libpq-be-fe-helpers.h

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/*-------------------------------------------------------------------------
 *
 * libpq-be-fe-helpers.h
 *    Helper functions for using libpq in extensions
 *
 * Code built directly into the backend is not allowed to link to libpq
 * directly. Extension code is allowed to use libpq however. However, libpq
 * used in extensions has to be careful not to block inside libpq, otherwise
 * interrupts will not be processed, leading to issues like unresolvable
 * deadlocks. Backend code also needs to take care to acquire/release an
 * external fd for the connection, otherwise fd.c's accounting of fd's is
 * broken.
 *
 * This file provides helper functions to make it easier to comply with these
 * rules. It is a header only library as it needs to be linked into each
 * extension using libpq, and it seems too small to be worth adding a
 * dedicated static library for.
 *
 * TODO: For historical reasons the connections established here are not put
 * into non-blocking mode. That can lead to blocking even when only the async
 * libpq functions are used. This should be fixed.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/libpq/libpq-be-fe-helpers.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef LIBPQ_BE_FE_HELPERS_H
#define LIBPQ_BE_FE_HELPERS_H
 
/*
 * Despite the name, BUILDING_DLL is set only when building code directly part
 * of the backend. Which also is where libpq isn't allowed to be
 * used. Obviously this doesn't protect against libpq-fe.h getting included
 * otherwise, but perhaps still protects against a few mistakes...
 */
#ifdef BUILDING_DLL
#error "libpq may not be used code directly built into the backend"
#endif
 
#include "libpq-fe.h"
#include "miscadmin.h"
#include "storage/fd.h"
#include "storage/latch.h"
#include "utils/timestamp.h"
#include "utils/wait_event.h"
 
 
static inline void libpqsrv_connect_prepare(void);
static inline void libpqsrv_connect_internal(PGconn *conn, uint32 wait_event_info);
static inline PGresult *libpqsrv_get_result_last(PGconn *conn, uint32 wait_event_info);
static inline PGresult *libpqsrv_get_result(PGconn *conn, uint32 wait_event_info);
 
 
/*
 * PQconnectdb() wrapper that reserves a file descriptor and processes
 * interrupts during connection establishment.
 *
 * Throws an error if AcquireExternalFD() fails, but does not throw if
 * connection establishment itself fails. Callers need to use PQstatus() to
 * check if connection establishment succeeded.
 */
static inline PGconn *
libpqsrv_connect(const char *conninfo, uint32 wait_event_info)
{
    PGconn     *conn = NULL;
 
    libpqsrv_connect_prepare();
 
    conn = PQconnectStart(conninfo);
 
    libpqsrv_connect_internal(conn, wait_event_info);
 
    return conn;
}
 
/*
 * Like libpqsrv_connect(), except that this is a wrapper for
 * PQconnectdbParams().
  */
static inline PGconn *
libpqsrv_connect_params(const char *const *keywords,
                        const char *const *values,
                        int expand_dbname,
                        uint32 wait_event_info)
{
    PGconn     *conn = NULL;
 
    libpqsrv_connect_prepare();
 
    conn = PQconnectStartParams(keywords, values, expand_dbname);
 
    libpqsrv_connect_internal(conn, wait_event_info);
 
    return conn;
}
 
/*
 * PQfinish() wrapper that additionally releases the reserved file descriptor.
 *
 * It is allowed to call this with a NULL pgconn iff NULL was returned by
 * libpqsrv_connect*.
 */
static inline void
libpqsrv_disconnect(PGconn *conn)
{
    /*
     * If no connection was established, we haven't reserved an FD for it (or
     * already released it). This rule makes it easier to write PG_CATCH()
     * handlers for this facility's users.
     *
     * See also libpqsrv_connect_internal().
     */
    if (conn == NULL)
        return;
 
    ReleaseExternalFD();
    PQfinish(conn);
}
 
 
/* internal helper functions follow */
 
 
/*
 * Helper function for all connection establishment functions.
 */
static inline void
libpqsrv_connect_prepare(void)
{
    /*
     * We must obey fd.c's limit on non-virtual file descriptors.  Assume that
     * a PGconn represents one long-lived FD.  (Doing this here also ensures
     * that VFDs are closed if needed to make room.)
     */
    if (!AcquireExternalFD())
    {
#ifndef WIN32                   /* can't write #if within ereport() macro */
        ereport(ERROR,
                (errcode(ERRCODE_SQLCLIENT_UNABLE_TO_ESTABLISH_SQLCONNECTION),
                 errmsg("could not establish connection"),
                 errdetail("There are too many open files on the local server."),
                 errhint("Raise the server's \"max_files_per_process\" and/or \"ulimit -n\" limits.")));
#else
        ereport(ERROR,
                (errcode(ERRCODE_SQLCLIENT_UNABLE_TO_ESTABLISH_SQLCONNECTION),
                 errmsg("could not establish connection"),
                 errdetail("There are too many open files on the local server."),
                 errhint("Raise the server's \"max_files_per_process\" setting.")));
#endif
    }
}
 
/*
 * Helper function for all connection establishment functions.
 */
static inline void
libpqsrv_connect_internal(PGconn *conn, uint32 wait_event_info)
{
    /*
     * With conn == NULL libpqsrv_disconnect() wouldn't release the FD. So do
     * that here.
     */
    if (conn == NULL)
    {
        ReleaseExternalFD();
        return;
    }
 
    /*
     * Can't wait without a socket. Note that we don't want to close the libpq
     * connection yet, so callers can emit a useful error.
     */
    if (PQstatus(conn) == CONNECTION_BAD)
        return;
 
    /*
     * WaitLatchOrSocket() can conceivably fail, handle that case here instead
     * of requiring all callers to do so.
     */
    PG_TRY();
    {
        PostgresPollingStatusType status;
 
        /*
         * Poll connection until we have OK or FAILED status.
         *
         * Per spec for PQconnectPoll, first wait till socket is write-ready.
         */
        status = PGRES_POLLING_WRITING;
        while (status != PGRES_POLLING_OK && status != PGRES_POLLING_FAILED)
        {
            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) == 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),
                                   0,
                                   wait_event_info);
 
            /* Interrupted? */
            if (rc & WL_LATCH_SET)
            {
                ResetLatch(MyLatch);
                CHECK_FOR_INTERRUPTS();
            }
 
            /* If socket is ready, advance the libpq state machine */
            if (rc & io_flag)
                status = PQconnectPoll(conn);
        }
    }
    PG_CATCH();
    {
        /*
         * If an error is thrown here, the callers won't call
         * libpqsrv_disconnect() with a conn, so release resources
         * immediately.
         */
        ReleaseExternalFD();
        PQfinish(conn);
 
        PG_RE_THROW();
    }
    PG_END_TRY();
}
 
/*
 * PQexec() wrapper that processes interrupts.
 *
 * Unless PQsetnonblocking(conn, 1) is in effect, this can't process
 * interrupts while pushing the query text to the server.  Consider that
 * setting if query strings can be long relative to TCP buffer size.
 *
 * This has the preconditions of PQsendQuery(), not those of PQexec().  Most
 * notably, PQexec() would silently discard any prior query results.
 */
static inline PGresult *
libpqsrv_exec(PGconn *conn, const char *query, uint32 wait_event_info)
{
    if (!PQsendQuery(conn, query))
        return NULL;
    return libpqsrv_get_result_last(conn, wait_event_info);
}
 
/*
 * PQexecParams() wrapper that processes interrupts.
 *
 * See notes at libpqsrv_exec().
 */
static inline PGresult *
libpqsrv_exec_params(PGconn *conn,
                     const char *command,
                     int nParams,
                     const Oid *paramTypes,
                     const char *const *paramValues,
                     const int *paramLengths,
                     const int *paramFormats,
                     int resultFormat,
                     uint32 wait_event_info)
{
    if (!PQsendQueryParams(conn, command, nParams, paramTypes, paramValues,
                           paramLengths, paramFormats, resultFormat))
        return NULL;
    return libpqsrv_get_result_last(conn, wait_event_info);
}
 
/*
 * Like PQexec(), loop over PQgetResult() until it returns NULL or another
 * terminal state.  Return the last non-NULL result or the terminal state.
 */
static inline PGresult *
libpqsrv_get_result_last(PGconn *conn, uint32 wait_event_info)
{
    PGresult   *volatile lastResult = NULL;
 
    /* In what follows, do not leak any PGresults on an error. */
    PG_TRY();
    {
        for (;;)
        {
            /* Wait for, and collect, the next PGresult. */
            PGresult   *result;
 
            result = libpqsrv_get_result(conn, wait_event_info);
            if (result == NULL)
                break;          /* query is complete, or failure */
 
            /*
             * Emulate PQexec()'s behavior of returning the last result when
             * there are many.
             */
            PQclear(lastResult);
            lastResult = result;
 
            if (PQresultStatus(lastResult) == PGRES_COPY_IN ||
                PQresultStatus(lastResult) == PGRES_COPY_OUT ||
                PQresultStatus(lastResult) == PGRES_COPY_BOTH ||
                PQstatus(conn) == CONNECTION_BAD)
                break;
        }
    }
    PG_CATCH();
    {
        PQclear(lastResult);
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    return lastResult;
}
 
/*
 * Perform the equivalent of PQgetResult(), but watch for interrupts.
 */
static inline PGresult *
libpqsrv_get_result(PGconn *conn, uint32 wait_event_info)
{
    /*
     * Collect data until PQgetResult is ready to get the result without
     * blocking.
     */
    while (PQisBusy(conn))
    {
        int         rc;
 
        rc = WaitLatchOrSocket(MyLatch,
                               WL_EXIT_ON_PM_DEATH | WL_LATCH_SET |
                               WL_SOCKET_READABLE,
                               PQsocket(conn),
                               0,
                               wait_event_info);
 
        /* Interrupted? */
        if (rc & WL_LATCH_SET)
        {
            ResetLatch(MyLatch);
            CHECK_FOR_INTERRUPTS();
        }
 
        /* Consume whatever data is available from the socket */
        if (PQconsumeInput(conn) == 0)
        {
            /* trouble; expect PQgetResult() to return NULL */
            break;
        }
    }
 
    /* Now we can collect and return the next PGresult */
    return PQgetResult(conn);
}
 
/*
 * Submit a cancel request to the given connection, waiting only until
 * the given time.
 *
 * We sleep interruptibly until we receive confirmation that the cancel
 * request has been accepted, and if it is, return NULL; if the cancel
 * request fails, return an error message string (which is not to be
 * freed).
 *
 * For other problems (to wit: OOM when strdup'ing an error message from
 * libpq), this function can ereport(ERROR).
 *
 * Note: this function leaks a string's worth of memory when reporting
 * libpq errors.  Make sure to call it in a transient memory context.
 */
static inline const char *
libpqsrv_cancel(PGconn *conn, TimestampTz endtime)
{
    PGcancelConn *cancel_conn;
    const char *error = NULL;
 
    cancel_conn = PQcancelCreate(conn);
    if (cancel_conn == NULL)
        return "out of memory";
 
    /* In what follows, do not leak any PGcancelConn on any errors. */
 
    PG_TRY();
    {
        if (!PQcancelStart(cancel_conn))
        {
            error = pchomp(PQcancelErrorMessage(cancel_conn));
            goto exit;
        }
 
        for (;;)
        {
            PostgresPollingStatusType pollres;
            TimestampTz now;
            long        cur_timeout;
            int         waitEvents = WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH;
 
            pollres = PQcancelPoll(cancel_conn);
            if (pollres == PGRES_POLLING_OK)
                break;          /* success! */
 
            /* If timeout has expired, give up, else get sleep time. */
            now = GetCurrentTimestamp();
            cur_timeout = TimestampDifferenceMilliseconds(now, endtime);
            if (cur_timeout <= 0)
            {
                error = "cancel request timed out";
                break;
            }
 
            switch (pollres)
            {
                case PGRES_POLLING_READING:
                    waitEvents |= WL_SOCKET_READABLE;
                    break;
                case PGRES_POLLING_WRITING:
                    waitEvents |= WL_SOCKET_WRITEABLE;
                    break;
                default:
                    error = pchomp(PQcancelErrorMessage(cancel_conn));
                    goto exit;
            }
 
            /* Sleep until there's something to do */
            WaitLatchOrSocket(MyLatch, waitEvents, PQcancelSocket(cancel_conn),
                              cur_timeout, PG_WAIT_CLIENT);
 
            ResetLatch(MyLatch);
 
            CHECK_FOR_INTERRUPTS();
        }
exit:   ;
    }
    PG_FINALLY();
    {
        PQcancelFinish(cancel_conn);
    }
    PG_END_TRY();
 
    return error;
}
 
#endif                          /* LIBPQ_BE_FE_HELPERS_H */