p2p: fix data race in MakeSwitch test helper (#5810)

Fixes #5809.

p2p/test_util.go

@@ -169,15 +169,15 @@ func MakeSwitch(
 	logger := log.TestingLogger().With("switch", i)
 	t := NewMConnTransport(logger, nodeInfo, nodeKey.PrivKey, MConnConfig(cfg))
 
-	if err := t.Listen(addr.Endpoint()); err != nil {
-		panic(err)
-	}
-
 	// TODO: let the config be passed in?
 	sw := initSwitch(i, NewSwitch(cfg, t, opts...))
 	sw.SetLogger(log.TestingLogger().With("switch", i))
 	sw.SetNodeKey(nodeKey)
 
+	if err := t.Listen(addr.Endpoint()); err != nil {
+		panic(err)
+	}
+
 	ni := nodeInfo
 	ni.Channels = []byte{}
 	for ch := range sw.reactorsByCh {

p2p/transport_mconn.go

@@ -129,6 +129,11 @@ func NewMConnTransport(
 }
 
 // SetChannelDescriptors implements Transport.
+//
+// This is not concurrency-safe, and must be called before listening.
+//
+// FIXME: This is here for compatibility with existing switch code,
+// it should be passed via the constructor instead.
 func (m *MConnTransport) SetChannelDescriptors(chDescs []*conn.ChannelDescriptor) {
 	m.channelDescs = chDescs
 }