Avoid calling writeStackCookie to early in worker.js

[sbc100]

When running a worker we don't want to write the stack cookie until the
worker is actually running a pthread. This is done in
establishStackSpace.

For this reason we don't want to run writeStackCookie during
run (which is used only to initiialize the worker, before its
running a thread).

The extra call to writeStackCookie was not harmfull but its writing
the default stack location (the one used by the main thread!).

[kripken]

IIUC pthreads will now only call writeStackCookie() - which they always have - but no longer call stackCheckInit() - which they used to before this PR, but that was wrong. But do we not need the other parts of stackCheckInit(), namely emscripten_stack_set_limits /emscripten_stack_init?

[sbc100]

IIUC pthreads will now only call writeStackCookie() - which they always have - but no longer call stackCheckInit() - which they used to before this PR, but that was wrong. But do we not need the other parts of stackCheckInit(), namely emscripten_stack_set_limits /emscripten_stack_init?

I think the answer is that pthreads do all this stuff in establishStackSpace:

emscripten/src/library_pthread.js

Lines 945 to 980 in 1f6b136

	$establishStackSpace: function() {
	var pthread_ptr = _pthread_self();
	var stackTop = {{{ makeGetValue('pthread_ptr', C_STRUCTS.pthread.stack, 'i32') }}};
	var stackSize = {{{ makeGetValue('pthread_ptr', C_STRUCTS.pthread.stack_size, 'i32') }}};
	var stackMax = stackTop - stackSize;
	#if PTHREADS_DEBUG
	err('establishStackSpace: ' + stackTop + ' -> ' + stackMax);
	#endif
	#if ASSERTIONS
	assert(stackTop != 0);
	assert(stackMax != 0);
	assert(stackTop > stackMax);
	#endif
	// Set stack limits used by `emscripten/stack.h` function. These limits are
	// cached in wasm-side globals to make checks as fast as possible.
	_emscripten_stack_set_limits(stackTop, stackMax);
	#if STACK_OVERFLOW_CHECK >= 2
	// Set stack limits used by binaryen's `StackCheck` pass.
	// TODO(sbc): Can this be combined with the above.
	___set_stack_limits(stackTop, stackMax);
	#if MAIN_MODULE
	// With dynamic linking we could have any number of pre-loaded libraries
	// that each need to have their stack limits set.
	setDylinkStackLimits(stackTop, stackMax);
	#endif
	#endif
	// Call inside wasm module to set up the stack frame for this pthread in wasm module scope
	stackRestore(stackTop);
	#if STACK_OVERFLOW_CHECK
	// Write the stack cookie last, after we have set up the proper bounds and
	// current position of the stack.
	writeStackCookie();
	#endif
	},

emscripten_stack_set_limits and emscripten_stack_init both do the same job so only one of them should be called.

In the case of a pthread its the explicit emscripten_stack_set_limits that is called from establishStackSpace.