FlattenUnordered: improve wakers behavior

[olegnn]

Fixes a rare case when the same waker (for stream or inner streams) was called twice and more times at the same time, and the stream was polled before the wakers ended their waking process, as this may lead to incorrect access to the UnsafeCell from some of the waking wakers.

[taiki-e]

Thanks!

this may lead to incorrect access to the UnsafeCell from some of the waking wakers.

Does this mean it will cause a data race?

[olegnn]

Does this mean it will cause a data race?

Well, the UnsafeCell may be potentially modified at the moment while it's being read, so yes.
However, to make it happen, we need the following sequence:

• Some waker (for ex. for inner streams) was called without delay several times from different threads
• We started to poll the stream before the last waker ends its waking process
• Some of the wakers reads potentially bad waker from the unsafe cell