Embind’ed processing with nested threads hangs.

The minimal example and build instruction is here: https://github.com/amukovozov/embind_threading.

The code does simple arithmetical computations (as an example) without using any features like filesystem, canvases, or calling JS functions from C++ (except for what emscripten does itself), it is just a number-cruncher.

The example initializes the WASM module from a web-worker; without the worker (doing the same from the main browser thread) the same issue is observed, but it also blocks the main thread which is not convenient.

The example has 3 modes of computation: “no threads”, “flat”, and “nested”.
All modes perform the same processing on 2 arrays of numbers.
“No threads” mode does the processing sequentially. It works fine.
“Flat” mode spawns a thread for each value to process. It works fine.
“Nested” mode spawns 2 threads for an array, and each of those threads spawns a thread for each value in the half of that array. It gives a warning in the console that says `Tried to spawn a new thread, but the thread pool is exhausted` and never finishes execution. However, the total number of spawned threads should never exceed the pool size by design of the example.