api: replaced Future.done with a sync.Cond

[laines-it]

This commit reduces allocations.

Future.done allocation replaced with

• Future.cond (sync.Cond)
• Future.finished (atomic.Bool)

Other code use Future.isDone() instead Future.done == nil check.

Added Future.finish() marks Future as done.
Future.WaitChan() now creates channel on demand.

Closes #496

• Tests (see documentation for a testing package)
• Changelog (see documentation for changelog format)

[oleg-jukovec]

Thanks for the patch. Overall, everything's fine.

I've added some implementation notes, but you need to do a proper rebase first.

[oleg-jukovec]

Please, rebase on the master branch.

[oleg-jukovec]

Thank you for the patch! I have just a one mirror comment:

[oleg-jukovec]

Thank you for the patch!

[bigbes]

Right now atomic.finished is called under lock everywhere (except isDone), that is once or twice called without it. It's better to replace with simple bool calling it with lock, than making it atomic (that will invalidate cache record on every access to this variable).

[laines-it]

Done. Thanks for recommendations! I wrapped finished in connection.go like:

fut.mutex.Lock()
is_done := fut.finished
fut.mutex.Unlock()

if is_done {
   ...
   return
}

[bigbes]

This kind of initialization is allowed (and preferred) when sync.Cond is stored by value rather than as a pointer:

Suggested change

	fut.done = nil
	fut.finished.Store(false)
	fut.cond = *sync.NewCond(&fut.mutex)
	fut.done = nil
	fut.finished.Store(false)
	fut.cond.L = &fut.Mutex

Moreover, it generates less code, is more performant, and helps the Go escape analyzer understand that the allocated memory does not escape (not that the analyzer couldn’t deduce this on its own, but the hint certainly doesn’t hurt).

[laines-it]

Fixed.