context.Context based cancellation

[at-wat]

Proposed at #147.

#147 (comment)

@Sean-Der:
What do you think of updating Client/Server to take a context? I need a way to be notified that an error has occurred, and it looks like taking a Context is the proper way to have a async way to bubble up an error. I talk about it in #151 a bit.

[at-wat]

It's also related to #163 (comment).

The new API could be:

func Dial(ctx context.Context, network string, raddr *net.UDPAddr, config *Config) (*Conn, error) {}

func Client(ctx context.Context, conn net.Conn, config *Config) (*Conn, error) {}

func Server(ctx context.Context, conn net.Conn, config *Config) (*Conn, error) {}

In this API, we might be able to drop ConnectTimeout option and ConnectTimeoutOption helper.

[at-wat]

Adding ctx context.Context makes the API different from crypto/tls.
We can also keep the current Dial/Client/Server and add DialWithContext/ClientWithContext/ServerWithContext.

[daenney]

I'd vote for the second option, adding the WithContext methods. There's a pretty decent precedent in net/http for doing that, and I'd prefer to not have our exported methods that share a name with crypto/tls diverge.

[daenney]

On net.Dialer though you can already pass in a context, so perhaps it's not needed?

[at-wat]

Dialer sounds that is only for Dial, but we have to do the same things on Cilent and Server. And, most of the things in Dialer can be provided by Config.
So, I think Dialer is not very useful for us.

[at-wat]

I opened a draft PR at #191.

One problem is that Listener.Accept internally calls Server, but Accept can't take context.Context as it waits until a new connection is requested from outside.

[at-wat]

To be discussed

ConnectTimeout config 06468a3

Having timeout both by context.Context and dtls.Config is confusing.
However, as Accept should internally start connection timeout timer, it can't receive context.Context. In addition, listener is an implementation of net.Listener now, adding a new method makes it diverged from tls.Conn.

Error value on timeout dbf9263

Currently, errConnectTimeout is returned on timeout, but returning an error that Is(context.DeadlineExceeded) would be better.
ref: golang/go#31449

[at-wat]

ConnectTimeout config

Adding a

	// function to make a context used in Dial(), Client(), Server(), and Accept().
	// If it is nil, default context with 30s timeout is used.
	ConnectContextMaker func() (context.Context, func())

to dtls.Config and doing

	ctxParent, cancelParent := context.WithCancel(context.Background())
	defer cancelParent()

	config := &dtls.Config{
		ConnectContextMaker: func() (context.Context, func()) {
			return context.WithTimeout(ctxParent, 30*time.Second)
		},
	}
	listener, err := dtls.Listen("udp", addr, config)
	conn, err := listener.Accept()

func (l *listener) Accept() (net.Conn, error) {
	c, err := l.parent.Accept()
	if err != nil {
		return nil, err
	}
	ctx, cancel := l.config.ConnectContextMaker()
	defer cancel()
	return ServerWithContext(ctx, c, l.config)
}

allows to Inherit the parent context to listener.Accept() internal context.
This should be helpful for servers usage to properly shutdown quickly.

(06468a3 implements this)

[at-wat]

@daenney @Sean-Der how do you think these APIs?