Substitution: thold_str_replace blanks the value zero, get_current_value falls back to the first data source

[somethingwithproof]

Two substitution helpers return the wrong value for inputs that occur in normal operation.

thold_str_replace() blanks the value zero (thold_functions.php:8314-8320)

function thold_str_replace($search, $replace, $subject) {
    if (empty($replace) || $replace === 0) {
        $replace = '';
    }

    return str_replace($search, $replace, $subject);
}

empty('0') is true, so a replacement value of 0 or '0' becomes an empty string. Confirmed:

thold_str_replace('<X>', 0,   'v=<X>')  =>  'v='
thold_str_replace('<X>', '0', 'v=<X>')  =>  'v='
thold_str_replace('<X>', 5,   'v=<X>')  =>  'v=5'

This helper performs every tag substitution in thold_replace_threshold_tags() and the <SUBJECT> substitution in thold_mail(). So an alert for a value that dropped to zero reads "Current value is " with a blank — exactly the case the operator most needs to see.

The same blanking applies to trigger commands, where a script invoked as --value <CURRENTVALUE> receives a missing argument and its remaining arguments shift position.

Testing $replace === null || $replace === false instead would keep zero intact.

get_current_value() returns the wrong data source's value (thold_functions.php:5075-5081)

$idx = array_search($data_template_rrd_id, $result['data_source_names'], true);

// Return Blank if the value was not found (Cache Cleared?)

if (!isset($result['values']) || $idx === null || !cacti_sizeof($result['values'][$idx])) {
    return 0;
}

$value = array_values($result['values'][$idx])[0];

array_search() returns false when there is no match, never null, so the $idx === null guard never fires. $result['values'][false] is $result['values'][0] under PHP's bool-to-int key cast, so a lookup for a data source that does not exist quietly returns the first data source's value.

This is reachable today. thold_expression_specialtype_rpn() (:723-747) calls get_current_value() with 'upper_limit', 'lower_limit', 'rrd_minimum', 'rrd_maximum' and 'hdd_total' — none of which are data source names — as do the CDEF substitutions at :4817-4823. All of them resolve to the first data source's current value instead of the graph or data source limit that was asked for.

$idx === false fixes the guard. The call sites then need to look up the actual column rather than passing it as a data source name.

[somethingwithproof]

Implementation in #790.