mcp: fix several goroutine leaks in tests (#494)

Fix some (but not all) goroutine leaks in tests. Others were nontrivial,
because they relate to streamable http.

For #489.

mcp/client_example_test.go

@@ -45,9 +45,12 @@ func Example_roots() {
 	if _, err := s.Connect(ctx, t1, nil); err != nil {
 		log.Fatal(err)
 	}
-	if _, err := c.Connect(ctx, t2, nil); err != nil {
+
+	clientSession, err := c.Connect(ctx, t2, nil)
+	if err != nil {
 		log.Fatal(err)
 	}
+	defer clientSession.Close()
 
 	// ...and add a root. The server is notified about the change.
 	c.AddRoots(&mcp.Root{URI: "file://b"})

mcp/mcp_test.go

@@ -592,7 +592,10 @@ func errorCode(err error) int64 {
 //
 // The caller should cancel either the client connection or server connection
 // when the connections are no longer needed.
-func basicConnection(t *testing.T, config func(*Server)) (*ClientSession, *ServerSession) {
+//
+// The returned func cleans up by closing the client and waiting for the server
+// to shut down.
+func basicConnection(t *testing.T, config func(*Server)) (*ClientSession, *ServerSession, func()) {
 	return basicClientServerConnection(t, nil, nil, config)
 }
 
@@ -604,7 +607,10 @@ func basicConnection(t *testing.T, config func(*Server)) (*ClientSession, *Serve
 //
 // The caller should cancel either the client connection or server connection
 // when the connections are no longer needed.
-func basicClientServerConnection(t *testing.T, client *Client, server *Server, config func(*Server)) (*ClientSession, *ServerSession) {
+//
+// The returned func cleans up by closing the client and waiting for the server
+// to shut down.
+func basicClientServerConnection(t *testing.T, client *Client, server *Server, config func(*Server)) (*ClientSession, *ServerSession, func()) {
 	t.Helper()
 
 	ctx := context.Background()
@@ -628,14 +634,17 @@ func basicClientServerConnection(t *testing.T, client *Client, server *Server, c
 	if err != nil {
 		t.Fatal(err)
 	}
-	return cs, ss
+	return cs, ss, func() {
+		cs.Close()
+		ss.Wait()
+	}
 }
 
 func TestServerClosing(t *testing.T) {
-	cs, ss := basicConnection(t, func(s *Server) {
+	cs, ss, cleanup := basicConnection(t, func(s *Server) {
 		AddTool(s, greetTool(), sayHi)
 	})
-	defer cs.Close()
+	defer cleanup()
 
 	ctx := context.Background()
 	var wg sync.WaitGroup
@@ -715,10 +724,10 @@ func TestCancellation(t *testing.T) {
 		}
 		return nil, nil, nil
 	}
-	cs, _ := basicConnection(t, func(s *Server) {
+	cs, _, cleanup := basicConnection(t, func(s *Server) {
 		AddTool(s, &Tool{Name: "slow", InputSchema: &jsonschema.Schema{Type: "object"}}, slowTool)
 	})
-	defer cs.Close()
+	defer cleanup()
 
 	ctx, cancel := context.WithCancel(context.Background())
 	go cs.CallTool(ctx, &CallToolParams{Name: "slow"})
@@ -741,13 +750,10 @@ func TestMiddleware(t *testing.T) {
 		t.Fatal(err)
 	}
 	// Wait for the server to exit after the client closes its connection.
-	var clientWG sync.WaitGroup
-	clientWG.Add(1)
-	go func() {
+	defer func() {
 		if err := ss.Wait(); err != nil {
 			t.Errorf("server failed: %v", err)
 		}
-		clientWG.Done()
 	}()
 
 	var sbuf, cbuf bytes.Buffer
@@ -767,6 +773,8 @@ func TestMiddleware(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	defer cs.Close()
+
 	if _, err := cs.ListTools(ctx, nil); err != nil {
 		t.Fatal(err)
 	}
@@ -1511,7 +1519,7 @@ func TestKeepAliveFailure(t *testing.T) {
 func TestAddTool_DuplicateNoPanicAndNoDuplicate(t *testing.T) {
 	// Adding the same tool pointer twice should not panic and should not
 	// produce duplicates in the server's tool list.
-	cs, _ := basicConnection(t, func(s *Server) {
+	cs, _, cleanup := basicConnection(t, func(s *Server) {
 		// Use two distinct Tool instances with the same name but different
 		// descriptions to ensure the second replaces the first
 		// This case was written specifically to reproduce a bug where duplicate tools where causing jsonschema errors
@@ -1520,7 +1528,7 @@ func TestAddTool_DuplicateNoPanicAndNoDuplicate(t *testing.T) {
 		s.AddTool(t1, nopHandler)
 		s.AddTool(t2, nopHandler)
 	})
-	defer cs.Close()
+	defer cleanup()
 
 	ctx := context.Background()
 	res, err := cs.ListTools(ctx, nil)
@@ -1568,7 +1576,7 @@ func TestSynchronousNotifications(t *testing.T) {
 		},
 	}
 	server := NewServer(testImpl, serverOpts)
-	cs, ss := basicClientServerConnection(t, client, server, func(s *Server) {
+	cs, ss, cleanup := basicClientServerConnection(t, client, server, func(s *Server) {
 		AddTool(s, &Tool{Name: "tool"}, func(ctx context.Context, req *CallToolRequest, args any) (*CallToolResult, any, error) {
 			if !rootsChanged.Load() {
 				return nil, nil, fmt.Errorf("didn't get root change notification")
@@ -1576,6 +1584,7 @@ func TestSynchronousNotifications(t *testing.T) {
 			return new(CallToolResult), nil, nil
 		})
 	})
+	defer cleanup()
 
 	t.Run("from client", func(t *testing.T) {
 		client.AddRoots(&Root{Name: "myroot", URI: "file://foo"})
@@ -1617,7 +1626,7 @@ func TestNoDistributedDeadlock(t *testing.T) {
 		},
 	}
 	client := NewClient(testImpl, clientOpts)
-	cs, _ := basicClientServerConnection(t, client, nil, func(s *Server) {
+	cs, _, cleanup := basicClientServerConnection(t, client, nil, func(s *Server) {
 		AddTool(s, &Tool{Name: "tool1"}, func(ctx context.Context, req *CallToolRequest, args any) (*CallToolResult, any, error) {
 			req.Session.CreateMessage(ctx, new(CreateMessageParams))
 			return new(CallToolResult), nil, nil
@@ -1627,7 +1636,7 @@ func TestNoDistributedDeadlock(t *testing.T) {
 			return new(CallToolResult), nil, nil
 		})
 	})
-	defer cs.Close()
+	defer cleanup()
 
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
@@ -1651,7 +1660,7 @@ func TestPointerArgEquivalence(t *testing.T) {
 	type output struct {
 		Out string
 	}
-	cs, _ := basicConnection(t, func(s *Server) {
+	cs, _, cleanup := basicConnection(t, func(s *Server) {
 		// Add two equivalent tools, one of which operates in the 'pointer' realm,
 		// the other of which does not.
 		//
@@ -1686,7 +1695,7 @@ func TestPointerArgEquivalence(t *testing.T) {
 			}
 		})
 	})
-	defer cs.Close()
+	defer cleanup()
 
 	ctx := context.Background()
 	tools, err := cs.ListTools(ctx, nil)
@@ -1758,7 +1767,9 @@ func TestComplete(t *testing.T) {
 		},
 	}
 	server := NewServer(testImpl, serverOpts)
-	cs, _ := basicClientServerConnection(t, nil, server, func(s *Server) {})
+	cs, _, cleanup := basicClientServerConnection(t, nil, server, func(s *Server) {})
+	defer cleanup()
+
 	result, err := cs.Complete(context.Background(), &CompleteParams{
 		Argument: CompleteParamsArgument{
 			Name:  "language",

mcp/server_example_test.go

@@ -55,6 +55,7 @@ func Example_prompts() {
 	if err != nil {
 		log.Fatal(err)
 	}
+	defer cs.Close()
 
 	// List the prompts.
 	for p, err := range cs.Prompts(ctx, nil) {

mcp/transport_example_test.go

@@ -24,16 +24,21 @@ func ExampleLoggingTransport() {
 	ctx := context.Background()
 	t1, t2 := mcp.NewInMemoryTransports()
 	server := mcp.NewServer(&mcp.Implementation{Name: "server", Version: "v0.0.1"}, nil)
-	if _, err := server.Connect(ctx, t1, nil); err != nil {
+	serverSession, err := server.Connect(ctx, t1, nil)
+	if err != nil {
 		log.Fatal(err)
 	}
+	defer serverSession.Wait()
 
 	client := mcp.NewClient(&mcp.Implementation{Name: "client", Version: "v0.0.1"}, nil)
 	var b bytes.Buffer
 	logTransport := &mcp.LoggingTransport{Transport: t2, Writer: &b}
-	if _, err := client.Connect(ctx, logTransport, nil); err != nil {
+	clientSession, err := client.Connect(ctx, logTransport, nil)
+	if err != nil {
 		log.Fatal(err)
 	}
+	defer clientSession.Close()
+
 	// Sort for stability: reads are concurrent to writes.
 	for _, line := range slices.Sorted(strings.SplitSeq(b.String(), "\n")) {
 		fmt.Println(line)

mcp/transport_test.go

@@ -25,6 +25,7 @@ func TestBatchFraming(t *testing.T) {
 	r, w := io.Pipe()
 	tport := newIOConn(rwc{r, w})
 	tport.outgoingBatch = make([]jsonrpc.Message, 0, 2)
+	defer tport.Close()
 
 	// Read the two messages into a channel, for easy testing later.
 	read := make(chan jsonrpc.Message)