Gzip filter: handling of empty flush buffers. Empty flush buffers are legitimate and may happen e.g. due to $r->flush() calls in embedded perl. If there are no data buffered in zlib, deflate() will return Z_BUF_ERROR (i.e. no progress possible) without adding anything to output. Don't treat Z_BUF_ERROR as fatal and correctly send empty flush buffer if we have no data in output at all. See this thread for details: http://mailman.nginx.org/pipermail/nginx/2010-November/023693.html