The context.Context passed to Handler.Handle or Preempter.Preempt is not canceled when the connection is closed. Each async (long-running) handler has to monitor the end of the connection by itself to stop running when this happens.

The goal of this proposal is to mimic net/http.Server.ConnContext and so to allow the user to provide its own context.Context derived from the "server" one.

To be backward compatible, a new ConnContext field can be added to ConnectionOptions as:

// ConnectionOptions holds the options for new connections.
type ConnectionOptions struct {
	…
	// ConnContext optionally specifies a function that modifies
	// the context used for a new connection conn.
	ConnContext func(ctx context.Context, conn *Connection) context.Context
}

This function will then be called in newConnection just after Binder.Bind is called as:

 	options, err := binder.Bind(ctx, c)
 	if err != nil {
 		return nil, err
 	}
+ 	if options.ConnContext != nil {
+ 		ctx = options.ConnContext(ctx, c)
+ 		if ctx == nil {
+ 			panic("ConnContext returned nil")
+ 		}
+ 	}
  	if options.Framer == nil {