mcp: add a explicit test for concurrent streamable sessions (#584)

We seem not to have a test that explicitly exercises concurrent
streamable sessions. Add one.

mcp/streamable_test.go

@@ -192,6 +192,57 @@ func TestStreamableTransports(t *testing.T) {
 	}
 }
 
+func TestStreamableConcurrentHandling(t *testing.T) {
+	// This test checks that the streamable server and client transports can
+	// communicate.
+	type count struct {
+		Count int
+	}
+
+	var mu sync.Mutex
+	counts := make(map[string]int)
+
+	server := NewServer(testImpl, nil)
+	AddTool(server, &Tool{Name: "inc"}, func(ctx context.Context, req *CallToolRequest, _ any) (*CallToolResult, count, error) {
+		id := req.Session.ID()
+		mu.Lock()
+		defer mu.Unlock()
+		c := counts[id]
+		counts[id] = c + 1
+		return nil, count{c}, nil
+	})
+	handler := NewStreamableHTTPHandler(func(req *http.Request) *Server { return server }, nil)
+	httpServer := httptest.NewServer(mustNotPanic(t, handler))
+	defer httpServer.Close()
+
+	ctx := context.Background()
+	client := NewClient(testImpl, nil)
+	var wg sync.WaitGroup
+	for range 100 {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			clientSession, err := client.Connect(ctx, &StreamableClientTransport{Endpoint: httpServer.URL}, nil)
+			if err != nil {
+				t.Errorf("Connect failed: %v", err)
+				return
+			}
+			defer clientSession.Close()
+			for i := range 10 {
+				res, err := clientSession.CallTool(ctx, &CallToolParams{Name: "inc"})
+				if err != nil {
+					t.Errorf("CallTool failed: %v", err)
+					return
+				}
+				if got := int(res.StructuredContent.(map[string]any)["Count"].(float64)); got != i {
+					t.Errorf("got count %d, want %d", got, i)
+				}
+			}
+		}()
+	}
+	wg.Wait()
+}
+
 func TestStreamableServerShutdown(t *testing.T) {
 	ctx := context.Background()