Make SendToChildrenInParallel asynchronous

[ineiti]

The current SendToChildrenInParallel is synchronous: it waits for each children to receive the message OR to return an error. Unfortunately some connections only return an error after a timeout. For the blscosi protocol, this timeout is too long for the protocol to work correctly.

So SendToChildrenInParallel should return immediately and still allow to be notified:

• when it's done
• eventual errors

@tharvik proposed the following code:

	ret := make(chan error)
	childsRets := make(chan error)
	go func() {
		for range p.Children() {
			if err := <-childsRets; err != nil {
				ret <- xerrors.Errorf("send to children: %v", err)
			}
		}
		close(ret)
	}()
	for _, c := range p.Children() {
		go func(tn *onet.TreeNode) {
			childsRets <- p.SendTo(tn, msg)
		}(c)
	}
	return ret

As this is a breaking change, one could add the following to SendToChildrenInParallel:

Deprecated: this is synchronous with eventual connection errors, so please use `SendToChildrenInBackground`.

Naming propositions (add yours):

• SendToChildrenInBackground

Once this is merged, the calls to SendToChildrenInParallel should be replaced with this one. Also the hacky ones in the cothority/blscosi/protocol/sub_protocol.go and cothority/messaging/propagate.go.

[nkcr]

With the proposed solution we are open to a zombie goroutine in case the caller is not fully reading the channel. I see two solutions:

1. Concatenate the errors, and return a channel with one error, if any
2. Return a buffered chan so that we are not concerned about blocking when filling the chan and avoid a loop

Solution 2 seems easier:

	ret := make(chan error, len(p.Children))

	wait := sync.WaitGroup{}
	wait.Add(len(p.Children))

	for _, c := range p.Children() {
		go func(tn *onet.TreeNode) {
			defer wait.Done()
			ret <- p.SendTo(tn, msg)
		}(c)
	}

	go func() {
		wait.Wait()
		close(ret)
	}()

	return ret

Then the caller can do:

ret := SendToChildrenInBackground(...)
for err, more := range ret {
	if more {
		// do something with err
	}
}

[ineiti]

@tharvik what do you think of @nkcr 's proposal?

[tharvik]

Return a buffered chan so that we are not concerned about blocking when filling the chan and avoid a loop

Right, good idea, that do avoid having non-terminating goroutine.

I would avoid sending nil error on the channel

func SendToChildrenInBackground(msg interface{}) <-chan error {
	ret := make(chan error, len(p.Children()))

	var wait sync.WaitGroup
	wait.Add(len(p.Children()))

	for _, c := range p.Children() {
		go func(tn *onet.TreeNode) {
			defer wait.Done()
			if err := p.SendTo(tn, msg); err != nil {
				ret <- err
			}
		}(c)
	}

	go func() {
		wait.Wait()
		close(ret)
	}()

	return ret
}

This way, the caller can simply

for err := range SendToChildrenInBackground(...) {
	// some error happened
}

[ineiti]

The nice thing about having a potential nil in the channel is that the caller can also count how many successful connections have been made. So in case he needs t out of n successful connections to continue, the caller can simply count the nils and then go on, dropping the rest of the results. That's something I think might be used in some cases.

[tharvik]

Alright, let's add the next helper then, to easily choose between the two behaviors

func FilterNilError(recv <-chan error) <-chan error {
	ret := make(chan error) // should we use cap(recv) here?

	go func() {
		for err := range recv {
			if err != nil {
				ret <- err
			}
		}
		close(ret)
	}()

	return ret
}

[ineiti]

That makes the main function quite small:

func (n *TreeNodeInstance) SendToChildrenInBackground(
	msg interface{}) <-chan error {
	ret := make(chan error, len(n.Children()))

	for _, c := range n.Children() {
		go func(tn *TreeNode) {
			ret <- n.SendTo(tn, msg)
		}(c)
	}

	return ret
}

Or we make a SendToChildrenInBackgroundErrorOnly method (better name needed) that calls the SendToChildrenInBackground.

[tharvik]

That makes the main function quite small:
...

No, it needs the WaitGroup, because we really have to close the channel, otherwise, we can't range on it.

Or we make a SendToChildrenInBackgroundErrorOnly method (better name needed) that calls the SendToChildrenInBackground.

Having a helper allows us to reuse this pattern without linking it to SendToChildrenInBackground.

And to come back to the need to have nil errors: that's quite a specific use case, do you have anywhere it's needed now? We can always add it latter should the need arise.

[ineiti]

I thought of using it in a

childrenErrors := onet.SendToChildrenInBackground(...)
for {
   select {
   case err := <- childrenErrors:
   case other := <- otherStuff:
   }
}

If I remember correctly, closing the channel would always choose the first case, correct? And the case other would never be called?

If that is true, then I prefer not to have the close in the method. Because that's how it's used in the blscosi protocol.

[nkcr]

If I remember correctly, closing the channel would always choose the first case

No, there are absolutely no priority in select statement. Use default if you want that.

I seems you're trying to be too smart there. The function should have a simple behavior: return result of sends on a channel and close it when its done. period. Let the caller implement its own utility function if needed.

[ineiti]

I like being smart...

Anyway - if you both think the channel should be closed, I'll add some additional logic in the 2 places (out of 6) where this is needed.

And for the nil use-case, it could be used in the propagate-protocol. But I'm not sure it's a good thing to do there.

Now I just need an easy way to write the test for this and some time...