Function ProcessXLogDataMsg

src/bin/pg_basebackup/receivelog.c:1047–1172

* Process XLogData message. */

 * 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;
		}