Add support for pause & resume of thread-bound items in coroutines

[jonwis]

We saw heap & stack corruptions with code like:

auto watcher = wil::ThreadFailureCallback([](wil::FailureInfo const&) { return false; });
auto q = co_await SomethingAsync();
THROW_HR_IF(E_FAIL, q != 6);

This type and types that use it are inherently thread-bound, and there's no guarantee that the continuation happens on the same thread.

Instead, this new helper lets you say:

auto watcher = wil::ThreadFailureCallback(...);
auto q = co_await wil::with_watcher(watcher, SomethingAsync());
THROW_HR_IF(E_FAIL, q != 6);

... and the awaitable will unhook the watcher on the source thread during suspend and reattach it on the continuation thread before processing exception propagation, etc.

[jonwis]

So I'm on these:

cmake version 4.2.3
CMake suite maintained and supported by Kitware (kitware.com/cmake).

clang version 20.1.8
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Program Files\Microsoft Visual Studio\18\Professional\VC\Tools\Llvm\x64\bin

Installed just today. I can undo the change to the cmake file - I can't reproduce it building from a vs x64 build window with the scripts or with manual "cmake --preset clang ; cmake --build --preset clang-debug" ...

[dunhor]

There's a wil::details::priority_tag<N> I added (moved) somewhat recently. May be better to just re-use it? Higher N gets higher priority.

[dunhor]

Wait, oops, it's not in the details namespace

[jonwis]

@copilot please fix and use the standard priority_tag<> type instead

[dunhor]

I thought that

Suggested change

	noexcept(std::declval<TChildAwaitable>().await_suspend(std::forward<T>(handle))) && noexcept(pausable.suspend()))
	noexcept(child_awaitable.await_suspend(std::forward<T>(handle))) && noexcept(pausable.suspend()))

should work... did that give you issues?

[dunhor]

Really wish noexcept(auto) was a thing 😭

[dunhor]

You check noexcept(pausable.suspend()) above, but not noexcept(pausable.resume()) here?

[dunhor]

Doesn't this need to call get_awaiter on the awaitable?

[dunhor]

Strictly for my own curiosity (the code as-is is probably fine to be safe) - is there a valid scenario where we end up calling resume() when IsWatching() is true? Seems like if that's the case, there was a mis-match with suspend()