feat: Bridge.begin waits to be greenlighted by MPU up to timeout

Author: eigen-value

src/bridge.h

@@ -22,6 +22,7 @@
 #define UPDATE_THREAD_PRIORITY      5
 
 #define DEFAULT_SERIAL_BAUD         115200
+#define BEGIN_TIMEOUT_MS               5000
 
 #include <zephyr/kernel.h>
 #include <zephyr/sys/atomic.h>
@@ -118,6 +119,8 @@ class BridgeClass {
     k_thread_stack_t *upd_stack_area{};
     struct k_thread upd_thread_data{};
 
+    static struct k_sem cleared_sem;
+
     bool started = false;
 
 public:
@@ -142,6 +145,7 @@ class BridgeClass {
         k_mutex_init(&read_mutex);
         k_mutex_init(&write_mutex);
         k_mutex_init(&bridge_mutex);
+        k_sem_init(&cleared_sem, 0, 1);     //
 
         if (is_started()) return true;
 
@@ -151,6 +155,9 @@ class BridgeClass {
         client = new RPCClient(*transport);
         server = new RPCServer(*transport);
 
+        // The service method greenLight is not registered to the MPU, but it is provided for signaling
+        server->bind("greenLight", greenLight);
+
         upd_stack_area = k_thread_stack_alloc(UPDATE_THREAD_STACK_SIZE, 0);
         upd_tid = k_thread_create(&upd_thread_data, upd_stack_area,
                                 UPDATE_THREAD_STACK_SIZE,
@@ -159,6 +166,10 @@ class BridgeClass {
                                 UPDATE_THREAD_PRIORITY, 0, K_NO_WAIT);
         k_thread_name_set(upd_tid, "bridge");
 
+        // This should not be mutexed and go before the call to RESET_METHOD
+        // BEGIN_TIMEOUT_MS ensures compatibility with older versions that do not use signaling
+        k_sem_take(&cleared_sem, K_MSEC(BEGIN_TIMEOUT_MS));     // wait to be cleared by the other side
+
         k_mutex_lock(&bridge_mutex, K_FOREVER);
         bool res = false;
         started = call(RESET_METHOD).result(res) && res;
@@ -234,6 +245,10 @@ class BridgeClass {
 
 private:
 
+    static void greenLight() {
+        k_sem_give(&cleared_sem);
+    }
+
     void update_safe() {
 
         // Lock read mutex