packer/rpc: don't use stream ID zero [GH-738]

packer/rpc/muxconn.go

@@ -189,6 +189,12 @@ func (m *MuxConn) NextId() uint32 {
 	m.muAccept.Lock()
 	defer m.muAccept.Unlock()
 
+	// We never use stream ID 0 because 0 is the zero value of a uint32
+	// and we want to reserve that for "not in use"
+	if m.curId == 0 {
+		m.curId = 1
+	}
+
 	for {
 		result := m.curId
 		m.curId += 1

packer/rpc/muxconn_test.go

@@ -241,14 +241,14 @@ func TestMuxConnNextId(t *testing.T) {
 	a := client.NextId()
 	b := client.NextId()
 
-	if a != 0 || b != 1 {
+	if a != 1 || b != 2 {
 		t.Fatalf("IDs should increment")
 	}
 
 	a = server.NextId()
 	b = server.NextId()
 
-	if a != 0 || b != 1 {
+	if a != 1 || b != 2 {
 		t.Fatalf("IDs should increment: %d %d", a, b)
 	}
 }