peers: allow access to underlying connection

This is a first iteration for @awlx. I think this does
require further investigation as I am also not very happy
with the handler interface. As soon as we try to send data
to HAProxy we do need a base implementation that tracks
sticktable ids and allows us to define new one and reference
known sticktables. This is required as we always have to send
HAProxy all sticktables we want to send updates for and can't
reference the ones we already got from that instance.

peers/peers.go

@@ -56,7 +56,10 @@ func (a *Peer) Serve(l net.Listener) error {
 			return fmt.Errorf("accepting conn: %w", err)
 		}
 
-		p := newProtocolClient(a.BaseContext, nc, a.HandlerSource())
+		// Wrap the context to provide access to the underlying connection.
+		// TODO(tim): Do we really want this?
+		ctx := context.WithValue(a.BaseContext, connectionKey, nc)
+		p := newProtocolClient(ctx, nc, a.HandlerSource())
 		go func() {
 			defer nc.Close()
 			defer p.Close()
@@ -67,3 +70,15 @@ func (a *Peer) Serve(l net.Listener) error {
 		}()
 	}
 }
+
+type contextKey string
+
+const (
+	connectionKey = contextKey("connection")
+)
+
+// Connection returns the underlying connection used in calls
+// to function in a Handler.
+func Connection(ctx context.Context) net.Conn {
+	return ctx.Value(connectionKey).(net.Conn)
+}