[FL-1922] BLE buffer overflow (#789)

* rpc: increase RPC buffer size. Add get available buffer size API
* bt: add flow control characteristic to serial service
* ble: change updating flow control characteristic logic
* rpc: add buffer is empty callback
* bt: add notification about empty RPC buffer
* ble: add more debug info
* serial_service: add mutex guarding available buffer size
* ble: remove debug logs in serial service

applications/bt/bt_service/bt.c

@@ -81,7 +81,7 @@ Bt* bt_alloc() {
 }
 
 // Called from GAP thread from Serial service
-static void bt_on_data_received_callback(uint8_t* data, uint16_t size, void* context) {
+static uint16_t bt_on_data_received_callback(uint8_t* data, uint16_t size, void* context) {
     furi_assert(context);
     Bt* bt = context;
 
@@ -89,6 +89,7 @@ static void bt_on_data_received_callback(uint8_t* data, uint16_t size, void* con
     if(bytes_processed != size) {
         FURI_LOG_E(BT_SERVICE_TAG, "Only %d of %d bytes processed by RPC", bytes_processed, size);
     }
+    return rpc_session_get_available_size(bt->rpc_session);
 }
 
 // Called from GAP thread from Serial service
@@ -118,6 +119,11 @@ static void bt_rpc_send_bytes_callback(void* context, uint8_t* bytes, size_t byt
     }
 }
 
+static void bt_rpc_buffer_is_empty_callback(void* context) {
+    furi_assert(context);
+    furi_hal_bt_notify_buffer_is_empty();
+}
+
 // Called from GAP thread
 static void bt_on_gap_event_callback(BleEvent event, void* context) {
     furi_assert(context);
@@ -132,9 +138,10 @@ static void bt_on_gap_event_callback(BleEvent event, void* context) {
         FURI_LOG_I(BT_SERVICE_TAG, "Open RPC connection");
         bt->rpc_session = rpc_session_open(bt->rpc);
         rpc_session_set_send_bytes_callback(bt->rpc_session, bt_rpc_send_bytes_callback);
+        rpc_session_set_buffer_is_empty_callback(bt->rpc_session, bt_rpc_buffer_is_empty_callback);
         rpc_session_set_context(bt->rpc_session, bt);
         furi_hal_bt_set_data_event_callbacks(
-            bt_on_data_received_callback, bt_on_data_sent_callback, bt);
+            RPC_BUFFER_SIZE, bt_on_data_received_callback, bt_on_data_sent_callback, bt);
         // Update battery level
         PowerInfo info;
         power_get_info(bt->power, &info);