interfaces: require PPP interface to be in up state (#7692)

Otherwise the check will "approve" an interface that is not yet up, and subsequent
calls to fetch the gateway of that interface might fail if issued to quickly thereafter.

(cherry picked from commit 62a09bf)
(cherry picked from commit 7c9b97a)
(cherry picked from commit 4b77d13)
(cherry picked from commit 10aa787)

src/etc/inc/interfaces.inc

@@ -1239,6 +1239,9 @@ EOD;
         @unlink('/var/spool/lock/LCK..' . basename($port));
     }
 
+    /* precaution for post-start 'up' check */
+    legacy_interface_flags($ifcfg['if'], 'down', false);
+
     /* fire up mpd */
     mwexecf(
         '/usr/local/sbin/mpd5 -b -d /var/etc -f %s -p %s -s ppp %s',
@@ -1248,17 +1251,21 @@ EOD;
     /* appease netgraph by setting the correct node name */
     shell_safe('/usr/sbin/daemon -f /usr/local/opnsense/scripts/interfaces/ppp-rename.sh %s %s %s', [$interface, $ifcfg['if'], $ppp['type']]);
 
-    /* wait for up to 10 seconds for the interface to appear (ppp(oe)) */
+    /* wait for functional device */
+    $max = 20;
     $i = 0;
-    while ($i < 10) {
-        exec("/sbin/ifconfig " . escapeshellarg($ppp['if']) . " 2>&1", $out, $ret);
-        if ($ret == 0) {
+    while ($i < $max) {
+        sleep(1);
+        if (does_interface_exist($ifcfg['if'], 'up')) {
             break;
         }
-        sleep(1);
         $i++;
     }
 
+    if ($i >= $max) {
+        log_msg("interface_ppps_configure() waiting threshold exceeded - device {$ifcfg['if']} is still not up", LOG_WARNING);
+    }
+
     switch ($ppp['type']) {
         case 'pppoe':
             /* automatically change MAC address if parent interface changes */