MM-56060: Create base scaffolding for websocket pub-sub (#25654)

We create a new websocket action called "presence" which
can contain the active_channel and the active_team for a given
client connection.

On the client side, for every channel or team switch, we send
out this message.

https://mattermost.atlassian.net/browse/MM-56060

```release-note
NONE
```

server/channels/api4/websocket_test.go

@@ -417,8 +417,29 @@ func TestWebSocketStatuses(t *testing.T) {
 	require.True(t, awayHit, "didn't get away event")
 
 	time.Sleep(500 * time.Millisecond)
+}
 
-	WebSocketClient.Close()
+func TestWebSocketPresence(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+
+	wsClient, err := th.CreateWebSocketClient()
+	require.NoError(t, err)
+	defer wsClient.Close()
+	wsClient.Listen()
+
+	resp := <-wsClient.ResponseChannel
+	require.Equal(t, resp.Status, model.StatusOk, "should have responded OK to authentication challenge")
+
+	wsClient.UpdateActiveChannel("chID")
+	resp = <-wsClient.ResponseChannel
+	require.Nil(t, resp.Error)
+	require.Equal(t, resp.SeqReply, wsClient.Sequence-1, "bad sequence number")
+
+	wsClient.UpdateActiveTeam("teamID")
+	resp = <-wsClient.ResponseChannel
+	require.Nil(t, resp.Error)
+	require.Equal(t, resp.SeqReply, wsClient.Sequence-1, "bad sequence number")
 }
 
 func TestWebSocketUpgrade(t *testing.T) {

server/channels/app/platform/web_conn.go

@@ -105,13 +105,15 @@ type WebConn struct {
 	// a reused connection.
 	// It's theoretically possible for this number to wrap around. But we
 	// leave that as an edge-case.
-	reuseCount   int
-	sessionToken atomic.Value
-	session      atomic.Pointer[model.Session]
-	connectionID atomic.Value
-	endWritePump chan struct{}
-	pumpFinished chan struct{}
-	pluginPosted chan pluginWSPostedHook
+	reuseCount      int
+	sessionToken    atomic.Value
+	session         atomic.Pointer[model.Session]
+	connectionID    atomic.Value
+	activeChannelID atomic.Value
+	activeTeamID    atomic.Value
+	endWritePump    chan struct{}
+	pumpFinished    chan struct{}
+	pluginPosted    chan pluginWSPostedHook
 
 	// These counters are to suppress spammy websocket.slow
 	// and websocket.full logs which happen continuously, if they
@@ -290,6 +292,26 @@ func (wc *WebConn) GetConnectionID() string {
 	return wc.connectionID.Load().(string)
 }
 
+// SetActiveChannelID sets the active channel id of the connection.
+func (wc *WebConn) SetActiveChannelID(id string) {
+	wc.activeChannelID.Store(id)
+}
+
+// GetActiveChannelID returns the active channel id of the connection.
+func (wc *WebConn) GetActiveChannelID() string {
+	return wc.activeChannelID.Load().(string)
+}
+
+// SetActiveTeamID sets the active team id of the connection.
+func (wc *WebConn) SetActiveTeamID(id string) {
+	wc.activeTeamID.Store(id)
+}
+
+// GetActiveTeamID returns the active team id of the connection.
+func (wc *WebConn) GetActiveTeamID() string {
+	return wc.activeTeamID.Load().(string)
+}
+
 // areAllInactive returns whether all of the connections
 // are inactive or not.
 func areAllInactive(conns []*WebConn) bool {

server/channels/app/platform/websocket_router.go

@@ -73,6 +73,25 @@ func (wr *WebSocketRouter) ServeWebSocket(conn *WebConn, r *model.WebSocketReque
 		return
 	}
 
+	if r.Action == string(model.WebsocketPresenceIndicator) {
+		if chID, ok := r.Data["channel_id"].(string); ok {
+			// Set active channel
+			conn.SetActiveChannelID(chID)
+		}
+		if teamID, ok := r.Data["team_id"].(string); ok {
+			// Set active team
+			conn.SetActiveTeamID(teamID)
+		}
+
+		resp := model.NewWebSocketResponse(model.StatusOk, r.Seq, nil)
+		hub := conn.Platform.GetHubForUserId(conn.UserId)
+		if hub == nil {
+			return
+		}
+		hub.SendMessage(conn, resp)
+		return
+	}
+
 	if !conn.IsAuthenticated() {
 		err := model.NewAppError("ServeWebSocket", "api.web_socket_router.not_authenticated.app_error", nil, "", http.StatusUnauthorized)
 		returnWebSocketError(conn.Platform, conn, r, err)

server/public/model/websocket_client.go

@@ -326,6 +326,22 @@ func (wsc *WebSocketClient) GetStatusesByIds(userIds []string) {
 	wsc.SendMessage("get_statuses_by_ids", data)
 }
 
+// UpdateActiveChannel sets the current channel that the user is viewing.
+func (wsc *WebSocketClient) UpdateActiveChannel(channelID string) {
+	data := map[string]any{
+		"channel_id": channelID,
+	}
+	wsc.SendMessage(string(WebsocketPresenceIndicator), data)
+}
+
+// UpdateActiveTeam sets the current team that the user is in.
+func (wsc *WebSocketClient) UpdateActiveTeam(teamID string) {
+	data := map[string]any{
+		"team_id": teamID,
+	}
+	wsc.SendMessage(string(WebsocketPresenceIndicator), data)
+}
+
 func (wsc *WebSocketClient) configurePingHandling() {
 	wsc.Conn.SetPingHandler(wsc.pingHandler)
 	wsc.pingTimeoutTimer = time.NewTimer(time.Second * (60 + PingTimeoutBufferSeconds))

server/public/model/websocket_message.go

@@ -86,6 +86,7 @@ const (
 	WebsocketEventAcknowledgementRemoved              WebsocketEventType = "post_acknowledgement_removed"
 	WebsocketEventPersistentNotificationTriggered     WebsocketEventType = "persistent_notification_triggered"
 	WebsocketEventHostedCustomerSignupProgressUpdated WebsocketEventType = "hosted_customer_signup_progress_updated"
+	WebsocketPresenceIndicator                        WebsocketEventType = "presence"
 )
 
 type WebSocketMessage interface {