WiFiClient/WiFiUDP list needs mutex protection

We track a list of all connections to allow the WiFi.stopAll() call
(used for updates).  This linked list needs to be protected from multiple
cores updating it in parallel under FreeRTOS.  Add a mutex around the ops.

Author: earlephilhower

libraries/WiFi/src/WiFiClient.cpp

@@ -57,7 +57,12 @@ bool WiFiClient::getDefaultSync() {
 
 template<>
 WiFiClient* SList<WiFiClient>::_s_first = 0;
-
+#ifdef __FREERTOS
+template<>
+SemaphoreHandle_t SList<WiFiClient>::_s_first_lock = 0;
+template<>
+bool SList<WiFiClient>::_s_first_lock_created = false;
+#endif
 
 WiFiClient::WiFiClient()
     : _client(0), _owned(0) {

libraries/WiFi/src/WiFiUdp.cpp

@@ -36,6 +36,13 @@
 
 template<>
 WiFiUDP* SList<WiFiUDP>::_s_first = 0;
+#ifdef __FREERTOS
+template<>
+SemaphoreHandle_t SList<WiFiUDP>::_s_first_lock = 0;
+template<>
+bool SList<WiFiUDP>::_s_first_lock_created = false;
+#endif
+
 
 /* Constructor */
 WiFiUDP::WiFiUDP() : _ctx(0), _multicast(false), _dirty(false) {

libraries/WiFi/src/include/slist.h

@@ -1,6 +1,11 @@
 #ifndef SLIST_H
 #define SLIST_H
 
+#ifdef __FREERTOS
+#include "FreeRTOS.h"
+#include "semphr.h"
+#endif
+
 template<typename T>
 class SList {
 public:
@@ -9,27 +14,58 @@ class SList {
 protected:
 
     static void _add(T* self) {
+#ifdef __FREERTOS
+        if (!_s_first_lock_created) {
+            _s_first_lock_created = true;
+            _s_first_lock = xSemaphoreCreateMutex();
+        }
+        xSemaphoreTake(_s_first_lock, portMAX_DELAY);
+#endif
         T* tmp = _s_first;
         _s_first = self;
         self->_next = tmp;
+#ifdef __FREERTOS
+        xSemaphoreGive(_s_first_lock);
+#endif
     }
 
     static void _remove(T* self) {
+#ifdef __FREERTOS
+        if (!_s_first_lock_created) {
+            _s_first_lock_created = true;
+            _s_first_lock = xSemaphoreCreateMutex();
+        }
+        xSemaphoreTake(_s_first_lock, portMAX_DELAY);
+#endif
+
         if (_s_first == self) {
             _s_first = self->_next;
             self->_next = 0;
+#ifdef __FREERTOS
+            xSemaphoreGive(_s_first_lock);
+#endif
             return;
         }
 
         for (T* prev = _s_first; prev->_next; prev = prev->_next) {
             if (prev->_next == self) {
                 prev->_next = self->_next;
                 self->_next = 0;
+#ifdef __FREERTOS
+                xSemaphoreGive(_s_first_lock);
+#endif
                 return;
             }
         }
+#ifdef __FREERTOS
+        xSemaphoreGive(_s_first_lock);
+#endif
     }
 
+#ifdef __FREERTOS
+    static SemaphoreHandle_t _s_first_lock;
+    static bool _s_first_lock_created;
+#endif
     static T* _s_first;
     T* _next;
 };