Add optional cleanup task for old bill_data

daily.php

@@ -115,6 +115,27 @@ if ($options['f'] === 'notifications') {
     include_once 'includes/notifications.php';
 }
 
+if ($options['f'] === 'bill_data') {
+    if (is_numeric($config['billing_data_purge']) && $config['billing_data_purge'] > 0) {
+        # Deletes data older than XX months before the start of the last complete billing period
+        $months = $config['billing_data_purge'];
+        echo "Deleting billing data more than $months month before the last completed billing cycle\n";
+        $sql = "DELETE bill_data
+                FROM bill_data
+                    INNER JOIN (SELECT bill_id, 
+                        SUBDATE(
+                            SUBDATE(
+                                ADDDATE(
+                                    subdate(curdate(), (day(curdate())-1)),             # Start of this month
+                                    bill_day - 1),                                      # Billing anniversary
+                                INTERVAL IF(bill_day > DAY(curdate()), 1, 0) MONTH),    # Deal with anniversary not yet happened this month
+                            INTERVAL ? MONTH) AS threshold                              # Adjust based on config threshold
+                FROM bills) q
+                ON bill_data.bill_id = q.bill_id AND bill_data.timestamp < q.threshold;";
+        dbQuery($sql, array($months));
+    }
+}
+
 if ($options['f'] === 'purgeusers') {
     $purge = 0;
     if (is_numeric($config['radius']['users_purge']) && $config['auth_mechanism'] === 'radius') {

daily.sh

@@ -74,6 +74,7 @@ else
             php daily.php -f callback
             php daily.php -f device_perf
             php daily.php -f purgeusers
+            php daily.php -f bill_data
         ;;
         submodules)
             # Init+Update our submodules

doc/Extensions/Billing-Module.md

@@ -16,3 +16,17 @@ Edit `/etc/cron.d/librenms` and add the following:
 ```
 
 Create billing graphs as required.
+
+## Options
+
+Billing data is stored in the MySQL database, and you may wish to purge the detailed 
+stats for old data (per-month totals will always be kept).  To enable this, add the 
+following to `config.php`:
+
+```php
+$config['billing_data_purge'] = 12;     // Number of months to retain
+```
+
+Data for the last complete billing cycle will always be retained - only data older than
+this by the configured number of months will be removed.  This task is performed in the
+daily cleanup tasks.