Lock scheduler items while modifying them (#4410)

* Cosmetic fixes to scheduler code

* Add generic Mutex API

* Lock scheduler items while modifying them

* Always defer MQTT callbacks on Arduino

esphome/core/helpers.cpp

@@ -393,6 +393,18 @@ void hsv_to_rgb(int hue, float saturation, float value, float &red, float &green
 }
 
 // System APIs
+#if defined(USE_ESP8266)
+// ESP8266 doesn't have mutexes, but that shouldn't be an issue as it's single-core and non-preemptive OS.
+Mutex::Mutex() {}
+void Mutex::lock() {}
+bool Mutex::try_lock() { return true; }
+void Mutex::unlock() {}
+#elif defined(USE_ESP32) || defined(USE_RP2040)
+Mutex::Mutex() { handle_ = xSemaphoreCreateMutex(); }
+void Mutex::lock() { xSemaphoreTake(this->handle_, portMAX_DELAY); }
+bool Mutex::try_lock() { return xSemaphoreTake(this->handle_, 0) == pdTRUE; }
+void Mutex::unlock() { xSemaphoreGive(this->handle_); }
+#endif
 
 #if defined(USE_ESP8266)
 IRAM_ATTR InterruptLock::InterruptLock() { state_ = xt_rsil(15); }