P1205R0
Teleportation via co_await

1. Issues

The C++ Coroutine TS [N4736] has issues 31 and 32 listed in [P0664R5]:

31. Add a note warning about thread switching near await and/or coroutine_handle wording.

Add a note warning about thread switching near await and/or coroutine_handle wording

32. Add a normative text making it UB to migrate coroutines between certain kind of execution agents.

Add a normative text making it UB to migrate coroutines between certain kind of execution agents. Clarify that migrating between std::threads is OK. But migrating between CPU and GPU is UB.

2. Discussion

Using co_await, one can teleport a suspended execution between execution agents:

thread::id get_an_id() {

  // here: acquire a lock, read thread_local
    
  co_yield std::this_thread::get_id(); //< one result

  // UB: release the lock, reuse the same thread_local
    
  co_return std::this_thread::get_id(); //< different result
}

We say "teleport" here because the code that relocates the coroutine is outside the coroutine, in a possibly unrelated part of the program. This teleportation can take your coroutine to many interesting places, for example:

1. the thread that runs main

2. threads from std::thread / std::async

3. elemental functions of std::par, std::par_unseq, std::unseq algorithms

4. global / thread_local constructors (see note)

5. global / thread_local / static destructors (see note)

6. functions registered with at_exit / quick_exit

7. signal handlers

8. future fibers_context of [P0876R3]

Note that it is presently implementation-defined whether many of these functions run in a specific thread, a single thread, or in many unspecified threads—see [CWG2046].

3. Proposed Resolution

After [N4736] [dcl.fct.def.coroutine] ❡6:

A suspended coroutine can be resumed to continue execution by invoking a resumption member function of an object of type coroutine_handle<P> associated with this instance of the coroutine. The function that invoked a resumption member function is called resumer. Invoking a resumption member function for a coroutine that is not suspended results in undefined behavior.

Add ❡7:

Resuming a coroutine on an execution agent other than the one it was suspended on has implementation-defined behavior unless both are instances of std::thread. [Note: a coroutine that is moved this way should avoid the use of thread_local or mutex objects. — End note.]