Android: the save on the way to the background looks unreachable (SDL doesn't queue SDL_EVENT_WILL_ENTER_BACKGROUND) #767

ui_run in runtime/nvn/vulkan/device.c (line 2150 at d5344d7, added in 202916d) saves the pipeline cache and the settings when the app goes to the background:

while (SDL_WaitEventTimeout(&e, 16)) {
    ...
#ifdef __ANDROID__
    if (e.type == SDL_EVENT_WILL_ENTER_BACKGROUND && g_vk.dev) {
        vk_pipeline_cache_save();
        settings_save();
    }
#endif

From reading SDL, that event never comes out of the queue, so the block is dead code.

Why

SDL_SendAppEvent (src/events/SDL_events.c) does not queue the six app lifecycle events (TERMINATING, LOW_MEMORY, WILL_ENTER_BACKGROUND, DID_ENTER_BACKGROUND, WILL_ENTER_FOREGROUND, DID_ENTER_FOREGROUND). It only calls the event watchers:

// We won't actually queue this event, it needs to be handled in this call stack by an event watcher
SDL_CallEventWatchers(&event);

The function is the same in 3.4.16, which android/deps/build_deps.sh builds, and in 3.4.18. On Android the path is Android_PumpEvents -> Android_OnPause -> SDL_OnApplicationWillEnterBackground -> SDL_SendAppEvent.

The Android build installs no watcher or filter: the only SDL_AddEventWatch in the runtime is the iOS one in runtime/nvn/metal.m (background_watch), which exists for this same reason.

What is lost

When the system ends the app in the background:

• options changed with the menu still open (the menu saves on close) - the case the comment there names;
• pipelines compiled since the last save: the cache is otherwise written at the end of the shader preparation (renderer_prepare_end) and at exit (backend_quit).

Not checked on a device

I have no Android device set up, so this is from the source only. A log line inside the block would settle it: send the app to the background and the line should be missing from lasrecomp.log.

Fix

The same two calls from an event watcher, installed where the window is made:

static bool SDLCALL background_watch(void *ud, SDL_Event *e) {
    if (e->type == SDL_EVENT_WILL_ENTER_BACKGROUND && g_vk.dev) {
        vk_pipeline_cache_save();
        settings_save();
    }
    return true;
}
...
SDL_AddEventWatch(background_watch, NULL);

On Android the watcher runs on the thread that pumps events, inside SDL_WaitEventTimeout (SDL_WaitEventTimeoutNS calls Android_PumpEvents directly), so it is the same thread as the block it replaces.

Sharing it with iOS

The review of #238 suggested one watcher under PLAT_TOUCH for both ports (settings, rumble, audio, the hang watch's pause), with a hook for the backend. I have left that out of the iOS follow-ups: it changes the Android path, which I can't test. If you'd like it shared, the iOS half is background_watch in metal.m; I can do that part once the Android side is confirmed on a device.

(Written with GitHub Copilot.)