receivelog.c

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/*-------------------------------------------------------------------------
 *
 * receivelog.c - receive WAL files using the streaming
 *                replication protocol.
 *
 * Author: Magnus Hagander <magnus@hagander.net>
 *
 * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *        src/bin/pg_basebackup/receivelog.c
 *-------------------------------------------------------------------------
 */
 
#include "postgres_fe.h"
 
#include <sys/select.h>
#include <sys/stat.h>
#include <unistd.h>
 
#include "access/xlog_internal.h"
#include "common/file_utils.h"
#include "common/logging.h"
#include "libpq-fe.h"
#include "receivelog.h"
#include "streamutil.h"
 
/* currently open WAL file */
static Walfile *walfile = NULL;
static bool reportFlushPosition = false;
static XLogRecPtr lastFlushPosition = InvalidXLogRecPtr;
 
static bool still_sending = true;   /* feedback still needs to be sent? */
 
static PGresult *HandleCopyStream(PGconn *conn, StreamCtl *stream,
                                  XLogRecPtr *stoppos);
static int  CopyStreamPoll(PGconn *conn, long timeout_ms, pgsocket stop_socket);
static int  CopyStreamReceive(PGconn *conn, long timeout, pgsocket stop_socket,
                              char **buffer);
static bool ProcessKeepaliveMsg(PGconn *conn, StreamCtl *stream, char *copybuf,
                                int len, XLogRecPtr blockpos, TimestampTz *last_status);
static bool ProcessXLogDataMsg(PGconn *conn, StreamCtl *stream, char *copybuf, int len,
                               XLogRecPtr *blockpos);
static PGresult *HandleEndOfCopyStream(PGconn *conn, StreamCtl *stream, char *copybuf,
                                       XLogRecPtr blockpos, XLogRecPtr *stoppos);
static bool CheckCopyStreamStop(PGconn *conn, StreamCtl *stream, XLogRecPtr blockpos);
static long CalculateCopyStreamSleeptime(TimestampTz now, int standby_message_timeout,
                                         TimestampTz last_status);
 
static bool ReadEndOfStreamingResult(PGresult *res, XLogRecPtr *startpos,
                                     uint32 *timeline);
 
static bool
mark_file_as_archived(StreamCtl *stream, const char *fname)
{
    Walfile    *f;
    static char tmppath[MAXPGPATH];
 
    snprintf(tmppath, sizeof(tmppath), "archive_status/%s.done",
             fname);
 
    f = stream->walmethod->ops->open_for_write(stream->walmethod, tmppath,
                                               NULL, 0);
    if (f == NULL)
    {
        pg_log_error("could not create archive status file \"%s\": %s",
                     tmppath, GetLastWalMethodError(stream->walmethod));
        return false;
    }
 
    if (stream->walmethod->ops->close(f, CLOSE_NORMAL) != 0)
    {
        pg_log_error("could not close archive status file \"%s\": %s",
                     tmppath, GetLastWalMethodError(stream->walmethod));
        return false;
    }
 
    return true;
}
 
/*
 * Open a new WAL file in the specified directory.
 *
 * Returns true if OK; on failure, returns false after printing an error msg.
 * On success, 'walfile' is set to the opened WAL file.
 *
 * The file will be padded to 16Mb with zeroes.
 */
static bool
open_walfile(StreamCtl *stream, XLogRecPtr startpoint)
{
    Walfile    *f;
    char       *fn;
    ssize_t     size;
    XLogSegNo   segno;
    char        walfile_name[MAXPGPATH];
 
    XLByteToSeg(startpoint, segno, WalSegSz);
    XLogFileName(walfile_name, stream->timeline, segno, WalSegSz);
 
    /* Note that this considers the compression used if necessary */
    fn = stream->walmethod->ops->get_file_name(stream->walmethod,
                                               walfile_name,
                                               stream->partial_suffix);
 
    /*
     * When streaming to files, if an existing file exists we verify that it's
     * either empty (just created), or a complete WalSegSz segment (in which
     * case it has been created and padded). Anything else indicates a corrupt
     * file. Compressed files have no need for padding, so just ignore this
     * case.
     *
     * When streaming to tar, no file with this name will exist before, so we
     * never have to verify a size.
     */
    if (stream->walmethod->compression_algorithm == PG_COMPRESSION_NONE &&
        stream->walmethod->ops->existsfile(stream->walmethod, fn))
    {
        size = stream->walmethod->ops->get_file_size(stream->walmethod, fn);
        if (size < 0)
        {
            pg_log_error("could not get size of write-ahead log file \"%s\": %s",
                         fn, GetLastWalMethodError(stream->walmethod));
            pg_free(fn);
            return false;
        }
        if (size == WalSegSz)
        {
            /* Already padded file. Open it for use */
            f = stream->walmethod->ops->open_for_write(stream->walmethod, walfile_name, stream->partial_suffix, 0);
            if (f == NULL)
            {
                pg_log_error("could not open existing write-ahead log file \"%s\": %s",
                             fn, GetLastWalMethodError(stream->walmethod));
                pg_free(fn);
                return false;
            }
 
            /* fsync file in case of a previous crash */
            if (stream->walmethod->ops->sync(f) != 0)
            {
                pg_log_error("could not fsync existing write-ahead log file \"%s\": %s",
                             fn, GetLastWalMethodError(stream->walmethod));
                stream->walmethod->ops->close(f, CLOSE_UNLINK);
                exit(1);
            }
 
            walfile = f;
            pg_free(fn);
            return true;
        }
        if (size != 0)
        {
            /* if write didn't set errno, assume problem is no disk space */
            if (errno == 0)
                errno = ENOSPC;
            pg_log_error(ngettext("write-ahead log file \"%s\" has %zd byte, should be 0 or %d",
                                  "write-ahead log file \"%s\" has %zd bytes, should be 0 or %d",
                                  size),
                         fn, size, WalSegSz);
            pg_free(fn);
            return false;
        }
        /* File existed and was empty, so fall through and open */
    }
 
    /* No file existed, so create one */
 
    f = stream->walmethod->ops->open_for_write(stream->walmethod,
                                               walfile_name,
                                               stream->partial_suffix,
                                               WalSegSz);
    if (f == NULL)
    {
        pg_log_error("could not open write-ahead log file \"%s\": %s",
                     fn, GetLastWalMethodError(stream->walmethod));
        pg_free(fn);
        return false;
    }
 
    pg_free(fn);
    walfile = f;
    return true;
}
 
/*
 * Close the current WAL file (if open), and rename it to the correct
 * filename if it's complete. On failure, prints an error message to stderr
 * and returns false, otherwise returns true.
 */
static bool
close_walfile(StreamCtl *stream, XLogRecPtr pos)
{
    char       *fn;
    off_t       currpos;
    int         r;
    char        walfile_name[MAXPGPATH];
 
    if (walfile == NULL)
        return true;
 
    strlcpy(walfile_name, walfile->pathname, MAXPGPATH);
    currpos = walfile->currpos;
 
    /* Note that this considers the compression used if necessary */
    fn = stream->walmethod->ops->get_file_name(stream->walmethod,
                                               walfile_name,
                                               stream->partial_suffix);
 
    if (stream->partial_suffix)
    {
        if (currpos == WalSegSz)
            r = stream->walmethod->ops->close(walfile, CLOSE_NORMAL);
        else
        {
            pg_log_info("not renaming \"%s\", segment is not complete", fn);
            r = stream->walmethod->ops->close(walfile, CLOSE_NO_RENAME);
        }
    }
    else
        r = stream->walmethod->ops->close(walfile, CLOSE_NORMAL);
 
    walfile = NULL;
 
    if (r != 0)
    {
        pg_log_error("could not close file \"%s\": %s",
                     fn, GetLastWalMethodError(stream->walmethod));
 
        pg_free(fn);
        return false;
    }
 
    pg_free(fn);
 
    /*
     * Mark file as archived if requested by the caller - pg_basebackup needs
     * to do so as files can otherwise get archived again after promotion of a
     * new node. This is in line with walreceiver.c always doing a
     * XLogArchiveForceDone() after a complete segment.
     */
    if (currpos == WalSegSz && stream->mark_done)
    {
        /* writes error message if failed */
        if (!mark_file_as_archived(stream, walfile_name))
            return false;
    }
 
    lastFlushPosition = pos;
    return true;
}
 
 
/*
 * Check if a timeline history file exists.
 */
static bool
existsTimeLineHistoryFile(StreamCtl *stream)
{
    char        histfname[MAXFNAMELEN];
 
    /*
     * Timeline 1 never has a history file. We treat that as if it existed,
     * since we never need to stream it.
     */
    if (stream->timeline == 1)
        return true;
 
    TLHistoryFileName(histfname, stream->timeline);
 
    return stream->walmethod->ops->existsfile(stream->walmethod, histfname);
}
 
static bool
writeTimeLineHistoryFile(StreamCtl *stream, char *filename, char *content)
{
    int         size = strlen(content);
    char        histfname[MAXFNAMELEN];
    Walfile    *f;
 
    /*
     * Check that the server's idea of how timeline history files should be
     * named matches ours.
     */
    TLHistoryFileName(histfname, stream->timeline);
    if (strcmp(histfname, filename) != 0)
    {
        pg_log_error("server reported unexpected history file name for timeline %u: %s",
                     stream->timeline, filename);
        return false;
    }
 
    f = stream->walmethod->ops->open_for_write(stream->walmethod,
                                               histfname, ".tmp", 0);
    if (f == NULL)
    {
        pg_log_error("could not create timeline history file \"%s\": %s",
                     histfname, GetLastWalMethodError(stream->walmethod));
        return false;
    }
 
    if ((int) stream->walmethod->ops->write(f, content, size) != size)
    {
        pg_log_error("could not write timeline history file \"%s\": %s",
                     histfname, GetLastWalMethodError(stream->walmethod));
 
        /*
         * If we fail to make the file, delete it to release disk space
         */
        stream->walmethod->ops->close(f, CLOSE_UNLINK);
 
        return false;
    }
 
    if (stream->walmethod->ops->close(f, CLOSE_NORMAL) != 0)
    {
        pg_log_error("could not close file \"%s\": %s",
                     histfname, GetLastWalMethodError(stream->walmethod));
        return false;
    }
 
    /* Maintain archive_status, check close_walfile() for details. */
    if (stream->mark_done)
    {
        /* writes error message if failed */
        if (!mark_file_as_archived(stream, histfname))
            return false;
    }
 
    return true;
}
 
/*
 * Send a Standby Status Update message to server.
 */
static bool
sendFeedback(PGconn *conn, XLogRecPtr blockpos, TimestampTz now, bool replyRequested)
{
    char        replybuf[1 + 8 + 8 + 8 + 8 + 1];
    int         len = 0;
 
    replybuf[len] = 'r';
    len += 1;
    fe_sendint64(blockpos, &replybuf[len]); /* write */
    len += 8;
    if (reportFlushPosition)
        fe_sendint64(lastFlushPosition, &replybuf[len]);    /* flush */
    else
        fe_sendint64(InvalidXLogRecPtr, &replybuf[len]);    /* flush */
    len += 8;
    fe_sendint64(InvalidXLogRecPtr, &replybuf[len]);    /* apply */
    len += 8;
    fe_sendint64(now, &replybuf[len]);  /* sendTime */
    len += 8;
    replybuf[len] = replyRequested ? 1 : 0; /* replyRequested */
    len += 1;
 
    if (PQputCopyData(conn, replybuf, len) <= 0 || PQflush(conn))
    {
        pg_log_error("could not send feedback packet: %s",
                     PQerrorMessage(conn));
        return false;
    }
 
    return true;
}
 
/*
 * Check that the server version we're connected to is supported by
 * ReceiveXlogStream().
 *
 * If it's not, an error message is printed to stderr, and false is returned.
 */
bool
CheckServerVersionForStreaming(PGconn *conn)
{
    int         minServerMajor,
                maxServerMajor;
    int         serverMajor;
 
    /*
     * The message format used in streaming replication changed in 9.3, so we
     * cannot stream from older servers. And we don't support servers newer
     * than the client; it might work, but we don't know, so err on the safe
     * side.
     */
    minServerMajor = 903;
    maxServerMajor = PG_VERSION_NUM / 100;
    serverMajor = PQserverVersion(conn) / 100;
    if (serverMajor < minServerMajor)
    {
        const char *serverver = PQparameterStatus(conn, "server_version");
 
        pg_log_error("incompatible server version %s; client does not support streaming from server versions older than %s",
                     serverver ? serverver : "'unknown'",
                     "9.3");
        return false;
    }
    else if (serverMajor > maxServerMajor)
    {
        const char *serverver = PQparameterStatus(conn, "server_version");
 
        pg_log_error("incompatible server version %s; client does not support streaming from server versions newer than %s",
                     serverver ? serverver : "'unknown'",
                     PG_VERSION);
        return false;
    }
    return true;
}
 
/*
 * Receive a log stream starting at the specified position.
 *
 * Individual parameters are passed through the StreamCtl structure.
 *
 * If sysidentifier is specified, validate that both the system
 * identifier and the timeline matches the specified ones
 * (by sending an extra IDENTIFY_SYSTEM command)
 *
 * All received segments will be written to the directory
 * specified by basedir. This will also fetch any missing timeline history
 * files.
 *
 * The stream_stop callback will be called every time data
 * is received, and whenever a segment is completed. If it returns
 * true, the streaming will stop and the function
 * return. As long as it returns false, streaming will continue
 * indefinitely.
 *
 * If stream_stop() checks for external input, stop_socket should be set to
 * the FD it checks.  This will allow such input to be detected promptly
 * rather than after standby_message_timeout (which might be indefinite).
 * Note that signals will interrupt waits for input as well, but that is
 * race-y since a signal received while busy won't interrupt the wait.
 *
 * standby_message_timeout controls how often we send a message
 * back to the primary letting it know our progress, in milliseconds.
 * Zero means no messages are sent.
 * This message will only contain the write location, and never
 * flush or replay.
 *
 * If 'partial_suffix' is not NULL, files are initially created with the
 * given suffix, and the suffix is removed once the file is finished. That
 * allows you to tell the difference between partial and completed files,
 * so that you can continue later where you left.
 *
 * If 'synchronous' is true, the received WAL is flushed as soon as written,
 * otherwise only when the WAL file is closed.
 *
 * Note: The WAL location *must* be at a log segment start!
 */
bool
ReceiveXlogStream(PGconn *conn, StreamCtl *stream)
{
    char        query[128];
    char        slotcmd[128];
    PGresult   *res;
    XLogRecPtr  stoppos;
 
    /*
     * The caller should've checked the server version already, but doesn't do
     * any harm to check it here too.
     */
    if (!CheckServerVersionForStreaming(conn))
        return false;
 
    /*
     * Decide whether we want to report the flush position. If we report the
     * flush position, the primary will know what WAL we'll possibly
     * re-request, and it can then remove older WAL safely. We must always do
     * that when we are using slots.
     *
     * Reporting the flush position makes one eligible as a synchronous
     * replica. People shouldn't include generic names in
     * synchronous_standby_names, but we've protected them against it so far,
     * so let's continue to do so unless specifically requested.
     */
    if (stream->replication_slot != NULL)
    {
        reportFlushPosition = true;
        sprintf(slotcmd, "SLOT \"%s\" ", stream->replication_slot);
    }
    else
    {
        if (stream->synchronous)
            reportFlushPosition = true;
        else
            reportFlushPosition = false;
        slotcmd[0] = 0;
    }
 
    if (stream->sysidentifier != NULL)
    {
        char       *sysidentifier = NULL;
        TimeLineID  servertli;
 
        /*
         * Get the server system identifier and timeline, and validate them.
         */
        if (!RunIdentifySystem(conn, &sysidentifier, &servertli, NULL, NULL))
        {
            pg_free(sysidentifier);
            return false;
        }
 
        if (strcmp(stream->sysidentifier, sysidentifier) != 0)
        {
            pg_log_error("system identifier does not match between base backup and streaming connection");
            pg_free(sysidentifier);
            return false;
        }
        pg_free(sysidentifier);
 
        if (stream->timeline > servertli)
        {
            pg_log_error("starting timeline %u is not present in the server",
                         stream->timeline);
            return false;
        }
    }
 
    /*
     * initialize flush position to starting point, it's the caller's
     * responsibility that that's sane.
     */
    lastFlushPosition = stream->startpos;
 
    while (1)
    {
        /*
         * Fetch the timeline history file for this timeline, if we don't have
         * it already. When streaming log to tar, this will always return
         * false, as we are never streaming into an existing file and
         * therefore there can be no pre-existing timeline history file.
         */
        if (!existsTimeLineHistoryFile(stream))
        {
            snprintf(query, sizeof(query), "TIMELINE_HISTORY %u", stream->timeline);
            res = PQexec(conn, query);
            if (PQresultStatus(res) != PGRES_TUPLES_OK)
            {
                /* FIXME: we might send it ok, but get an error */
                pg_log_error("could not send replication command \"%s\": %s",
                             "TIMELINE_HISTORY", PQresultErrorMessage(res));
                PQclear(res);
                return false;
            }
 
            /*
             * The response to TIMELINE_HISTORY is a single row result set
             * with two fields: filename and content
             */
            if (PQnfields(res) != 2 || PQntuples(res) != 1)
            {
                pg_log_warning("unexpected response to TIMELINE_HISTORY command: got %d rows and %d fields, expected %d rows and %d fields",
                               PQntuples(res), PQnfields(res), 1, 2);
            }
 
            /* Write the history file to disk */
            writeTimeLineHistoryFile(stream,
                                     PQgetvalue(res, 0, 0),
                                     PQgetvalue(res, 0, 1));
 
            PQclear(res);
        }
 
        /*
         * Before we start streaming from the requested location, check if the
         * callback tells us to stop here.
         */
        if (stream->stream_stop(stream->startpos, stream->timeline, false))
            return true;
 
        /* Initiate the replication stream at specified location */
        snprintf(query, sizeof(query), "START_REPLICATION %s%X/%X TIMELINE %u",
                 slotcmd,
                 LSN_FORMAT_ARGS(stream->startpos),
                 stream->timeline);
        res = PQexec(conn, query);
        if (PQresultStatus(res) != PGRES_COPY_BOTH)
        {
            pg_log_error("could not send replication command \"%s\": %s",
                         "START_REPLICATION", PQresultErrorMessage(res));
            PQclear(res);
            return false;
        }
        PQclear(res);
 
        /* Stream the WAL */
        res = HandleCopyStream(conn, stream, &stoppos);
        if (res == NULL)
            goto error;
 
        /*
         * Streaming finished.
         *
         * There are two possible reasons for that: a controlled shutdown, or
         * we reached the end of the current timeline. In case of
         * end-of-timeline, the server sends a result set after Copy has
         * finished, containing information about the next timeline. Read
         * that, and restart streaming from the next timeline. In case of
         * controlled shutdown, stop here.
         */
        if (PQresultStatus(res) == PGRES_TUPLES_OK)
        {
            /*
             * End-of-timeline. Read the next timeline's ID and starting
             * position. Usually, the starting position will match the end of
             * the previous timeline, but there are corner cases like if the
             * server had sent us half of a WAL record, when it was promoted.
             * The new timeline will begin at the end of the last complete
             * record in that case, overlapping the partial WAL record on the
             * old timeline.
             */
            uint32      newtimeline;
            bool        parsed;
 
            parsed = ReadEndOfStreamingResult(res, &stream->startpos, &newtimeline);
            PQclear(res);
            if (!parsed)
                goto error;
 
            /* Sanity check the values the server gave us */
            if (newtimeline <= stream->timeline)
            {
                pg_log_error("server reported unexpected next timeline %u, following timeline %u",
                             newtimeline, stream->timeline);
                goto error;
            }
            if (stream->startpos > stoppos)
            {
                pg_log_error("server stopped streaming timeline %u at %X/%X, but reported next timeline %u to begin at %X/%X",
                             stream->timeline, LSN_FORMAT_ARGS(stoppos),
                             newtimeline, LSN_FORMAT_ARGS(stream->startpos));
                goto error;
            }
 
            /* Read the final result, which should be CommandComplete. */
            res = PQgetResult(conn);
            if (PQresultStatus(res) != PGRES_COMMAND_OK)
            {
                pg_log_error("unexpected termination of replication stream: %s",
                             PQresultErrorMessage(res));
                PQclear(res);
                goto error;
            }
            PQclear(res);
 
            /*
             * Loop back to start streaming from the new timeline. Always
             * start streaming at the beginning of a segment.
             */
            stream->timeline = newtimeline;
            stream->startpos = stream->startpos -
                XLogSegmentOffset(stream->startpos, WalSegSz);
            continue;
        }
        else if (PQresultStatus(res) == PGRES_COMMAND_OK)
        {
            PQclear(res);
 
            /*
             * End of replication (ie. controlled shut down of the server).
             *
             * Check if the callback thinks it's OK to stop here. If not,
             * complain.
             */
            if (stream->stream_stop(stoppos, stream->timeline, false))
                return true;
            else
            {
                pg_log_error("replication stream was terminated before stop point");
                goto error;
            }
        }
        else
        {
            /* Server returned an error. */
            pg_log_error("unexpected termination of replication stream: %s",
                         PQresultErrorMessage(res));
            PQclear(res);
            goto error;
        }
    }
 
error:
    if (walfile != NULL && stream->walmethod->ops->close(walfile, CLOSE_NO_RENAME) != 0)
        pg_log_error("could not close file \"%s\": %s",
                     walfile->pathname, GetLastWalMethodError(stream->walmethod));
    walfile = NULL;
    return false;
}
 
/*
 * Helper function to parse the result set returned by server after streaming
 * has finished. On failure, prints an error to stderr and returns false.
 */
static bool
ReadEndOfStreamingResult(PGresult *res, XLogRecPtr *startpos, uint32 *timeline)
{
    uint32      startpos_xlogid,
                startpos_xrecoff;
 
    /*----------
     * The result set consists of one row and two columns, e.g:
     *
     *  next_tli | next_tli_startpos
     * ----------+-------------------
     *         4 | 0/9949AE0
     *
     * next_tli is the timeline ID of the next timeline after the one that
     * just finished streaming. next_tli_startpos is the WAL location where
     * the server switched to it.
     *----------
     */
    if (PQnfields(res) < 2 || PQntuples(res) != 1)
    {
        pg_log_error("unexpected result set after end-of-timeline: got %d rows and %d fields, expected %d rows and %d fields",
                     PQntuples(res), PQnfields(res), 1, 2);
        return false;
    }
 
    *timeline = atoi(PQgetvalue(res, 0, 0));
    if (sscanf(PQgetvalue(res, 0, 1), "%X/%X", &startpos_xlogid,
               &startpos_xrecoff) != 2)
    {
        pg_log_error("could not parse next timeline's starting point \"%s\"",
                     PQgetvalue(res, 0, 1));
        return false;
    }
    *startpos = ((uint64) startpos_xlogid << 32) | startpos_xrecoff;
 
    return true;
}
 
/*
 * The main loop of ReceiveXlogStream. Handles the COPY stream after
 * initiating streaming with the START_REPLICATION command.
 *
 * If the COPY ends (not necessarily successfully) due a message from the
 * server, returns a PGresult and sets *stoppos to the last byte written.
 * On any other sort of error, returns NULL.
 */
static PGresult *
HandleCopyStream(PGconn *conn, StreamCtl *stream,
                 XLogRecPtr *stoppos)
{
    char       *copybuf = NULL;
    TimestampTz last_status = -1;
    XLogRecPtr  blockpos = stream->startpos;
 
    still_sending = true;
 
    while (1)
    {
        int         r;
        TimestampTz now;
        long        sleeptime;
 
        /*
         * Check if we should continue streaming, or abort at this point.
         */
        if (!CheckCopyStreamStop(conn, stream, blockpos))
            goto error;
 
        now = feGetCurrentTimestamp();
 
        /*
         * If synchronous option is true, issue sync command as soon as there
         * are WAL data which has not been flushed yet.
         */
        if (stream->synchronous && lastFlushPosition < blockpos && walfile != NULL)
        {
            if (stream->walmethod->ops->sync(walfile) != 0)
                pg_fatal("could not fsync file \"%s\": %s",
                         walfile->pathname, GetLastWalMethodError(stream->walmethod));
            lastFlushPosition = blockpos;
 
            /*
             * Send feedback so that the server sees the latest WAL locations
             * immediately.
             */
            if (!sendFeedback(conn, blockpos, now, false))
                goto error;
            last_status = now;
        }
 
        /*
         * Potentially send a status message to the primary
         */
        if (still_sending && stream->standby_message_timeout > 0 &&
            feTimestampDifferenceExceeds(last_status, now,
                                         stream->standby_message_timeout))
        {
            /* Time to send feedback! */
            if (!sendFeedback(conn, blockpos, now, false))
                goto error;
            last_status = now;
        }
 
        /*
         * Calculate how long send/receive loops should sleep
         */
        sleeptime = CalculateCopyStreamSleeptime(now, stream->standby_message_timeout,
                                                 last_status);
 
        r = CopyStreamReceive(conn, sleeptime, stream->stop_socket, &copybuf);
        while (r != 0)
        {
            if (r == -1)
                goto error;
            if (r == -2)
            {
                PGresult   *res = HandleEndOfCopyStream(conn, stream, copybuf, blockpos, stoppos);
 
                if (res == NULL)
                    goto error;
                else
                    return res;
            }
 
            /* Check the message type. */
            if (copybuf[0] == 'k')
            {
                if (!ProcessKeepaliveMsg(conn, stream, copybuf, r, blockpos,
                                         &last_status))
                    goto error;
            }
            else if (copybuf[0] == 'w')
            {
                if (!ProcessXLogDataMsg(conn, stream, copybuf, r, &blockpos))
                    goto error;
 
                /*
                 * Check if we should continue streaming, or abort at this
                 * point.
                 */
                if (!CheckCopyStreamStop(conn, stream, blockpos))
                    goto error;
            }
            else
            {
                pg_log_error("unrecognized streaming header: \"%c\"",
                             copybuf[0]);
                goto error;
            }
 
            /*
             * Process the received data, and any subsequent data we can read
             * without blocking.
             */
            r = CopyStreamReceive(conn, 0, stream->stop_socket, &copybuf);
        }
    }
 
error:
    PQfreemem(copybuf);
    return NULL;
}
 
/*
 * Wait until we can read a CopyData message,
 * or timeout, or occurrence of a signal or input on the stop_socket.
 * (timeout_ms < 0 means wait indefinitely; 0 means don't wait.)
 *
 * Returns 1 if data has become available for reading, 0 if timed out
 * or interrupted by signal or stop_socket input, and -1 on an error.
 */
static int
CopyStreamPoll(PGconn *conn, long timeout_ms, pgsocket stop_socket)
{
    int         ret;
    fd_set      input_mask;
    int         connsocket;
    int         maxfd;
    struct timeval timeout;
    struct timeval *timeoutptr;
 
    connsocket = PQsocket(conn);
    if (connsocket < 0)
    {
        pg_log_error("invalid socket: %s", PQerrorMessage(conn));
        return -1;
    }
 
    FD_ZERO(&input_mask);
    FD_SET(connsocket, &input_mask);
    maxfd = connsocket;
    if (stop_socket != PGINVALID_SOCKET)
    {
        FD_SET(stop_socket, &input_mask);
        maxfd = Max(maxfd, stop_socket);
    }
 
    if (timeout_ms < 0)
        timeoutptr = NULL;
    else
    {
        timeout.tv_sec = timeout_ms / 1000L;
        timeout.tv_usec = (timeout_ms % 1000L) * 1000L;
        timeoutptr = &timeout;
    }
 
    ret = select(maxfd + 1, &input_mask, NULL, NULL, timeoutptr);
 
    if (ret < 0)
    {
        if (errno == EINTR)
            return 0;           /* Got a signal, so not an error */
        pg_log_error("%s() failed: %m", "select");
        return -1;
    }
    if (ret > 0 && FD_ISSET(connsocket, &input_mask))
        return 1;               /* Got input on connection socket */
 
    return 0;                   /* Got timeout or input on stop_socket */
}
 
/*
 * Receive CopyData message available from XLOG stream, blocking for
 * maximum of 'timeout' ms.
 *
 * 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 CopyStreamReceive call.
 *
 * Returns 0 if no data was available within timeout, or if wait was
 * interrupted by signal or stop_socket input.
 * -1 on error. -2 if the server ended the COPY.
 */
static int
CopyStreamReceive(PGconn *conn, long timeout, pgsocket stop_socket,
                  char **buffer)
{
    char       *copybuf = NULL;
    int         rawlen;
 
    PQfreemem(*buffer);
    *buffer = NULL;
 
    /* Try to receive a CopyData message */
    rawlen = PQgetCopyData(conn, &copybuf, 1);
    if (rawlen == 0)
    {
        int         ret;
 
        /*
         * No data available.  Wait for some to appear, but not longer than
         * the specified timeout, so that we can ping the server.  Also stop
         * waiting if input appears on stop_socket.
         */
        ret = CopyStreamPoll(conn, timeout, stop_socket);
        if (ret <= 0)
            return ret;
 
        /* Now there is actually data on the socket */
        if (PQconsumeInput(conn) == 0)
        {
            pg_log_error("could not receive data from WAL stream: %s",
                         PQerrorMessage(conn));
            return -1;
        }
 
        /* Now that we've consumed some input, try again */
        rawlen = PQgetCopyData(conn, &copybuf, 1);
        if (rawlen == 0)
            return 0;
    }
    if (rawlen == -1)           /* end-of-streaming or error */
        return -2;
    if (rawlen == -2)
    {
        pg_log_error("could not read COPY data: %s", PQerrorMessage(conn));
        return -1;
    }
 
    /* Return received messages to caller */
    *buffer = copybuf;
    return rawlen;
}
 
/*
 * Process the keepalive message.
 */
static bool
ProcessKeepaliveMsg(PGconn *conn, StreamCtl *stream, char *copybuf, int len,
                    XLogRecPtr blockpos, TimestampTz *last_status)
{
    int         pos;
    bool        replyRequested;
    TimestampTz now;
 
    /*
     * Parse the keepalive message, enclosed in the CopyData message. We just
     * check if the server requested a reply, and ignore the rest.
     */
    pos = 1;                    /* skip msgtype 'k' */
    pos += 8;                   /* skip walEnd */
    pos += 8;                   /* skip sendTime */
 
    if (len < pos + 1)
    {
        pg_log_error("streaming header too small: %d", len);
        return false;
    }
    replyRequested = copybuf[pos];
 
    /* If the server requested an immediate reply, send one. */
    if (replyRequested && still_sending)
    {
        if (reportFlushPosition && lastFlushPosition < blockpos &&
            walfile != NULL)
        {
            /*
             * If a valid flush location needs to be reported, flush the
             * current WAL file so that the latest flush location is sent back
             * to the server. This is necessary to see whether the last WAL
             * data has been successfully replicated or not, at the normal
             * shutdown of the server.
             */
            if (stream->walmethod->ops->sync(walfile) != 0)
                pg_fatal("could not fsync file \"%s\": %s",
                         walfile->pathname, GetLastWalMethodError(stream->walmethod));
            lastFlushPosition = blockpos;
        }
 
        now = feGetCurrentTimestamp();
        if (!sendFeedback(conn, blockpos, now, false))
            return false;
        *last_status = now;
    }
 
    return true;
}
 
/*
 * Process XLogData message.
 */
static bool
ProcessXLogDataMsg(PGconn *conn, StreamCtl *stream, char *copybuf, int len,
                   XLogRecPtr *blockpos)
{
    int         xlogoff;
    int         bytes_left;
    int         bytes_written;
    int         hdr_len;
 
    /*
     * Once we've decided we don't want to receive any more, just ignore any
     * subsequent XLogData messages.
     */
    if (!(still_sending))
        return true;
 
    /*
     * Read the header of the XLogData message, enclosed in the CopyData
     * message. We only need the WAL location field (dataStart), the rest of
     * the header is ignored.
     */
    hdr_len = 1;                /* msgtype 'w' */
    hdr_len += 8;               /* dataStart */
    hdr_len += 8;               /* walEnd */
    hdr_len += 8;               /* sendTime */
    if (len < hdr_len)
    {
        pg_log_error("streaming header too small: %d", len);
        return false;
    }
    *blockpos = fe_recvint64(&copybuf[1]);
 
    /* Extract WAL location for this block */
    xlogoff = XLogSegmentOffset(*blockpos, WalSegSz);
 
    /*
     * Verify that the initial location in the stream matches where we think
     * we are.
     */
    if (walfile == NULL)
    {
        /* No file open yet */
        if (xlogoff != 0)
        {
            pg_log_error("received write-ahead log record for offset %u with no file open",
                         xlogoff);
            return false;
        }
    }
    else
    {
        /* More data in existing segment */
        if (walfile->currpos != xlogoff)
        {
            pg_log_error("got WAL data offset %08x, expected %08x",
                         xlogoff, (int) walfile->currpos);
            return false;
        }
    }
 
    bytes_left = len - hdr_len;
    bytes_written = 0;
 
    while (bytes_left)
    {
        int         bytes_to_write;
 
        /*
         * If crossing a WAL boundary, only write up until we reach wal
         * segment size.
         */
        if (xlogoff + bytes_left > WalSegSz)
            bytes_to_write = WalSegSz - xlogoff;
        else
            bytes_to_write = bytes_left;
 
        if (walfile == NULL)
        {
            if (!open_walfile(stream, *blockpos))
            {
                /* Error logged by open_walfile */
                return false;
            }
        }
 
        if (stream->walmethod->ops->write(walfile,
                                          copybuf + hdr_len + bytes_written,
                                          bytes_to_write) != bytes_to_write)
        {
            pg_log_error("could not write %d bytes to WAL file \"%s\": %s",
                         bytes_to_write, walfile->pathname,
                         GetLastWalMethodError(stream->walmethod));
            return false;
        }
 
        /* Write was successful, advance our position */
        bytes_written += bytes_to_write;
        bytes_left -= bytes_to_write;
        *blockpos += bytes_to_write;
        xlogoff += bytes_to_write;
 
        /* Did we reach the end of a WAL segment? */
        if (XLogSegmentOffset(*blockpos, WalSegSz) == 0)
        {
            if (!close_walfile(stream, *blockpos))
                /* Error message written in close_walfile() */
                return false;
 
            xlogoff = 0;
 
            if (still_sending && stream->stream_stop(*blockpos, stream->timeline, true))
            {
                if (PQputCopyEnd(conn, NULL) <= 0 || PQflush(conn))
                {
                    pg_log_error("could not send copy-end packet: %s",
                                 PQerrorMessage(conn));
                    return false;
                }
                still_sending = false;
                return true;    /* ignore the rest of this XLogData packet */
            }
        }
    }
    /* No more data left to write, receive next copy packet */
 
    return true;
}
 
/*
 * Handle end of the copy stream.
 */
static PGresult *
HandleEndOfCopyStream(PGconn *conn, StreamCtl *stream, char *copybuf,
                      XLogRecPtr blockpos, XLogRecPtr *stoppos)
{
    PGresult   *res = PQgetResult(conn);
 
    /*
     * The server closed its end of the copy stream.  If we haven't closed
     * ours already, we need to do so now, unless the server threw an error,
     * in which case we don't.
     */
    if (still_sending)
    {
        if (!close_walfile(stream, blockpos))
        {
            /* Error message written in close_walfile() */
            PQclear(res);
            return NULL;
        }
        if (PQresultStatus(res) == PGRES_COPY_IN)
        {
            if (PQputCopyEnd(conn, NULL) <= 0 || PQflush(conn))
            {
                pg_log_error("could not send copy-end packet: %s",
                             PQerrorMessage(conn));
                PQclear(res);
                return NULL;
            }
            res = PQgetResult(conn);
        }
        still_sending = false;
    }
    PQfreemem(copybuf);
    *stoppos = blockpos;
    return res;
}
 
/*
 * Check if we should continue streaming, or abort at this point.
 */
static bool
CheckCopyStreamStop(PGconn *conn, StreamCtl *stream, XLogRecPtr blockpos)
{
    if (still_sending && stream->stream_stop(blockpos, stream->timeline, false))
    {
        if (!close_walfile(stream, blockpos))
        {
            /* Potential error message is written by close_walfile */
            return false;
        }
        if (PQputCopyEnd(conn, NULL) <= 0 || PQflush(conn))
        {
            pg_log_error("could not send copy-end packet: %s",
                         PQerrorMessage(conn));
            return false;
        }
        still_sending = false;
    }
 
    return true;
}
 
/*
 * Calculate how long send/receive loops should sleep
 */
static long
CalculateCopyStreamSleeptime(TimestampTz now, int standby_message_timeout,
                             TimestampTz last_status)
{
    TimestampTz status_targettime = 0;
    long        sleeptime;
 
    if (standby_message_timeout && still_sending)
        status_targettime = last_status +
            (standby_message_timeout - 1) * ((int64) 1000);
 
    if (status_targettime > 0)
    {
        long        secs;
        int         usecs;
 
        feTimestampDifference(now,
                              status_targettime,
                              &secs,
                              &usecs);
        /* Always sleep at least 1 sec */
        if (secs <= 0)
        {
            secs = 1;
            usecs = 0;
        }
 
        sleeptime = secs * 1000 + usecs / 1000;
    }
    else
        sleeptime = -1;
 
    return sleeptime;
}