Prevent port from having two connections (#138)

* Prevent port from having two connections

* regenerate mock files

sim/connection.go

@@ -11,6 +11,7 @@ func NewSendError() *SendError {
 
 // A Connection is responsible for delivering messages to its destination.
 type Connection interface {
+	Named
 	Hookable
 
 	// PlugIn connects a port to the connection. The connection should reserve

sim/directconnection/builder.go

@@ -25,7 +25,11 @@ func (b Builder) WithFreq(f sim.Freq) Builder {
 func (b Builder) Build(name string) *Comp {
 	c := &Comp{}
 	c.TickingComponent = sim.NewSecondaryTickingComponent(name, b.engine, b.freq, c)
-	middleware := &middleware{Comp: c}
+
+	middleware := &middleware{
+		Comp: c,
+	}
 	c.AddMiddleware(middleware)
+
 	return c
 }

sim/directconnection/directconnection.go

@@ -80,7 +80,9 @@ func (m *middleware) forwardMany(
 			break
 		}
 
-		err := head.Meta().Dst.Deliver(head)
+		dst := head.Meta().Dst
+
+		err := dst.Deliver(head)
 		if err != nil {
 			break
 		}

sim/port.go

@@ -1,6 +1,7 @@
 package sim
 
 import (
+	"fmt"
 	"sync"
 )
 
@@ -51,6 +52,16 @@ var HookPosPortMsgRetrieve = &HookPos{Name: "Port Msg Retrieve"}
 
 // SetConnection sets which connection plugged in to this port.
 func (p *LimitNumMsgPort) SetConnection(conn Connection) {
+	if p.conn != nil {
+		connName := p.conn.Name()
+		newConnName := conn.Name()
+		panicMsg := fmt.Sprintf(
+			"connection already set to %s, now connecting to %s",
+			connName, newConnName,
+		)
+		panic(panicMsg)
+	}
+
 	p.conn = conn
 }