mcp: fix race condition in ServerSession.startKeepalive (#856)

begelundmuller · maciej-kisiel · web-flow · commit 862d78a265cb · 2026-04-03T20:17:49.000+02:00
This PR fixes a race condition where `ServerSession.keepaliveCancel` was accessed in `Close` concurrently with its initialization in `ServerSession.startKeepalive`. It also removes a now redundant test. The fix moves keepalive initialization into `ServerSession.Connect` similar to the implementation in `ClientSession.Connect`. This is safe because the MCP spec allows keepalive pings before the initialization messages. From the MCP [spec](https://modelcontextprotocol.io/specification/2025-11-25/basic/lifecycle) (emphasis added): > The client SHOULD NOT send requests **other than pings** before the server has responded to the initialize request. The PR includes a reproducer test case, which fails with the following error without this fix: ``` > go test -count=10 -race -run ^TestStreamableStatelessKeepaliveRace$ github.com/modelcontextprotocol/go-sdk/mcp 8s ================== WARNING: DATA RACE Write at 0x00c00007fbf0 by goroutine 1095: github.com/modelcontextprotocol/go-sdk/mcp.startKeepalive() /Users/benjamin/Documents/go-sdk/mcp/shared.go:590 +0x58 github.com/modelcontextprotocol/go-sdk/mcp.(*ServerSession).startKeepalive() /Users/benjamin/Documents/go-sdk/mcp/server.go:1526 +0x130 github.com/modelcontextprotocol/go-sdk/mcp.(*ServerSession).initialized() /Users/benjamin/Documents/go-sdk/mcp/server.go:1059 +0xf8 github.com/modelcontextprotocol/go-sdk/mcp.init.serverSessionMethod[go.shape.*uint8,go.shape.interface { GetMeta() map[string]interface {}; SetMeta(map[string]interface {}); github.com/modelcontextprotocol/go-sdk/mcp.isResult() }].func27() /Users/benjamin/Documents/go-sdk/mcp/shared.go:332 +0x98 github.com/modelcontextprotocol/go-sdk/mcp.newServerMethodInfo[go.shape.*github.com/modelcontextprotocol/go-sdk/mcp.InitializedParams,go.shape.interface { GetMeta() map[string]interface {}; SetMeta(map[string]interface {}); github.com/modelcontextprotocol/go-sdk/mcp.isResult() },go.shape.struct { github.com/modelcontextprotocol/go-sdk/mcp.Meta "json:\"_meta,omitempty\"" }].func2() /Users/benjamin/Documents/go-sdk/mcp/shared.go:272 +0x84 github.com/modelcontextprotocol/go-sdk/mcp.defaultReceivingMethodHandler[go.shape.*uint8]() /Users/benjamin/Documents/go-sdk/mcp/shared.go:152 +0xe8 github.com/modelcontextprotocol/go-sdk/mcp.defaultReceivingMethodHandler[*github.com/modelcontextprotocol/go-sdk/mcp.ServerSession]() /Users/benjamin/Documents/go-sdk/mcp/shared.go:146 +0x60 github.com/modelcontextprotocol/go-sdk/mcp.handleReceive[go.shape.*uint8]() /Users/benjamin/Documents/go-sdk/mcp/shared.go:169 +0x228 github.com/modelcontextprotocol/go-sdk/mcp.(*ServerSession).handle() /Users/benjamin/Documents/go-sdk/mcp/server.go:1445 +0x370 github.com/modelcontextprotocol/go-sdk/mcp.connect[go.shape.*uint8,go.shape.*uint8].func1.1() /Users/benjamin/Documents/go-sdk/mcp/transport.go:170 +0x5c github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.HandlerFunc.Handle() /Users/benjamin/Documents/go-sdk/internal/jsonrpc2/jsonrpc2.go:84 +0x48 github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.(*Connection).handleAsync.func3() /Users/benjamin/Documents/go-sdk/internal/jsonrpc2/conn.go:717 +0xd8 Previous read at 0x00c00007fbf0 by goroutine 1089: github.com/modelcontextprotocol/go-sdk/mcp.(*ServerSession).Close() /Users/benjamin/Documents/go-sdk/mcp/server.go:1502 +0x34 github.com/modelcontextprotocol/go-sdk/mcp.(*StreamableHTTPHandler).ServeHTTP.deferwrap1() /Users/benjamin/Documents/go-sdk/mcp/streamable.go:519 +0x34 runtime.deferreturn() /opt/homebrew/Cellar/go/1.25.4/libexec/src/runtime/panic.go:589 +0x5c github.com/modelcontextprotocol/go-sdk/mcp.TestStreamableStatelessKeepaliveRace.mustNotPanic.func2() /Users/benjamin/Documents/go-sdk/mcp/streamable_test.go:2156 +0x94 net/http.HandlerFunc.ServeHTTP() /opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:2322 +0x48 net/http.serverHandler.ServeHTTP() /opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:3340 +0x270 net/http.(*conn).serve() /opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:2109 +0x9b0 net/http.(*Server).Serve.gowrap3() /opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:3493 +0x48 Goroutine 1095 (running) created at: github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.(*Connection).handleAsync() /Users/benjamin/Documents/go-sdk/internal/jsonrpc2/conn.go:715 +0x334 github.com/modelcontextprotocol/go-sdk/internal/jsonrpc2.(*Connection).acceptRequest.func2.gowrap1() /Users/benjamin/Documents/go-sdk/internal/jsonrpc2/conn.go:676 +0x34 Goroutine 1089 (running) created at: net/http.(*Server).Serve() /opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/server.go:3493 +0x5dc net/http/httptest.(*Server).goServe.func1() /opt/homebrew/Cellar/go/1.25.4/libexec/src/net/http/httptest/server.go:311 +0x98 ================== --- FAIL: TestStreamableStatelessKeepaliveRace (0.14s) testing.go:1617: race detected during execution of test FAIL FAIL github.com/modelcontextprotocol/go-sdk/mcp 1.727s FAIL ``` --------- Co-authored-by: Maciej Kisiel <mkisiel@google.com>
diff --git a/mcp/client.go b/mcp/client.go
@@ -340,7 +340,8 @@ func (cs *ClientSession) ID() string {
 // Close is idempotent and concurrency safe.
 func (cs *ClientSession) Close() error {
 	// Note: keepaliveCancel access is safe without a mutex because:
-	// 1. keepaliveCancel is only written once during startKeepalive (happens-before all Close calls)
+	// 1. keepaliveCancel is only written once during Client.Connect (through startKeepalive),
+	//    which happens before any code that may call Close from another goroutine
 	// 2. context.CancelFunc is safe to call multiple times and from multiple goroutines
 	// 3. The keepalive goroutine calls Close on ping failure, but this is safe since
 	//    Close is idempotent and conn.Close() handles concurrent calls correctly
diff --git a/mcp/server.go b/mcp/server.go
@@ -1031,6 +1031,13 @@ func (s *Server) Connect(ctx context.Context, t Transport, opts *ServerSessionOp
 		s.opts.Logger.Error("server connect error", "error", err)
 		return nil, err
 	}
+
+	// Start keepalive before returning the session to avoid race conditions with Close.
+	// This is safe because the spec allows sending pings before initialization (see ServerSession.handle for details).
+	if s.opts.KeepAlive > 0 {
+		ss.startKeepalive(ss.server.opts.KeepAlive)
+	}
+
 	return ss, nil
 }
 
@@ -1058,9 +1065,6 @@ func (ss *ServerSession) initialized(ctx context.Context, params *InitializedPar
 		ss.server.opts.Logger.Error("duplicate initialized notification")
 		return nil, fmt.Errorf("duplicate %q received", notificationInitialized)
 	}
-	if ss.server.opts.KeepAlive > 0 {
-		ss.startKeepalive(ss.server.opts.KeepAlive)
-	}
 	if h := ss.server.opts.InitializedHandler; h != nil {
 		h(ctx, serverRequestFor(ss, params))
 	}
@@ -1110,7 +1114,7 @@ type ServerSession struct {
 	server          *Server
 	conn            *jsonrpc2.Connection
 	mcpConn         Connection
-	keepaliveCancel context.CancelFunc // TODO: theory around why keepaliveCancel need not be guarded
+	keepaliveCancel context.CancelFunc
 
 	mu    sync.Mutex
 	state ServerSessionState
@@ -1504,7 +1508,8 @@ func (ss *ServerSession) setLevel(_ context.Context, params *SetLoggingLevelPara
 func (ss *ServerSession) Close() error {
 	if ss.keepaliveCancel != nil {
 		// Note: keepaliveCancel access is safe without a mutex because:
-		// 1. keepaliveCancel is only written once during startKeepalive (happens-before all Close calls)
+		// 1. keepaliveCancel is only written once during Server.Connect (through startKeepalive),
+		//    which happens before any code that may call Close from another goroutine
 		// 2. context.CancelFunc is safe to call multiple times and from multiple goroutines
 		// 3. The keepalive goroutine calls Close on ping failure, but this is safe since
 		//    Close is idempotent and conn.Close() handles concurrent calls correctly
diff --git a/mcp/server_test.go b/mcp/server_test.go
@@ -508,54 +508,6 @@ func TestServerAddResourceTemplate(t *testing.T) {
 	}
 }
 
-// TestServerSessionkeepaliveCancelOverwritten is to verify that `ServerSession.keepaliveCancel` is assigned exactly once,
-// ensuring that only a single goroutine is responsible for the session's keepalive ping mechanism.
-func TestServerSessionkeepaliveCancelOverwritten(t *testing.T) {
-	// Set KeepAlive to a long duration to ensure the keepalive
-	// goroutine stays alive for the duration of the test without actually sending
-	// ping requests, since we don't have a real client connection established.
-	server := NewServer(testImpl, &ServerOptions{KeepAlive: 5 * time.Second})
-	ss := &ServerSession{server: server}
-
-	// 1. Initialize the session.
-	_, err := ss.initialize(context.Background(), &InitializeParams{})
-	if err != nil {
-		t.Fatalf("ServerSession initialize failed: %v", err)
-	}
-
-	// 2. Call 'initialized' for the first time. This should start the keepalive mechanism.
-	_, err = ss.initialized(context.Background(), &InitializedParams{})
-	if err != nil {
-		t.Fatalf("First initialized call failed: %v", err)
-	}
-	if ss.keepaliveCancel == nil {
-		t.Fatalf("expected ServerSession.keepaliveCancel to be set after the first call of initialized")
-	}
-
-	// Save the cancel function and use defer to ensure resources are cleaned up.
-	firstCancel := ss.keepaliveCancel
-	defer firstCancel()
-
-	// 3. Manually set the field to nil.
-	// Do this to facilitate the test's core assertion. The goal is to verify that
-	// 'ss.keepaliveCancel' is not assigned a second time. By setting it to nil,
-	// we can easily check after the next call if a new keepalive goroutine was started.
-	ss.keepaliveCancel = nil
-
-	// 4. Call 'initialized' for the second time. This should return an error.
-	_, err = ss.initialized(context.Background(), &InitializedParams{})
-	if err == nil {
-		t.Fatalf("Expected 'duplicate initialized received' error on second call, got nil")
-	}
-
-	// 5. Re-check the field to ensure it remains nil.
-	// Since 'initialized' correctly returned an error and did not call
-	// 'startKeepalive', the field should remain unchanged.
-	if ss.keepaliveCancel != nil {
-		t.Fatal("expected ServerSession.keepaliveCancel to be nil after we manually niled it and re-initialized")
-	}
-}
-
 // panicks reports whether f() panics.
 func panics(f func()) (b bool) {
 	defer func() {
diff --git a/mcp/streamable_test.go b/mcp/streamable_test.go
@@ -363,6 +363,34 @@ func TestStreamableServerShutdown(t *testing.T) {
 	}
 }
 
+// TestStreamableStatelessKeepaliveRace verifies that there is no data race between
+// ServerSession.startKeepalive and ServerSession.Close in stateless servers.
+func TestStreamableStatelessKeepaliveRace(t *testing.T) {
+	ctx := context.Background()
+	server := NewServer(testImpl, &ServerOptions{KeepAlive: time.Hour})
+	AddTool(server, &Tool{Name: "greet"}, sayHi)
+	handler := NewStreamableHTTPHandler(
+		func(*http.Request) *Server { return server },
+		&StreamableHTTPOptions{Stateless: true},
+	)
+	httpServer := httptest.NewServer(mustNotPanic(t, handler))
+	defer httpServer.Close()
+
+	for range 50 {
+		cs, err := NewClient(testImpl, nil).Connect(ctx, &StreamableClientTransport{
+			Endpoint: httpServer.URL,
+		}, nil)
+		if err != nil {
+			t.Fatalf("NewClient() failed: %v", err)
+		}
+		_, _ = cs.CallTool(ctx, &CallToolParams{
+			Name:      "greet",
+			Arguments: map[string]any{"Name": "world"},
+		})
+		_ = cs.Close()
+	}
+}
+
 // TestClientReplay verifies that the client can recover from a mid-stream
 // network failure and receive replayed messages (if replay is configured). It
 // uses a proxy that is killed and restarted to simulate a recoverable network
