Add eventDownstreamConnected

In a later commit, we'll be able to move part of downstreamConn.register
into the user goroutine to prevent races.

References: https://todo.sr.ht/~emersion/soju/22

downstream.go

@@ -660,7 +660,9 @@ func (dc *downstreamConn) register() error {
 	dc.username = dc.user.Username
 	dc.logger.Printf("registration complete for user %q", dc.username)
 
-	firstDownstream := dc.user.addDownstream(dc)
+	dc.user.lock.Lock()
+	firstDownstream := len(dc.user.downstreamConns) == 0
+	dc.user.lock.Unlock()
 
 	dc.SendMessage(&irc.Message{
 		Prefix:  dc.srv.prefix(),

server.go

@@ -119,6 +119,7 @@ func (s *Server) Serve(ln net.Listener) error {
 			if err := dc.runUntilRegistered(); err != nil {
 				dc.logger.Print(err)
 			} else {
+				dc.user.events <- eventDownstreamConnected{dc}
 				if err := dc.readMessages(dc.user.events); err != nil {
 					dc.logger.Print(err)
 				}

user.go

@@ -19,6 +19,10 @@ type eventDownstreamMessage struct {
 	dc  *downstreamConn
 }
 
+type eventDownstreamConnected struct {
+	dc *downstreamConn
+}
+
 type network struct {
 	Network
 	user *user
@@ -160,6 +164,11 @@ func (u *user) run() {
 			if err := uc.handleMessage(msg); err != nil {
 				uc.logger.Printf("failed to handle message %q: %v", msg, err)
 			}
+		case eventDownstreamConnected:
+			dc := e.dc
+			u.lock.Lock()
+			u.downstreamConns = append(u.downstreamConns, dc)
+			u.lock.Unlock()
 		case eventDownstreamMessage:
 			msg, dc := e.msg, e.dc
 			if dc.isClosed() {
@@ -180,14 +189,6 @@ func (u *user) run() {
 	}
 }
 
-func (u *user) addDownstream(dc *downstreamConn) (first bool) {
-	u.lock.Lock()
-	first = len(dc.user.downstreamConns) == 0
-	u.downstreamConns = append(u.downstreamConns, dc)
-	u.lock.Unlock()
-	return first
-}
-
 func (u *user) removeDownstream(dc *downstreamConn) {
 	u.lock.Lock()
 	for i := range u.downstreamConns {