gui, menu: one LVGL lock, shared by the launcher and the pause menu

components/gui/include/gui.hpp

@@ -94,5 +94,5 @@ class Gui {
       .callback = std::bind(&Gui::update, this),
   }};
   espp::Logger logger_;
-  mutable std::recursive_mutex mutex_;
+  std::recursive_mutex &mutex_{Tab5Emu::lvgl_mutex()}; // shared: LVGL is not thread-safe
 };

components/gui/src/gui.cpp

@@ -39,6 +39,8 @@ Gui::~Gui() {
 void Gui::pause() {
   paused_ = true;
   task_.stop();
+  // wait out a timer callback already in lv_timer_handler() (stop() does not)
+  std::lock_guard<std::recursive_mutex> lk(mutex_);
 }
 
 void Gui::resume() {

components/menu/include/menu.hpp

@@ -92,5 +92,5 @@ class Menu {
       .callback = std::bind(&Menu::update, this),
   }};
   espp::Logger logger_;
-  std::recursive_mutex mutex_;
+  std::recursive_mutex &mutex_{Tab5Emu::lvgl_mutex()}; // shared: LVGL is not thread-safe
 };

components/menu/src/menu.cpp

@@ -36,6 +36,8 @@ void Menu::select_slot(int slot) {
 void Menu::pause() {
   paused_ = true;
   task_.stop();
+  // wait out a timer callback already in lv_timer_handler() (stop() does not)
+  std::lock_guard<std::recursive_mutex> lk(mutex_);
 }
 
 void Menu::resume() {

components/tab5-emu/include/tab5-emu.hpp

@@ -194,6 +194,13 @@ class Tab5Emu : public espp::BaseComponent {
   /// focused slider / dropdown), A = enter, B = escape. Menus attach their
   /// lv_group to it (lv_indev_set_group) when shown.
   lv_indev_t *keypad_indev() const { return keypad_indev_; }
+  /// The one lock for LVGL (built without LV_USE_OS, so it is not
+  /// thread-safe): the launcher's and the pause menu's timers and their
+  /// event-manager callbacks (volume) all touch it from different tasks.
+  static std::recursive_mutex &lvgl_mutex() {
+    static std::recursive_mutex m;
+    return m;
+  }
   /// Stop the USB host library (keyboard / mouse stop working). The USB
   /// controller is shared with device mode (see initialize_usb_msc()).
   void deinitialize_usb_host();