Can't exit the runtime immediately after `emscripten_cancel_main_loop` without errors when using assertions

[NoctemCat]

Version of emscripten/emsdk:
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 6.0.5 (1db5137)
clang version 24.0.0git (https:/github.com/llvm/llvm-project abd3b3a1445b5a8eeffae5c3912883faa9287fb7)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/noctemcat/programs/emsdk/upstream/bin

Failing example:

emcmake cmake -S . -B build && cmake --build build

cmake file

cmake_minimum_required(VERSION 3.19)
project(main_loop_exit LANGUAGES CXX)
add_executable(app main.cpp)
set_target_properties(app 
  PROPERTIES 
    CXX_STANDARD 17
    CXX_STANDARD_REQUIRED ON
    CXX_EXTENSIONS OFF
    SUFFIX ".html"
    COMPILE_OPTIONS "-Wall;-Wextra;-O3"
    LINK_OPTIONS "-sEXIT_RUNTIME=1;-sASSERTIONS=2;-O3"
)

Edit: Forgot to test with -O0 when creating the issue, after testing the results are the same as -O3.

---

Error 1:

Similar pattern is used in test_emscripten_main_loop.c, test_emscripten_main_loop_setimmediate.c, test_emscripten_main_loop_settimeout.c, canvas_animate_resize.c, test_sdl2_mixer_wav.c, emscripten_api_browser.c, webgl_sample_query.c, emscripten_api_browser_infloop.cpp, test_sockets_select_server_closes_connection_client_rw.c, test_emscripten_async_wget2.cpp, test/core/pthread/exceptions.cpp, test_sockets_select_server_down_client.c, webgl_timer_query.c, etc. (I got tired of searching for them). It errored, but I think it exited the runtime after it, not fully sure.

Error:
app.js:1 program exited (with status: 0), but keepRuntimeAlive() is set (counter=1) due to an async operation, so halting execution but not exiting the runtime or preventing further async execution (you can use emscripten_force_exit, if you want to force a true shutdown)

code

void loop() {
    std::cout << "called loop \n";
    emscripten_cancel_main_loop();
    exit(0);
}
int main() {
    emscripten_set_main_loop(&loop, -1, false);
    return 99;
}

---

Error 2:

Similar pattern is used in main_thread_join.c, test_sdl2_mixer_music.c. I really expected the forced version to work better instead of abort.

app.js:1 Aborted(Assertion failed)
app.js:1 Uncaught RuntimeError: Aborted(Assertion failed)
    at abort (app.js:1:11170)
    at assert (app.js:1:5020)
    at runtimeKeepalivePop (app.js:1:21495)
    at checkIsRunning (app.js:1:21899)
    at MainLoop_runner (app.js:1:23080)

code

void loop() {
    std::cout << "called loop \n";
    emscripten_cancel_main_loop();
    emscripten_force_exit(0);
}
int main() {
    emscripten_set_main_loop(&loop, -1, false);
    return 99;
}

---

Error 3:

The error text is the same as error 1, but it loops. I kinda missed this possible usage myself, but it was used in webrtc_host.c.

code

void loop() {
    std::cout << "called loop \n";
    exit(0);
    emscripten_cancel_main_loop();
}
int main() {
    emscripten_set_main_loop(&loop, -1, false);
    return 99;
}

---

Working 1:

code

void loop() {
    std::cout << "called loop \n";
    emscripten_force_exit(0);
}
int main() {
    emscripten_set_main_loop(&loop, -1, false);
    return 99;
}

---

Working 2:

code

void loop() {
    std::cout << "called loop \n";
    emscripten_cancel_main_loop();
    emscripten_async_call([](void *) { exit(0); }, nullptr, 0)
}
int main() {
    emscripten_set_main_loop(&loop, -1, false);
    return 99;
}

---

Working 3:

This one is weird, it was used in test_async_mainloop.c and it seems to work fine?

code

void loop() {
    std::cout << "called loop \n";
    emscripten_force_exit(0);
    emscripten_cancel_main_loop();
}
int main() {
    emscripten_set_main_loop(&loop, -1, false);
    return 99;
}

---

All of the working examples seems to work fine, but I want to know which of them should be preferred.

[sbc100]

Yes, those all looks like real issues worth addressing.

I seems that the way emscripten_cancel_main_loop currently works is that it schedules to timer loop to exit on the next timeout. See

emscripten/src/lib/libeventloop.js

Lines 422 to 425 in 39a0a85

	#if RUNTIME_DEBUG
	dbg('main loop exiting');
	#endif
	{{{ runtimeKeepalivePop() }}}

[sbc100]

Oops, that webrtc test should should have been removed years ago: #27458.

[sbc100]

I believe #27468 addresses all the errors above.

Thanks for the detailed breakdown!