Fix #753: Enable CDEF/calculated data source value alerting

[bmfmancini]

Summary

Fixes #753

Problem

Thresholds configured against data sources that use CDEF (calculated) values do not trigger alerts. The threshold evaluation compares against the raw RRD value rather than the CDEF-transformed value, so alerts never fire when the threshold condition is only met after the CDEF calculation.

Root Cause

thold_modify_values_by_cdef() in thold_functions.php had two logic bugs:

1. Contradictory CDEF lookup condition

The original code:

if ($thold_data['data_type'] != 1 || empty($thold_data['cdef'])) {
    // Query graph items for a CDEF
    $cdef = db_fetch_cell_prepared(...);
}

if ($cdef !== false && $cdef > 0 && $thold_data['data_type'] == 1) {
    // Apply CDEF to threshold values
}

When a user explicitly configures a CDEF on a threshold (data_type=1, cdef set), the first if evaluates to false || false = false, so the DB query is skipped and $cdef remains false. The second if then checks $cdef !== false which fails, so the CDEF is never applied to the threshold values (thold_hi, thold_low, etc.).

This means the already-CDEF-transformed lastread was compared against raw threshold values, causing alerts to never fire (or fire incorrectly).

2. Double transformation of lastread

lastread is already CDEF-transformed by thold_poller_output() (or thold_process.php in daemon mode) before being stored in the database. thold_modify_values_by_cdef() then applied the CDEF to lastread again, producing an incorrect (double-transformed) value.

Fix

Restructured thold_modify_values_by_cdef():

1. Use the explicitly configured CDEF when data_type=1 and cdef is set, falling back to auto-detection from graph items only when cdef is empty.
2. Removed the redundant lastread transformation — lastread is already CDEF-transformed before storage. The CDEF is now applied only to the threshold values (thold_hi, thold_low, thold_warning_hi, thold_warning_low, time_hi, time_low, etc.) so the comparison is consistent.

Testing

• Verified both the standard poller path (thold_poller_output → thold_check_threshold) and the daemon path (thold_process.php → thold_check_threshold) apply the CDEF to lastread before storage, confirming the double-transformation was real.
• No new lint/compile errors introduced.

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