When Emscripten exits a thread that is still running

My Unreal Engine 5.8 browser build ran in Chrome, but in Safari 27, and in a macOS app that embeds the same WebKit, it never got past the loading screen. The engine’s workers kept reporting one line: TypeError: Invalid argument type in ToBigInt operation.

Some context first. The build runs main() on a Web Worker (-sPROXY_TO_PTHREAD), so the engine loop can block as it does on other platforms. When it waits for the GPU, JSPI suspends the WebAssembly stack and hands control back to the worker’s event loop. Pointers are 64-bit at the JavaScript boundary (-sMEMORY64=2), so the generated glue passes them as BigInt.

A worker’s error event carries only a message, a file and a line, and the line pointed at Emscripten’s thread start. To get a real stack, I wrapped Worker before the engine’s script ran, so each engine worker registered its own error and rejection handlers first and posted the stack back to the page. Safari’s stack ended in a plain BigInt(...) call, reached from Emscripten’s callback wrapper after a WebGPU onSubmittedWorkDone promise had resolved.

Emscripten runs such callbacks through callUserCallback, which finishes with maybeExit. When nothing holds a runtime keepalive, maybeExit treats the pthread as done and calls _emscripten_thread_exit(EXITSTATUS). But this thread was not done: its main() was suspended in a GPU wait, and that wait held no keepalive. EXITSTATUS was still undefined, and the 64-bit wrapper’s BigInt(undefined) threw. Chrome happened to avoid this path, and I have not traced why. With 32-bit pointers, I would expect the same path to exit the game thread silently instead of throwing.

I confirmed it twice before touching the engine. Patching the loader in Safari to hold a keepalive around the thread’s entry point let the scene render, with the first frame at 12.2 s. Then a minimal Emscripten 6.0.10 program with the engine’s flags, where a timer callback completes while main() waits in an async library function that holds no keepalive, failed the same way under Node, in V8’s wording: Cannot convert undefined to a BigInt. With a keepalive, it resumed and returned normally.

The fix is small. In the HTML5 launch code, main() calls emscripten_runtime_keepalive_push() on entry and releases it with ON_SCOPE_EXIT, so every return path, early failures included, still exits the thread and reports its exit code to the page.

The lesson I took from it: with JSPI, a suspended main() is still a running thread, but Emscripten’s exit check only counts keepalives. Anything that suspends without one is at the mercy of whichever callback finishes next, and a single browser’s timing can hide that for a long time.

The build guide and source-access instructions are in UnrealEngine-WebGPU on GitHub.