text: /*-------------------------------------------------------------------------
   *
   * receivelog.c - receive WAL files using the streaming
   *				  replication protocol.
   *
   * Author: Magnus Hagander <magnus@hagander.net>
   *
   * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
   *
   * IDENTIFICATION
   *		  src/bin/pg_basebackup/receivelog.c
   *-------------------------------------------------------------------------
   */
  
  #include "postgres_fe.h"
  
  #include <sys/stat.h>
  #include <unistd.h>
  #ifdef HAVE_SYS_SELECT_H
  #include <sys/select.h>
  #endif
  
  /* local includes */
  #include "receivelog.h"
  #include "streamutil.h"
  
  #include "libpq-fe.h"
  #include "access/xlog_internal.h"
  #include "common/file_utils.h"
  #include "common/logging.h"
  
  
  /* fd and filename for currently open WAL file */
  static Walfile *walfile = NULL;
  static char current_walfile_name[MAXPGPATH] = "";
  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,
  								XLogRecPtr *stoppos);
  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->open_for_write(tmppath, NULL, 0);
  	if (f == NULL)
  	{
  		pg_log_error("could not create archive status file \"%s\": %s",
  					 tmppath, stream->walmethod->getlasterror());
  		return false;
  	}
  
  	stream->walmethod->close(f, CLOSE_NORMAL);
  
  	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 FD for the file, and the base filename
   * (without partial_suffix) is stored in 'current_walfile_name'.
   *
   * The file will be padded to 16Mb with zeroes.
   */
  static bool
  open_walfile(StreamCtl *stream, XLogRecPtr startpoint)
  {
  	Walfile    *f;
  	char		fn[MAXPGPATH];
  	ssize_t		size;
  	XLogSegNo	segno;
  
  	XLByteToSeg(startpoint, segno, WalSegSz);
  	XLogFileName(current_walfile_name, stream->timeline, segno, WalSegSz);
  
  	snprintf(fn, sizeof(fn), "%s%s", current_walfile_name,
  			 stream->partial_suffix ? 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.
  	 *
  	 * When streaming to tar, no file with this name will exist before, so we
  	 * never have to verify a size.
  	 */
  	if (stream->walmethod->existsfile(fn))
  	{
  		size = stream->walmethod->get_file_size(fn);
  		if (size < 0)
  		{
  			pg_log_error("could not get size of write-ahead log file \"%s\": %s",
  						 fn, stream->walmethod->getlasterror());
  			return false;
  		}
  		if (size == WalSegSz)
  		{
  			/* Already padded file. Open it for use */
  			f = stream->walmethod->open_for_write(current_walfile_name, stream->partial_suffix, 0);
  			if (f == NULL)
  			{
  				pg_log_error("could not open existing write-ahead log file \"%s\": %s",
  							 fn, stream->walmethod->getlasterror());
  				return false;
  			}
  
  			/* fsync file in case of a previous crash */
  			if (stream->walmethod->sync(f) != 0)
  			{
  				pg_log_error("could not fsync existing write-ahead log file \"%s\": %s",
  							 fn, stream->walmethod->getlasterror());
  				stream->walmethod->close(f, CLOSE_UNLINK);
  				return false;
  			}
  
  			walfile = f;
  			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 %d byte, should be 0 or %d",
  								  "write-ahead log file \"%s\" has %d bytes, should be 0 or %d",
  								  size),
  						 fn, (int) size, WalSegSz);
  			return false;
  		}
  		/* File existed and was empty, so fall through and open */
  	}
  
  	/* No file existed, so create one */
  
  	f = stream->walmethod->open_for_write(current_walfile_name,
  										  stream->partial_suffix, WalSegSz);
  	if (f == NULL)
  	{
  		pg_log_error("could not open write-ahead log file \"%s\": %s",
  					 fn, stream->walmethod->getlasterror());
  		return false;
  	}
  
  	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)
  {
  	off_t		currpos;
  	int			r;
  
  	if (walfile == NULL)
  		return true;
  
  	currpos = stream->walmethod->get_current_pos(walfile);
  	if (currpos == -1)
  	{
  		pg_log_error("could not determine seek position in file \"%s\": %s",
  					 current_walfile_name, stream->walmethod->getlasterror());
  		stream->walmethod->close(walfile, CLOSE_UNLINK);
  		walfile = NULL;
  
  		return false;
  	}
  
  	if (stream->partial_suffix)
  	{
  		if (currpos == WalSegSz)
  			r = stream->walmethod->close(walfile, CLOSE_NORMAL);
  		else
  		{
  			pg_log_info("not renaming \"%s%s\", segment is not complete",
  						current_walfile_name, stream->partial_suffix);
  			r = stream->walmethod->close(walfile, CLOSE_NO_RENAME);
  		}
  	}
  	else
  		r = stream->walmethod->close(walfile, CLOSE_NORMAL);
  
  	walfile = NULL;
  
  	if (r != 0)
  	{
  		pg_log_error("could not close file \"%s\": %s",
  					 current_walfile_name, stream->walmethod->getlasterror());
  		return false;
  	}
  
  	/*
  	 * 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, current_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->existsfile(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->open_for_write(histfname, ".tmp", 0);
  	if (f == NULL)
  	{
  		pg_log_error("could not create timeline history file \"%s\": %s",
  					 histfname, stream->walmethod->getlasterror());
  		return false;
  	}
  
  	if ((int) stream->walmethod->write(f, content, size) != size)
  	{
  		pg_log_error("could not write timeline history file \"%s\": %s",
  					 histfname, stream->walmethod->getlasterror());
  
  		/*
  		 * If we fail to make the file, delete it to release disk space
  		 */
  		stream->walmethod->close(f, CLOSE_UNLINK);
  
  		return false;
  	}
  
  	if (stream->walmethod->close(f, CLOSE_NORMAL) != 0)
  	{
  		pg_log_error("could not close file \"%s\": %s",
  					 histfname, stream->walmethod->getlasterror());
  		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 master 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)
  	{
  		/* Validate system identifier hasn't changed */
  		res = PQexec(conn, "IDENTIFY_SYSTEM");
  		if (PQresultStatus(res) != PGRES_TUPLES_OK)
  		{
  			pg_log_error("could not send replication command \"%s\": %s",
  						 "IDENTIFY_SYSTEM", PQerrorMessage(conn));
  			PQclear(res);
  			return false;
  		}
  		if (PQntuples(res) != 1 || PQnfields(res) < 3)
  		{
  			pg_log_error("could not identify system: got %d rows and %d fields, expected %d rows and %d or more fields",
  						 PQntuples(res), PQnfields(res), 1, 3);
  			PQclear(res);
  			return false;
  		}
  		if (strcmp(stream->sysidentifier, PQgetvalue(res, 0, 0)) != 0)
  		{
  			pg_log_error("system identifier does not match between base backup and streaming connection");
  			PQclear(res);
  			return false;
  		}
  		if (stream->timeline > atoi(PQgetvalue(res, 0, 1)))
  		{
  			pg_log_error("starting timeline %u is not present in the server",
  						 stream->timeline);
  			PQclear(res);
  			return false;
  		}
  		PQclear(res);
  	}
  
  	/*
  	 * 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,
  				 (uint32) (stream->startpos >> 32), (uint32) 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, (uint32) (stoppos >> 32), (uint32) stoppos,
  							 newtimeline, (uint32) (stream->startpos >> 32), (uint32) 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->close(walfile, CLOSE_NO_RENAME) != 0)
  		pg_log_error("could not close file \"%s\": %s",
  					 current_walfile_name, stream->walmethod->getlasterror());
  	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, stoppos))
  			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->sync(walfile) != 0)
  			{
  				pg_log_error("could not fsync file \"%s\": %s",
  							 current_walfile_name, stream->walmethod->getlasterror());
  				goto error;
  			}
  			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 master
  		 */
  		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, stoppos))
  					goto error;
  			}
  			else if (copybuf[0] == 'E')
  			{
  				/*
  				 * GPDB: message for extension, do nothing.
  				 * NOTE: extension may has own pg_basebackup binary.
  				 */
  			}
  			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:
  	if (copybuf != NULL)
  		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("select() failed: %m");
  		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;
  
  	if (*buffer != NULL)
  		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->sync(walfile) != 0)
  			{
  				pg_log_error("could not fsync file \"%s\": %s",
  							 current_walfile_name, stream->walmethod->getlasterror());
  				return false;
  			}
  			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 (stream->walmethod->get_current_pos(walfile) != xlogoff)
  		{
  			pg_log_error("got WAL data offset %08x, expected %08x",
  						 xlogoff, (int) stream->walmethod->get_current_pos(walfile));
  			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->write(walfile, copybuf + hdr_len + bytes_written,
  									 bytes_to_write) != bytes_to_write)
  		{
  			pg_log_error("could not write %u bytes to WAL file \"%s\": %s",
  						 bytes_to_write, current_walfile_name,
  						 stream->walmethod->getlasterror());
  			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;
  	}
  	if (copybuf != NULL)
  		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,
  					XLogRecPtr *stoppos)
  {
  	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;
  }
  