Add documentation on threading issues with WiFi.onEvent() to examples (#8081)

* Compile error if CONFIG_FREERTOS_HZ != 1000

* add a check at the CMake level, per feedback

* fix a punctuation glitch

* Remove `_Static_assert` per feedback

* add documentation on threading issues with WiFi.onEvent()

* more comments

* thin out comments, add docs

* Update WiFiProv.ino merge conflict issue fixed

* Added the CLK type and MAC from eFuse to Ethernet begin

* Fixed the order and arguments on the Ethernet begin function

---------
Co-authored-by: Pedro Minatel <pedro.minatel@espressif.com>
Co-authored-by: Pedro Minatel <pminatel@gmail.com>

docs/source/api/wifi.rst

@@ -50,6 +50,102 @@ Common API
 
 Here are the common APIs that are used for both modes, AP and STA.
 
+onEvent (and removeEvent)
+*************************
+
+Registers a caller-supplied function to be called when WiFi events
+occur. Several forms are available.
+
+Function pointer callback taking the event ID:
+
+.. code-block:: arduino
+
+    typedef void (*WiFiEventCb)(arduino_event_id_t);
+    wifi_event_id_t onEvent(WiFiEventCb, arduino_event_id_t = ARDUINO_EVENT_MAX);
+
+Function pointer callback taking an event-ID-and-info struct:
+
+.. code-block:: arduino
+
+    typedef struct{
+        arduino_event_id_t event_id;
+        arduino_event_info_t event_info;
+    } arduino_event_t;
+
+    typedef void (*WiFiEventSysCb)(arduino_event_t *);
+    wifi_event_id_t onEvent(WiFiEventSysCb, arduino_event_id_t = ARDUINO_EVENT_MAX);
+
+Callback using ``std::function`` taking event ID and info separately:
+
+.. code-block:: arduino
+
+    typedef std::function<void(arduino_event_id_t, arduino_event_info_t)> WiFiEventFuncCb;
+    wifi_event_id_t onEvent(WiFiEventFuncCb, arduino_event_id_t = ARDUINO_EVENT_MAX);
+
+A similar set of functions are available to remove callbacks:
+
+.. code-block:: arduino
+
+    void removeEvent(WiFiEventCb, arduino_event_id_t = ARDUINO_EVENT_MAX);
+    void removeEvent(WiFiEventSysCb, arduino_event_id_t = ARDUINO_EVENT_MAX);
+    void removeEvent(wifi_event_id_t = ARDUINO_EVENT_MAX);
+
+In all cases, the subscribing function accepts an optional event type to
+invoke the callback only for that specific event; with the default
+``ARDUINO_EVENT_MAX``, the callback will be invoked for all WiFi events.
+
+Any callback function is given the event type in a parameter.
+Some of the possible callback function formats also take an
+``arduino_event_info_t`` (or use ``arduino_event_t`` which includes both
+ID and info) which is a union of structs with additional information
+about different event types.
+
+See
+`WiFiGeneric.h <https://github.com/espressif/arduino-esp32/blob/master/libraries/WiFi/src/WiFiGeneric.h>`_
+for the list of event types and "info" substructures, and also see a full
+example of event handling: `events example`_.
+
+.. warning::
+
+    Event callback functions are invoked on a separate
+    `thread <https://en.wikipedia.org/wiki/Thread_(computing)>`_
+    (`FreeRTOS task <https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/system/freertos_idf.html#tasks>`_)
+    independent of the main application thread that runs ``setup()`` and
+    ``loop()``. Callback functions must therefore be
+    `thread-safe <https://en.wikipedia.org/wiki/Thread_safety>`_;
+    they must not access shared/global variables directly without locking,
+    and must only call similarly thread-safe functions.
+
+    Some core operations like ``Serial.print()`` are thread-safe but many
+    functions are not. Notably, ``WiFi.onEvent()`` and ``WiFi.removeEvent()``
+    are not thread-safe and should never be invoked from a callback thread.
+
+setHostname (and getHostname)
+*****************************
+
+Sets the name the DHCP client uses to identify itself. In a typical network
+setup this will be the name that shows up in the Wi-Fi router's device list.
+The hostname must be no longer than 32 characters.
+
+.. code-block:: arduino
+
+    setHostname(const char *hostname);
+
+If the hostname is never specified, a default one will be assigned based
+on the chip type and MAC address. The current hostname (default or custom)
+may be retrieved:
+
+.. code-block:: arduino
+
+    const char *getHostname();
+
+.. warning::
+
+    The ``setHostname()`` function must be called BEFORE WiFi is started with
+    ``WiFi.begin()``, ``WiFi.softAP()``, ``WiFi.mode()``, or ``WiFi.run()``.
+    To change the name, reset WiFi with ``WiFi.mode(WIFI_MODE_NULL)``,
+    then proceed with ``WiFi.setHostname(...)`` and restart WiFi from scratch.
+
 useStaticBuffers
 ****************
 
@@ -552,6 +648,8 @@ To see how to use the ``WiFiScan``, take a look at the ``WiFiScan.ino`` example
 Examples
 --------
 
+`Complete list of WiFi examples <https://github.com/espressif/arduino-esp32/tree/master/libraries/WiFi/examples>`_.
+
 .. _ap example:
 
 Wi-Fi AP Example
@@ -568,5 +666,10 @@ Wi-Fi STA Example
 .. literalinclude:: ../../../libraries/WiFi/examples/WiFiClient/WiFiClient.ino
     :language: arduino
 
-References
-----------
+.. _events example:
+
+Wi-Fi Events Example
+********************
+
+.. literalinclude:: ../../../libraries/WiFi/examples/WiFiClientEvents/WiFiClientEvents.ino
+    :language: arduino

libraries/Ethernet/examples/ETH_LAN8720/ETH_LAN8720.ino

@@ -16,12 +16,14 @@
 
 static bool eth_connected = false;
 
-void onEvent(arduino_event_id_t event, arduino_event_info_t info)
+// WARNING: WiFiEvent is called from a separate FreeRTOS task (thread)!
+void WiFiEvent(WiFiEvent_t event)
 {
   switch (event) {
     case ARDUINO_EVENT_ETH_START:
       Serial.println("ETH Started");
-      //set eth hostname here
+      // The hostname must be set after the interface is started, but needs
+      // to be set before DHCP, so set it from the event handler thread.
       ETH.setHostname("esp32-ethernet");
       break;
     case ARDUINO_EVENT_ETH_CONNECTED:
@@ -72,11 +74,10 @@ void testClient(const char * host, uint16_t port)
 void setup()
 {
   Serial.begin(115200);
-  WiFi.onEvent(onEvent);
+  WiFi.onEvent(WiFiEvent);  // Will call WiFiEvent() from another thread.
   ETH.begin();
 }
 
-
 void loop()
 {
   if (eth_connected) {

libraries/Ethernet/examples/ETH_TLK110/ETH_TLK110.ino

@@ -14,12 +14,14 @@
 
 static bool eth_connected = false;
 
-void onEvent(arduino_event_id_t event, arduino_event_info_t info)
+// WARNING: WiFiEvent is called from a separate FreeRTOS task (thread)!
+void WiFiEvent(WiFiEvent_t event)
 {
   switch (event) {
     case ARDUINO_EVENT_ETH_START:
       Serial.println("ETH Started");
-      //set eth hostname here
+      // The hostname must be set after the interface is started, but needs
+      // to be set before DHCP, so set it from the event handler thread.
       ETH.setHostname("esp32-ethernet");
       break;
     case ARDUINO_EVENT_ETH_CONNECTED:
@@ -70,7 +72,7 @@ void testClient(const char * host, uint16_t port)
 void setup()
 {
   Serial.begin(115200);
-  WiFi.onEvent(onEvent);
+  WiFi.onEvent(WiFiEvent);  // Will call WiFiEvent() from another thread.
   ETH.begin(ETH_TYPE, ETH_ADDR, ETH_MDC_PIN, ETH_MDIO_PIN, ETH_POWER_PIN, ETH_CLK_MODE);
 }
 

libraries/RainMaker/examples/RMakerCustom/RMakerCustom.ino

@@ -24,6 +24,7 @@ bool dimmer_state = true;
 // But, you can also define custom devices using the 'Device' base class object, as shown here
 static Device *my_device = NULL;
 
+// WARNING: sysProvEvent is called from a separate FreeRTOS task (thread)!
 void sysProvEvent(arduino_event_t *sys_event)
 {
     switch (sys_event->event_id) {
@@ -105,7 +106,7 @@ void setup()
 
     RMaker.start();
 
-    WiFi.onEvent(sysProvEvent);
+    WiFi.onEvent(sysProvEvent);  // Will call sysProvEvent() from another thread.
 #if CONFIG_IDF_TARGET_ESP32S2
     WiFiProv.beginProvision(WIFI_PROV_SCHEME_SOFTAP, WIFI_PROV_SCHEME_HANDLER_NONE, WIFI_PROV_SECURITY_1, pop, service_name);
 #else

libraries/RainMaker/examples/RMakerCustomAirCooler/RMakerCustomAirCooler.ino

@@ -40,6 +40,7 @@ bool power_state = true;
 // But, you can also define custom devices using the 'Device' base class object, as shown here
 static Device *my_device = NULL;
 
+// WARNING: sysProvEvent is called from a separate FreeRTOS task (thread)!
 void sysProvEvent(arduino_event_t *sys_event)
 {
     switch (sys_event->event_id) {
@@ -170,7 +171,7 @@ void setup()
 
     RMaker.start();
 
-    WiFi.onEvent(sysProvEvent);
+    WiFi.onEvent(sysProvEvent);  // Will call sysProvEvent() from another thread.
 #if CONFIG_IDF_TARGET_ESP32S2
     WiFiProv.beginProvision(WIFI_PROV_SCHEME_SOFTAP, WIFI_PROV_SCHEME_HANDLER_NONE, WIFI_PROV_SECURITY_1, pop, service_name);
 #else

libraries/RainMaker/examples/RMakerSonoffDualR3/RMakerSonoffDualR3.ino

@@ -34,6 +34,7 @@ LightSwitch switch_ch2 = {gpio_switch2, false};
 static Switch *my_switch1 = NULL;
 static Switch *my_switch2 = NULL;
 
+// WARNING: sysProvEvent is called from a separate FreeRTOS task (thread)!
 void sysProvEvent(arduino_event_t *sys_event)
 {
     switch (sys_event->event_id) {
@@ -160,7 +161,7 @@ void setup()
     Serial.printf("\nStarting ESP-RainMaker\n");
     RMaker.start();
 
-    WiFi.onEvent(sysProvEvent);
+    WiFi.onEvent(sysProvEvent);  // Will call sysProvEvent() from another thread.
 #if CONFIG_IDF_TARGET_ESP32
     WiFiProv.beginProvision(WIFI_PROV_SCHEME_BLE, WIFI_PROV_SCHEME_HANDLER_FREE_BTDM, WIFI_PROV_SECURITY_1, pop, service_name);
 #else

libraries/RainMaker/examples/RMakerSwitch/RMakerSwitch.ino

@@ -25,6 +25,7 @@ bool switch_state = true;
 // fan, temperaturesensor.
 static Switch *my_switch = NULL;
 
+// WARNING: sysProvEvent is called from a separate FreeRTOS task (thread)!
 void sysProvEvent(arduino_event_t *sys_event)
 {
     switch (sys_event->event_id) {
@@ -107,7 +108,7 @@ void setup()
 
     RMaker.start();
 
-    WiFi.onEvent(sysProvEvent);
+    WiFi.onEvent(sysProvEvent);  // Will call sysProvEvent() from another thread.
 #if CONFIG_IDF_TARGET_ESP32S2
     WiFiProv.beginProvision(WIFI_PROV_SCHEME_SOFTAP, WIFI_PROV_SCHEME_HANDLER_NONE,
                             WIFI_PROV_SECURITY_1, pop, service_name);

libraries/WiFi/examples/FTM/FTM_Initiator/FTM_Initiator.ino

@@ -28,6 +28,7 @@ SemaphoreHandle_t ftmSemaphore;
 bool ftmSuccess = true;
 
 // FTM report handler with the calculated data from the round trip
+// WARNING: This function is called from a separate FreeRTOS task (thread)!
 void onFtmReport(arduino_event_t *event) {
   const char * status_str[5] = {"SUCCESS", "UNSUPPORTED", "CONF_REJECTED", "NO_RESPONSE", "FAIL"};
   wifi_event_ftm_report_t * report = &event->event_info.wifi_ftm_report;
@@ -62,7 +63,7 @@ void setup() {
   // Create binary semaphore (initialized taken and can be taken/given from any thread/ISR)
   ftmSemaphore = xSemaphoreCreateBinary();
   
-  // Listen for FTM Report events
+  // Will call onFtmReport() from another thread with FTM Report events.
   WiFi.onEvent(onFtmReport, ARDUINO_EVENT_WIFI_FTM_REPORT);
   
   // Connect to AP that has FTM Enabled

libraries/WiFi/examples/WPS/WPS.ino

@@ -61,6 +61,7 @@ String wpspin2string(uint8_t a[]){
   return (String)wps_pin;
 }
 
+// WARNING: WiFiEvent is called from a separate FreeRTOS task (thread)!
 void WiFiEvent(WiFiEvent_t event, arduino_event_info_t info){
   switch(event){
     case ARDUINO_EVENT_WIFI_STA_START:
@@ -103,7 +104,7 @@ void setup(){
   Serial.begin(115200);
   delay(10);
   Serial.println();
-  WiFi.onEvent(WiFiEvent);
+  WiFi.onEvent(WiFiEvent);  // Will call WiFiEvent() from another thread.
   WiFi.mode(WIFI_MODE_STA);
   Serial.println("Starting WPS");
   wpsInitConfig();

libraries/WiFi/examples/WiFiBlueToothSwitch/WiFiBlueToothSwitch.ino

@@ -73,6 +73,7 @@ void onButton(){
   delay(100);
 }
 
+// WARNING: WiFiEvent is called from a separate FreeRTOS task (thread)!
 void WiFiEvent(WiFiEvent_t event){
     switch(event) {
         case ARDUINO_EVENT_WIFI_AP_START:
@@ -112,7 +113,7 @@ void WiFiEvent(WiFiEvent_t event){
 void setup() {
     Serial.begin(115200);
     pinMode(0, INPUT_PULLUP);
-    WiFi.onEvent(WiFiEvent);
+    WiFi.onEvent(WiFiEvent);  // Will call WiFiEvent() from another thread.
     Serial.print("ESP32 SDK: ");
     Serial.println(ESP.getSdkVersion());
     Serial.println("Press the button to select the next mode");

libraries/WiFi/examples/WiFiClientEvents/WiFiClientEvents.ino

@@ -41,7 +41,7 @@
 const char* ssid     = "your-ssid";
 const char* password = "your-password";
 
-
+// WARNING: This function is called from a separate FreeRTOS task (thread)!
 void WiFiEvent(WiFiEvent_t event)
 {
     Serial.printf("[WiFi-event] event: %d\n", event);
@@ -132,6 +132,7 @@ void WiFiEvent(WiFiEvent_t event)
         default: break;
     }}
 
+// WARNING: This function is called from a separate FreeRTOS task (thread)!
 void WiFiGotIP(WiFiEvent_t event, WiFiEventInfo_t info)
 {
     Serial.println("WiFi connected");
@@ -148,7 +149,8 @@ void setup()
 
     delay(1000);
 
-    // Examples of different ways to register wifi events
+    // Examples of different ways to register wifi events;
+    // these handlers will be called from another thread.
     WiFi.onEvent(WiFiEvent);
     WiFi.onEvent(WiFiGotIP, WiFiEvent_t::ARDUINO_EVENT_WIFI_STA_GOT_IP);
     WiFiEventId_t eventID = WiFi.onEvent([](WiFiEvent_t event, WiFiEventInfo_t info){

libraries/WiFi/examples/WiFiIPv6/WiFiIPv6.ino

@@ -64,16 +64,15 @@ void wifiConnectedLoop(){
   delay(9000);
 }
 
+// WARNING: WiFiEvent is called from a separate FreeRTOS task (thread)!
 void WiFiEvent(WiFiEvent_t event){
     switch(event) {
-
         case ARDUINO_EVENT_WIFI_AP_START:
             //can set ap hostname here
             WiFi.softAPsetHostname(AP_SSID);
             //enable ap ipv6 here
             WiFi.softAPenableIpV6();
             break;
-
         case ARDUINO_EVENT_WIFI_STA_START:
             //set sta hostname here
             WiFi.setHostname(AP_SSID);
@@ -106,7 +105,7 @@ void WiFiEvent(WiFiEvent_t event){
 void setup(){
     Serial.begin(115200);
     WiFi.disconnect(true);
-    WiFi.onEvent(WiFiEvent);
+    WiFi.onEvent(WiFiEvent);  // Will call WiFiEvent() from another thread.
     WiFi.mode(WIFI_MODE_APSTA);
     WiFi.softAP(AP_SSID);
     WiFi.begin(STA_SSID, STA_PASS);

libraries/WiFi/examples/WiFiUDPClient/WiFiUDPClient.ino

@@ -47,7 +47,7 @@ void connectToWiFi(const char * ssid, const char * pwd){
   // delete old config
   WiFi.disconnect(true);
   //register event handler
-  WiFi.onEvent(WiFiEvent);
+  WiFi.onEvent(WiFiEvent);  // Will call WiFiEvent() from another thread.
   
   //Initiate connection
   WiFi.begin(ssid, pwd);
@@ -55,7 +55,7 @@ void connectToWiFi(const char * ssid, const char * pwd){
   Serial.println("Waiting for WIFI connection...");
 }
 
-//wifi event handler
+// WARNING: WiFiEvent is called from a separate FreeRTOS task (thread)!
 void WiFiEvent(WiFiEvent_t event){
     switch(event) {
       case ARDUINO_EVENT_WIFI_STA_GOT_IP:

libraries/WiFiProv/examples/WiFiProv/README.md

@@ -10,7 +10,8 @@ This example allows Arduino users to choose either BLE or SOFTAP as the mode of
 
 ### WiFi.onEvent()
 
-Using this API, users can register to receive WiFi Events and Provisioning Events.
+This API can be used to register a function to be called from another
+thread for WiFi Events and Provisioning Events.
 
 ### WiFi.beginProvision()
 

libraries/WiFiProv/examples/WiFiProv/WiFiProv.ino

@@ -11,13 +11,13 @@ Note: This sketch takes up a lot of space for the app and may not be able to fla
 #include "WiFiProv.h"
 #include "WiFi.h"
 
-// #define USE_SOFT_AP // Uncomment if you want to enforce using Soft AP method instead of BLE
-
-const char * pop = "abcd1234"; // Proof of possession - otherwise called a PIN - string provided by the device, entered by user in the phone app
+// #define USE_SOFT_AP // Uncomment if you want to enforce using the Soft AP method instead of BLE
+const char * pop = "abcd1234"; // Proof of possession - otherwise called a PIN - string provided by the device, entered by the user in the phone app
 const char * service_name = "PROV_123"; // Name of your device (the Espressif apps expects by default device name starting with "Prov_")
 const char * service_key = NULL; // Password used for SofAP method (NULL = no password needed)
 bool reset_provisioned = true; // When true the library will automatically delete previously provisioned data.
 
+// WARNING: SysProvEvent is called from a separate FreeRTOS task (thread)!
 void SysProvEvent(arduino_event_t *sys_event)
 {
     switch (sys_event->event_id) {