[Bug libstdc++/60966] std::call_once sometime hangs

[hideaki.kimura at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60966

--- Comment #17 from Hideaki Kimura <hideaki.kimura at gmail dot com> ---
(In reply to Jonathan Wakely from comment #16)
> promise::get_future() is a non-const function that modifies the promise
> object, therefore it must not be called while any other object is accessing
> the promise.
Oh, is it the design of promise::get_future/set_value?
I so far don't see any reference that clarifies either way (most document just
mentions about difference between future/shared_future)

Taking a glance at boost::promise code, I got an impression that their code is
safe against concurrent get_future() and set_value() because get_future()
either does atomic swap in lazy_init() or does nothing (allocated in
constructor).
I guess that's why Thomas observed that the issue doesn't happen in
boost::promise.

I'm not sure what the C++ standard says. I haven't seen a single human being
who has read it through.