Fix emscripten_current_thread_is_wasm_worker return value.

[sbc100]

This function return ENVIRONMENT_IS_WASM_WORKER which was being initialized to Module['$ww'], i.e. the worker id.

Because emscripten_current_thread_is_wasm_worker, is defined to return the bool type its only valid values are 0 and 1. Returning values larger than 1 will result in undefined behaviour.

Fixes: #23479

[kripken]

This increases code size - is it necessary? Can we not fix the C method on the C side? (if not, perhaps rephrase the PR title/description)

[sbc100]

The C function is declared as returning bool which allows the compiler assume its going be be either a 1 or 0. Anything else is undefined behaviour.

We could change the return type to int, but this fix seems better, since it also means that the ENVIRONMENT_IS_WASM_WORKER is only ever true or false which is is what its name suggests. Storing the thread in this variable doesn't make much sense.

[kripken]

Sorry, I'm not following. Perhaps my confusion is about Module['$ww']. I cannot find where in the code that is ever written to. Where is that done?

From the context of what you are saying, I am guessing that the output of emscripten_current_thread_is_wasm_worker() is written there. (Otherwise, how does this line connect to the topic of the PR?) Assuming my guess is right, then it should already be either 0 or 1, since it is a bool in C?

[sbc100]

I tried to connect the docs in the PR description but perhaps more context is needed.

Module['$ww'] is the wasm workers ID. As each new worker is created its gets its own unique ID in Module['$ww'].

ENVIRONMENT_IS_WASM_WORKER was previously initialized to Module['$ww'] which means its value would laos be the worker ID. This works in most cases because zero is falsely and all other IDs would be truthy.

emscripten_current_thread_is_wasm_worker is the JS library that we expose to native code which is the equivalent of checking for ENVIRONMENT_IS_WASM_WORKER. However in C/C++ its defined as returning a bool which must be either 1 or 0.