Make fiber_context::can_resume() const

We propose to change the signature of function fiber_context::can_resume() so that it is a const member function.

[P0876R17] proposes a great library, fiber_context, providing an API for stackful context switching, based on Boost.Context. It has one unusual design point in its interface. Member function fiber_context::can_resume(), which tells if the given fiber can be resumed by the calling thread, which is used as a precondition for other member functions in fiber_context is not declared const, implying that it cannot be called concurrently, without external synchronization, from multiple threads.

The older revision of the paper, [P0876R6], gives the rationale for making this function non-const.

can_resume() is not marked const because in at least one implementation, it requires an internal context switch. However, the stack operations are effectively read-only. Nonetheless, if it is possible for more than one thread to call can_resume() concurrently on the same non-empty std::fiber_context instance, locking is the caller’s responsibility.

Given this potential data race, it is conceptually difficult to accept that it is a predicate suitable for use as a function precondition. In the context of the proposed contract support framework ([P2900R9]), function fiber_context::can_resume() would not be usable in contract assertions, as they always treat objects used in the predicates as const.

We believe that this "not const" provision is not necessary. The functionality of can_resume() can be implemented under a hosted implementation by storing the result of std::this_thread::get_id() upon the first fiber resumption and comparing it against the result of std::this_thread::get_id() performed inside can_resume(). This technique is used in the reference implementation. Under a freestanding implementation can_resume() can be implemented as !empty().

References

• [P0876R6] — Oliver Kowalke, Nat Goodspeed, "fiber_context - fibers without scheduler",
  ("https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0876r6.pdf").
• [P0876R17] — Oliver Kowalke, Nat Goodspeed, "fiber_context - fibers without scheduler",
  ("https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p0876r17.pdf").
• [P2900R9] — Joshua Berne, Timur Doumler, Andrzej Krzemieński et al., "Contracts for C++",
  ("https://isocpp.org/files/papers/P2900R9.pdf").