Can you redo this for 1.2.x branch or is it fine there?

I have been using Cacti 1.3 (Dev) for long time on Windows and did not test this issue on 1.2.x

I presume it is the same issue in Cacti 1.2.x, just checked the 1.2.x branch:

function rrdtool_trim_output(&$output) {
global $config;

/* When using RRDtool with proc_open for long strings
 * and using the '-' to handle standard in from inside
 * the process, RRDtool automatically appends stderr
 * to stdout for batch programs to parse the output
 * string.  So, therefore, we have to prune that
 * output.
 */
if ($config['cacti_server_os'] == 'win32') {
	$output = rtrim($output, "OK \n\r");

/* MSS: missing variable here */
} else {
$okpos = strrpos($output, 'OK u:');
if ($okpos !== false) {
$output = substr($output, 0, $okpos);
}
}
}

Okay, I fixed this in 1.2.x and will cherry pick into 1.3 shortly.