Add WifiPollDns to reduce blocking

Add WifiPollDns to reduce blocking resolving NTP and/or MQTT server names (#14398)

tasmota/tasmota_support/support_wifi.ino

@@ -729,6 +729,30 @@ void wifiKeepAlive(void) {
 }
 #endif  // ESP8266
 
+bool WifiPollDns(void) {
+  // WiFi.hostByName takes over ten seconds if no DNS server found
+  // This function checks to find the DNS server within 1 second
+  // This is an alternative for ping using less resources
+  WiFiClient DnsClient;
+
+#ifdef ESP8266
+  DnsClient.setTimeout(1000);
+#else
+  DnsClient.setTimeout(1);
+#endif
+  uint32_t i = 3;            // Check DNS1 only (to keep blocking to a minimum of 1 second)
+//  for (i = 3; i < 5; i++) {  // Check DNS1 and DNS2
+    uint32_t dns_address = (!TasmotaGlobal.global_state.eth_down) ? Settings->eth_ipv4_address[i] : Settings->ipv4_address[i];
+    if (DnsClient.connect((IPAddress)dns_address, 53)) {
+      DnsClient.stop();
+      return true;
+    }
+//    AddLog(LOG_LEVEL_DEBUG, PSTR("DNS: Disconnected %_I"), dns_address);
+    AddLog(LOG_LEVEL_DEBUG, PSTR("DNS: Disconnected"));
+//  }
+  return false;
+}
+
 void WifiPollNtp() {
   static uint8_t ntp_sync_minute = 0;
   static uint32_t ntp_run_time = 0;
@@ -775,6 +799,7 @@ uint32_t WifiGetNtp(void) {
 
   char* ntp_server;
   bool resolved_ip = false;
+  if (WifiPollDns()) {
     for (uint32_t i = 0; i <= MAX_NTP_SERVERS; i++) {
       if (ntp_server_id > 2) { ntp_server_id = 0; }
       if (i < MAX_NTP_SERVERS) {
@@ -794,6 +819,7 @@ uint32_t WifiGetNtp(void) {
       }
       ntp_server_id++;
     }
+  }
   if (!resolved_ip) {
     AddLog(LOG_LEVEL_DEBUG, PSTR("NTP: Unable to resolve IP address"));
     return 0;

tasmota/tasmota_xdrv_driver/xdrv_02_9_mqtt.ino

@@ -193,6 +193,17 @@ void MqttDisableLogging(bool state) {
 
 PubSubClient MqttClient;
 
+void MqttSetClientTimeout(void) {
+#ifdef ESP8266
+  // setTimeout in msecs
+  EspClient.setTimeout(Settings->mqtt_wifi_timeout * 100);
+#else
+  // setTimeout in secs
+  uint32_t timeout = (Settings->mqtt_wifi_timeout < 10) ? 1 : Settings->mqtt_wifi_timeout / 10;
+  EspClient.setTimeout(timeout);
+#endif
+}
+
 void MqttInit(void) {
   // Force buffer size since the #define may not be visible from Arduino lib
   MqttClient.setBufferSize(MQTT_MAX_PACKET_SIZE);
@@ -860,6 +871,20 @@ uint16_t MqttConnectCount(void) {
 }
 
 void MqttDisconnected(int state) {
+  /*
+  // Possible values for state - PubSubClient.h
+  Tasmota MQTT_DNS_DISCONNECTED       -5
+  #define MQTT_CONNECTION_TIMEOUT     -4
+  #define MQTT_CONNECTION_LOST        -3
+  #define MQTT_CONNECT_FAILED         -2
+  #define MQTT_DISCONNECTED           -1
+  #define MQTT_CONNECTED               0
+  #define MQTT_CONNECT_BAD_PROTOCOL    1
+  #define MQTT_CONNECT_BAD_CLIENT_ID   2
+  #define MQTT_CONNECT_UNAVAILABLE     3
+  #define MQTT_CONNECT_BAD_CREDENTIALS 4
+  #define MQTT_CONNECT_UNAUTHORIZED    5
+  */
   Mqtt.connected = false;
 
   Mqtt.retry_counter = Settings->mqtt_retry * Mqtt.retry_counter_delay;
@@ -1028,7 +1053,9 @@ void MqttReconnect(void) {
   Response_P(S_LWT_OFFLINE);
 
   if (MqttClient.connected()) { MqttClient.disconnect(); }
-  EspClient.setTimeout(Settings->mqtt_wifi_timeout * 100);
+
+  MqttSetClientTimeout();
+
 #ifdef USE_MQTT_TLS
   if (Mqtt.mqtt_tls) {
     tlsClient->stop();
@@ -1056,6 +1083,11 @@ void MqttReconnect(void) {
     }
   }
 #endif
+
+  if (!WifiPollDns()) {
+    MqttDisconnected(-5);  // MQTT_DNS_DISCONNECTED
+    return;
+  }
   MqttClient.setServer(SettingsText(SET_MQTT_HOST), Settings->mqtt_port);
 
   uint32_t mqtt_connect_time = millis();
@@ -1363,7 +1395,7 @@ void CmndMqttWifiTimeout(void) {
   // Set timeout between 100 and 20000 mSec
   if ((XdrvMailbox.payload >= 100) && (XdrvMailbox.payload <= 20000)) {
     Settings->mqtt_wifi_timeout = XdrvMailbox.payload / 100;
-    EspClient.setTimeout(Settings->mqtt_wifi_timeout * 100);
+    MqttSetClientTimeout();
   }
   ResponseCmndNumber(Settings->mqtt_wifi_timeout * 100);
 }