Restore Win7 compat by using old versions of threadpool functions

[oldnewthing]

Fortunately, the Ex versions behave the same as the Win7 versions, so we can just switch everybody to the Win7 versions.

Fixes #894

[kennykerr]

Sweet!

[ysc3839]

According to SetThreadpoolTimer docs, it returns void instead of BOOL, is it OK to use this return value?

[kennykerr]

Well spotted - @oldnewthing I'm guessing this means we don't have test coverage for this one. 😟

[oldnewthing]

Strange, because there's a cancellation test. Will have to figure out how this slipped through. Probably because I messed up.

[oldnewthing]

Okay, the deal is that this works on Win8+ due to how SetThreadpoolTimer is implemented (it calls SetThreadpoolTimerEx and throws away the result, but the result is still lying around in eax so we can still act on it). But this won't work on Win7, where eax does not contain anything meaningful. It looks like timer cancellation propagation will not work on Win7. Should I do GetProcAddress/dynamic light-up?

[kennykerr]

Yes, we'll have to do that. You can (almost) use load_runtime_function except that its currently only used for combase functions and the thread pool is implemented by kernel32.

cppwinrt/strings/base_agile_ref.h

Lines 124 to 140 in 1502e29

	template <typename F, typename L>
	void load_runtime_function(char const* name, F& result, L fallback) noexcept
	{
	if (result)
	{
	return;
	}
	result = reinterpret_cast<F>(WINRT_IMPL_GetProcAddress(WINRT_IMPL_LoadLibraryW(L"combase.dll"), name));
	if (result)
	{
	return;
	}
	result = fallback;
	}

This is the way Win7 fallback is handled, for example:

cppwinrt/strings/base_activation.h

Lines 21 to 38 in 1502e29

	inline int32_t __stdcall fallback_RoGetActivationFactory(void*, guid const&, void** factory) noexcept
	{
	*factory = nullptr;
	return error_class_not_available;
	}
	template <bool isSameInterfaceAsIActivationFactory>
	WINRT_IMPL_NOINLINE hresult get_runtime_activation_factory_impl(param::hstring const& name, winrt::guid const& guid, void** result) noexcept
	{
	if (winrt_activation_handler)
	{
	return winrt_activation_handler(*(void**)(&name), guid, result);
	}
	static int32_t(__stdcall * handler)(void* classId, winrt::guid const& iid, void** factory) noexcept;
	impl::load_runtime_function("RoGetActivationFactory", handler, fallback_RoGetActivationFactory);
	hresult hr = handler(*(void**)(&name), guid, result);

[oldnewthing]

Can't quite do it that way because I want to disable a feature if the GetProcAddress fails. There is no "fallback" available. (I guess I could see if result == fallback_Blah.)

[kennykerr]

Whatever you think is best.

[oldnewthing]

#898