Revert "(legacy) remove cache stuff from find_interface_subnet"

This reverts commit d4f27bc.

src/etc/inc/interfaces.inc

@@ -2839,6 +2839,7 @@ function interface_vlan_adapt_mtu($vlanifs, $mtu) {
 function interface_configure($interface = 'wan', $reloadall = false, $linkupevent = false)
 {
 	global $config;
+	global $interface_sn_arr_cache;
 	global $interface_snv6_arr_cache;
 
 	$wancfg = $config['interfaces'][$interface];
@@ -2955,6 +2956,7 @@ function interface_configure($interface = 'wan', $reloadall = false, $linkupeven
 	}
 
 	/* invalidate interface/ip/sn cache */
+	unset($interface_sn_arr_cache[$realif]);
 	unset($interface_snv6_arr_cache[$realif]);
 
 	$tunnelif = substr($realif, 0, 3);
@@ -4542,15 +4544,27 @@ function find_interface_ipv6_ll($interface) {
 		return null;
 }
 
-function find_interface_subnet($interface) {
-		$interface = trim($interface);
-		if (does_interface_exist($interface)) {
-				$ifinfo = pfSense_get_interface_addresses($interface);
-				if (isset($ifinfo['subnetbits'])) {
-						return $ifinfo['subnetbits'];
-				}
+function find_interface_subnet($interface, $flush = false) {
+	global $interface_sn_arr_cache;
+
+	if (!isset($interface_sn_arr_cache) || !is_array($interface_sn_arr_cache)){
+		$interface_sn_arr_cache = array();
+	}
+
+	$interface = str_replace("\n", "", $interface);
+	if (does_interface_exist($interface) == false)
+		return;
+
+	if (!isset($interface_sn_arr_cache[$interface]['subnetbits']) || $flush) {
+		$ifinfo = pfSense_get_interface_addresses($interface);
+		if (isset($ifinfo['subnetbits'])) {
+			$interface_sn_arr_cache[$interface] = $ifinfo['subnetbits'];
+		} else {
+			return null;
 		}
-		return null;
+	}
+
+	return $interface_sn_arr_cache[$interface];
 }
 
 function find_interface_subnetv6($interface, $flush = false) {