbuf, which must be of size
 * errbufsize (recommended size is 256 bytes).  *errbuf is not changed on
 * success return.
 *
 * CAUTION: we want this routine to be safely callable from a signal handler
 * (for example, an application might want to call it in a SIGINT handler).
 * This means we cannot use any C library routine that might be non-reentrant.
 * malloc/free are often non-reentrant, and anything that might call them is
 * just as dangerous.  We avoid sprintf here for that reason.  Building up
 * error messages with strcpy/strcat is tedious but should be quite safe.
 * We also save/restore errno in case the signal handler support doesn't.
 */
int
PQcancel(PGcancel *cancel, char *errbuf, int errbufsize)
{
    int         save_errno = SOCK_ERRNO;
    pgsocket    tmpsock = PGINVALID_SOCKET;
    int         maxlen;
    char        recvbuf;
    int         cancel_pkt_len;
 
    if (!cancel)
    {
        strlcpy(errbuf, "PQcancel() -- no cancel object supplied", errbufsize);
        /* strlcpy probably doesn't change errno, but be paranoid */
        SOCK_ERRNO_SET(save_errno);
        return false;
    }
 
    if (cancel->cancel_pkt_len == 0)
    {
        /* This is a dummy PGcancel object, see PQgetCancel */
        strlcpy(errbuf, "PQcancel() -- no cancellation key received", errbufsize);
        /* strlcpy probably doesn't change errno, but be paranoid */
        SOCK_ERRNO_SET(save_errno);
        return false;
    }
 
    /*
     * We need to open a temporary connection to the postmaster. Do this with
     * only kernel calls.
     */
    if ((tmpsock = socket(cancel->raddr.addr.ss_family, SOCK_STREAM, 0)) == PGINVALID_SOCKET)
    {
        strlcpy(errbuf, "PQcancel() -- socket() failed: ", errbufsize);
        goto cancel_errReturn;
    }
 
    /*
     * Since this connection will only be used to send a single packet of
     * data, we don't need NODELAY.  We also don't set the socket to
     * nonblocking mode, because the API definition of PQcancel requires the
     * cancel to be sent in a blocking way.
     *
     * We do set socket options related to keepalives and other TCP timeouts.
     * This ensures that this function does not block indefinitely when
     * reasonable keepalive and timeout settings have been provided.
     */
    if (cancel->raddr.addr.ss_family != AF_UNIX &&
        cancel->keepalives != 0)
    {
#ifndef WIN32
        if (!optional_setsockopt(tmpsock, SOL_SOCKET, SO_KEEPALIVE, 1))
        {
            strlcpy(errbuf, "PQcancel() -- setsockopt(SO_KEEPALIVE) failed: ", errbufsize);
            goto cancel_errReturn;
        }
 
#ifdef PG_TCP_KEEPALIVE_IDLE
        if (!optional_setsockopt(tmpsock, IPPROTO_TCP, PG_TCP_KEEPALIVE_IDLE,
                                 cancel->keepalives_idle))
        {
            strlcpy(errbuf, "PQcancel() -- setsockopt(" PG_TCP_KEEPALIVE_IDLE_STR ") failed: ", errbufsize);
            goto cancel_errReturn;
        }
#endif
 
#ifdef TCP_KEEPINTVL
        if (!optional_setsockopt(tmpsock, IPPROTO_TCP, TCP_KEEPINTVL,
                                 cancel->keepalives_interval))
        {
            strlcpy(errbuf, "PQcancel() -- setsockopt(TCP_KEEPINTVL) failed: ", errbufsize);
            goto cancel_errReturn;
        }
#endif
 
#ifdef TCP_KEEPCNT
        if (!optional_setsockopt(tmpsock, IPPROTO_TCP, TCP_KEEPCNT,
                                 cancel->keepalives_count))
        {
            strlcpy(errbuf, "PQcancel() -- setsockopt(TCP_KEEPCNT) failed: ", errbufsize);
            goto cancel_errReturn;
        }
#endif
 
#else                           /* WIN32 */
 
#ifdef SIO_KEEPALIVE_VALS
        if (!pqSetKeepalivesWin32(tmpsock,
                                  cancel->keepalives_idle,
                                  cancel->keepalives_interval))
        {
            strlcpy(errbuf, "PQcancel() -- WSAIoctl(SIO_KEEPALIVE_VALS) failed: ", errbufsize);
            goto cancel_errReturn;
        }
#endif                          /* SIO_KEEPALIVE_VALS */
#endif                          /* WIN32 */
 
        /* TCP_USER_TIMEOUT works the same way on Unix and Windows */
#ifdef TCP_USER_TIMEOUT
        if (!optional_setsockopt(tmpsock, IPPROTO_TCP, TCP_USER_TIMEOUT,
                                 cancel->pgtcp_user_timeout))
        {
            strlcpy(errbuf, "PQcancel() -- setsockopt(TCP_USER_TIMEOUT) failed: ", errbufsize);
            goto cancel_errReturn;
        }
#endif
    }
 
retry3:
    if (connect(tmpsock, (struct sockaddr *) &cancel->raddr.addr,
                cancel->raddr.salen) < 0)
    {
        if (SOCK_ERRNO == EINTR)
            /* Interrupted system call - we'll just try again */
            goto retry3;
        strlcpy(errbuf, "PQcancel() -- connect() failed: ", errbufsize);
        goto cancel_errReturn;
    }
 
    cancel_pkt_len = pg_ntoh32(cancel->cancel_pkt_len);
 
retry4:
 
    /*
     * Send the cancel request packet. It starts with the message length at
     * cancel_pkt_len, followed by the actual packet.
     */
    if (send(tmpsock, (char *) &cancel->cancel_pkt_len, cancel_pkt_len, 0) != cancel_pkt_len)
    {
        if (SOCK_ERRNO == EINTR)
            /* Interrupted system call - we'll just try again */
            goto retry4;
        strlcpy(errbuf, "PQcancel() -- send() failed: ", errbufsize);
        goto cancel_errReturn;
    }
 
    /*
     * Wait for the postmaster to close the connection, which indicates that
     * it's processed the request.  Without this delay, we might issue another
     * command only to find that our cancel zaps that command instead of the
     * one we thought we were canceling.  Note we don't actually expect this
     * read to obtain any data, we are just waiting for EOF to be signaled.
     */
retry5:
    if (recv(tmpsock, &recvbuf, 1, 0) < 0)
    {
        if (SOCK_ERRNO == EINTR)
            /* Interrupted system call - we'll just try again */
            goto retry5;
        /* we ignore other error conditions */
    }
 
    /* All done */
    closesocket(tmpsock);
    SOCK_ERRNO_SET(save_errno);
    return true;
 
cancel_errReturn:
 
    /*
     * Make sure we don't overflow the error buffer. Leave space for the \n at
     * the end, and for the terminating zero.
     */
    maxlen = errbufsize - strlen(errbuf) - 2;
    if (maxlen >= 0)
    {
        /*
         * We can't invoke strerror here, since it's not signal-safe.  Settle
         * for printing the decimal value of errno.  Even that has to be done
         * the hard way.
         */
        int         val = SOCK_ERRNO;
        char        buf[32];
        char       *bufp;
 
        bufp = buf + sizeof(buf) - 1;
        *bufp = '\0';
        do
        {
            *(--bufp) = (val % 10) + '0';
            val /= 10;
        } while (val > 0);
        bufp -= 6;
        memcpy(bufp, "error ", 6);
        strncat(errbuf, bufp, maxlen);
        strcat(errbuf, "\n");
    }
    if (tmpsock != PGINVALID_SOCKET)
        closesocket(tmpsock);
    SOCK_ERRNO_SET(save_errno);
    return false;
}
 
/*
 * PQrequestCancel: old, not thread-safe function for requesting query cancel
 *
 * Returns true if able to send the cancel request, false if not.
 *
 * On failure, the error message is saved in conn->errorMessage; this means
 * that this can't be used when there might be other active operations on
 * the connection object.
 *
 * NOTE: error messages will be cut off at the current size of the
 * error message buffer, since we dare not try to expand conn->errorMessage!
 */
int
PQrequestCancel(PGconn *conn)
{
    int         r;
    PGcancel   *cancel;
 
    /* Check we have an open connection */
    if (!conn)
        return false;
 
    if (conn->sock == PGINVALID_SOCKET)
    {
        strlcpy(conn->errorMessage.data,
                "PQrequestCancel() -- connection is not open\n",
                conn->errorMessage.maxlen);
        conn->errorMessage.len = strlen(conn->errorMessage.data);
        conn->errorReported = 0;
 
        return false;
    }
 
    cancel = PQgetCancel(conn);
    if (cancel)
    {
        r = PQcancel(cancel, conn->errorMessage.data,
                     conn->errorMessage.maxlen);
        PQfreeCancel(cancel);
    }
    else
    {
        strlcpy(conn->errorMessage.data, "out of memory",
                conn->errorMessage.maxlen);
        r = false;
    }
 
    if (!r)
    {
        conn->errorMessage.len = strlen(conn->errorMessage.data);
        conn->errorReported = 0;
    }
 
    return r;
}