Drop per-task port blocks; previews use any free loopback port

internal/agents/preview.go

@@ -22,10 +22,6 @@ import (
 
 const maxPreviewName = 40
 
-func portEnv(t api.Task) []string {
-	return []string{"PORT=" + strconv.Itoa(t.PortBase)}
-}
-
 type exposePortArgs struct {
 	Name string `json:"name"`
 	Port int    `json:"port"`
@@ -35,7 +31,7 @@ func (m *Manager) exposePortTool(l *live) mcp.Tool {
 	return mcp.Tool{
 		Name: "expose_port",
 		Description: "Show the human a server you are running, such as a dev server, as a link above the task's tabs that opens it in a new window. " +
-			"Start the server yourself first (in the background), listening on 127.0.0.1 (not 0.0.0.0 or localhost) on a port in this task's block. " +
+			"Start the server yourself first (in the background), listening on 127.0.0.1 (not 0.0.0.0 or localhost) on any free port. " +
 			"byre then serves it on the same port of byre's tailnet node, and the human's app opens it through a local link. " +
 			"name labels it in the tab (\"web\", \"storybook\"); exposing a name or port again replaces the earlier one. " +
 			"The server stops when your process does (a byre restart, compaction, the task closing); the human sees it as unreachable until you start it again.",
@@ -43,7 +39,7 @@ func (m *Manager) exposePortTool(l *live) mcp.Tool {
 			"type": "object",
 			"properties": {
 				"name": {"type": "string", "description": "Short label, shown as the link's text."},
-				"port": {"type": "integer", "description": "The port the server listens on, at 127.0.0.1, within this task's block."}
+				"port": {"type": "integer", "description": "The port the server listens on, at 127.0.0.1."}
 			},
 			"required": ["name", "port"]
 		}`),
@@ -65,13 +61,8 @@ func (m *Manager) exposePort(ctx context.Context, l *live, raw json.RawMessage)
 		return "", fmt.Errorf("name must be one non-empty line of at most %d characters", maxPreviewName)
 	}
 
-	task, err := m.store.GetTask(ctx, l.taskID)
-	if err != nil {
-		return "", err
-	}
-	last := task.PortBase + store.PortBlock - 1
-	if args.Port < task.PortBase || args.Port > last {
-		return "", fmt.Errorf("port %d is outside this task's block, %d to %d", args.Port, task.PortBase, last)
+	if args.Port < 1 || args.Port > 65535 {
+		return "", fmt.Errorf("port %d is not a TCP port", args.Port)
 	}
 
 	addr := net.JoinHostPort("127.0.0.1", strconv.Itoa(args.Port))
@@ -81,14 +72,23 @@ func (m *Manager) exposePort(ctx context.Context, l *live, raw json.RawMessage)
 	}
 	conn.Close()
 
-	err = m.servePreview(args.Port)
-	if err != nil {
-		return "", err
+	m.exposeMu.Lock()
+	defer m.exposeMu.Unlock()
+	m.previewMu.Lock()
+	serving := m.previewListeners[args.Port] != nil
+	m.previewMu.Unlock()
+	if !serving {
+		err = m.servePreview(args.Port)
+		if err != nil {
+			return "", err
+		}
 	}
 
-	dropped, err := m.store.SetPreview(ctx, api.Preview{TaskId: task.Id, Name: name, Port: args.Port, CreatedAt: time.Now().UTC()}, agentActor)
+	dropped, err := m.store.SetPreview(ctx, api.Preview{TaskId: l.taskID, Name: name, Port: args.Port, CreatedAt: time.Now().UTC()}, agentActor)
 	if errors.Is(err, store.ErrNotFound) {
-		m.stopPreview(args.Port)
+		if !serving {
+			m.stopPreview(args.Port)
+		}
 		return "", errors.New("this task is no longer active")
 	}
 	if err != nil {
@@ -263,12 +263,28 @@ func (m *Manager) recoverPreviews(ctx context.Context) error {
 }
 
 func (m *Manager) Unexpose(ctx context.Context, taskID string) {
+	m.exposeMu.Lock()
+	defer m.exposeMu.Unlock()
 	previews, err := m.store.ListPreviews(ctx, taskID)
 	if err != nil {
 		slog.Error("list previews", "task", taskID, "err", err)
 		return
 	}
+	active, err := m.store.ActivePreviews(ctx)
+	if err != nil {
+		slog.Error("list active previews", "err", err)
+		return
+	}
+	held := map[int]bool{}
+	for _, p := range active {
+		if p.TaskId != taskID {
+			held[p.Port] = true
+		}
+	}
 	for _, p := range previews {
+		if held[p.Port] {
+			continue
+		}
 		m.stopPreview(p.Port)
 		slog.Info("stopped serving preview", "task", taskID, "name", p.Name, "port", p.Port)
 	}