ntp: more checks when using DHCP servers

- make sure server is never empty
- disable dhcp option when server *is* empty
- update configure() to detect both tz and server changes

code/espurna/config/general.h

@@ -1629,6 +1629,10 @@
 #define NTP_WAIT_FOR_SYNC 1 // Do not report any datetime until NTP sync'ed
 #endif
 
+#ifndef NTP_DHCP_SERVER
+#define NTP_DHCP_SERVER 1 // Automatically replace the NTP server value with the one received with the DHCP packet
+#endif
+
 // -----------------------------------------------------------------------------
 // ALEXA
 // -----------------------------------------------------------------------------

code/espurna/ntp.cpp

@@ -166,13 +166,15 @@ void _ntpWebSocketOnConnected(JsonObject& root) {
 
 #endif
 
-// TODO: mention possibility of multiple servers
 String _ntpGetServer() {
  String server;
 
  server = sntp_getservername(0);
  if (!server.length()) {
- server = IPAddress(sntp_getserver(0)).toString();
+ auto ip = IPAddress(sntp_getserver(0));
+ if (ip) {
+ server = ip.toString();
+ }
  }
 
  return server;
@@ -220,19 +222,32 @@ void _ntpReport() {
 }
 
 void _ntpConfigure() {
+ // Ignore or accept the DHCP SNTP option
+ // When enabled, it is possible that lwip will replace the NTP server pointer from under us
+ sntp_servermode_dhcp(getSetting("ntpDhcp", 1 == NTP_DHCP_SERVER));
+
  // Note: TZ_... provided by the Core are already wrapped with PSTR(...)
+ // but, String() already handles every char pointer as a flash-string
  const auto cfg_tz = getSetting("ntpTZ", NTP_TIMEZONE);
  const char* active_tz = getenv("TZ");
- if (cfg_tz != active_tz) {
+
+ bool changed { cfg_tz != active_tz };
+ if (changed) {
  setenv("TZ", cfg_tz.c_str(), 1);
  tzset();
  }
 
  const auto cfg_server = getSetting("ntpServer", F(NTP_SERVER));
  const auto active_server = _ntpGetServer();
- if (cfg_tz != active_tz) {
+ changed = (cfg_server != active_server) || changed;
+
+ // We skip configTime() API since we already set the TZ just above
+ // (and most of the time we expect NTP server to proxy to multiple servers instead of defining more than one here)
+ if (changed) {
+ sntp_stop();
  _ntp_server = cfg_server;
- configTime(cfg_tz.c_str(), _ntp_server.c_str());
+ sntp_setservername(0, _ntp_server.c_str());
+ sntp_init();
  DEBUG_MSG_P(PSTR("[NTP] Server: %s, TZ: %s\n"), cfg_server.c_str(), cfg_tz.length() ? cfg_tz.c_str() : "UTC0");
  }
 }
@@ -428,8 +443,20 @@ void ntpSetup() {
  // make sure our logic does know about the actual server
  // in case dhcp sends out ntp settings
  static WiFiEventHandler on_sta = WiFi.onStationModeGotIP([](WiFiEventStationModeGotIP) {
- const auto server = _ntpGetServer();
- if (sntp_enabled() && (!_ntp_server.length() || (server != _ntp_server))) {
+ if (!sntp_enabled()) {
+ return;
+ }
+
+ auto server = _ntpGetServer();
+ if (!server.length()) {
+ DEBUG_MSG_P(PSTR("[NTP] Updating `ntpDhcp` to ignore the DHCP values\n"));
+ setSetting("ntpDhcp", "0");
+ sntp_servermode_dhcp(0);
+ schedule_function(_ntpConfigure);
+ return;
+ }
+
+ if (!_ntp_server.length() || (server != _ntp_server)) {
  DEBUG_MSG_P(PSTR("[NTP] Updating `ntpServer` setting from DHCP: %s\n"), server.c_str());
  _ntp_server = server;
  setSetting("ntpServer", server);