mcp: fix panics with nil logger; ensure test handlers do not panic (#557)

PR 501 added logging to the streamable server, but didn't pass a non-nil
logger to the streamable transport (an oversight). However, tests still
passed due to net/http's panic recovery.

Update test servers to enforce that handlers do not panic, and fix the
panic by:
1. Passing along the configured logger to the session transport.
2. Using ensureLogger in Connect, since we technically allow
StreamableServerTransports to be constructed by the user.

Fixes #556

mcp/streamable.go

@@ -236,6 +236,7 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 			SessionID:    sessionID,
 			Stateless:    h.opts.Stateless,
 			jsonResponse: h.opts.JSONResponse,
+			logger:       h.opts.Logger,
 		}
 
 		// To support stateless mode, we initialize the session with a default
@@ -377,6 +378,10 @@ type StreamableServerTransport struct {
 	// StreamableHTTPOptions.JSONResponse is exported.
 	jsonResponse bool
 
+	// optional logger provided through the [StreamableHTTPOptions.Logger].
+	//
+	// TODO(rfindley): logger should be exported, since we want to allow people
+	// to write their own streamable HTTP handler.
 	logger *slog.Logger
 
 	// connection is non-nil if and only if the transport has been connected.
@@ -393,7 +398,7 @@ func (t *StreamableServerTransport) Connect(ctx context.Context) (Connection, er
 		stateless:      t.Stateless,
 		eventStore:     t.EventStore,
 		jsonResponse:   t.jsonResponse,
-		logger:         t.logger,
+		logger:         ensureLogger(t.logger), // see #556: must be non-nil
 		incoming:       make(chan jsonrpc.Message, 10),
 		done:           make(chan struct{}),
 		streams:        make(map[string]*stream),

mcp/streamable_test.go

@@ -18,6 +18,8 @@ import (
 	"net/http/httptest"
 	"net/http/httputil"
 	"net/url"
+	"os"
+	"runtime"
 	"sort"
 	"strings"
 	"sync"
@@ -91,7 +93,7 @@ func TestStreamableTransports(t *testing.T) {
 				headerMu   sync.Mutex
 				lastHeader http.Header
 			)
-			httpServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			httpServer := httptest.NewServer(mustNotPanic(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 				headerMu.Lock()
 				lastHeader = r.Header
 				headerMu.Unlock()
@@ -102,7 +104,7 @@ func TestStreamableTransports(t *testing.T) {
 					t.Errorf("got cookie %q, want %q", cookie.Value, "test-value")
 				}
 				handler.ServeHTTP(w, r)
-			}))
+			})))
 			defer httpServer.Close()
 
 			// Create a client and connect it to the server using our StreamableClientTransport.
@@ -315,7 +317,7 @@ func testClientReplay(t *testing.T, test clientReplayTest) {
 			return new(CallToolResult), nil, nil
 		})
 
-	realServer := httptest.NewServer(NewStreamableHTTPHandler(func(*http.Request) *Server { return server }, nil))
+	realServer := httptest.NewServer(mustNotPanic(t, NewStreamableHTTPHandler(func(*http.Request) *Server { return server }, nil)))
 	defer realServer.Close()
 	realServerURL, err := url.Parse(realServer.URL)
 	if err != nil {
@@ -324,6 +326,7 @@ func testClientReplay(t *testing.T, test clientReplayTest) {
 
 	// Configure a proxy that sits between the client and the real server.
 	proxyHandler := httputil.NewSingleHostReverseProxy(realServerURL)
+	// note: don't use mustNotPanic here as the proxy WILL panic when killed.
 	proxy := httptest.NewServer(proxyHandler)
 	proxyAddr := proxy.Listener.Addr().String() // Get the address to restart it later.
 
@@ -434,7 +437,7 @@ func TestServerTransportCleanup(t *testing.T) {
 		chans[sessionID] <- struct{}{}
 	}
 
-	httpServer := httptest.NewServer(handler)
+	httpServer := httptest.NewServer(mustNotPanic(t, handler))
 	defer httpServer.Close()
 
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
@@ -484,7 +487,7 @@ func TestServerInitiatedSSE(t *testing.T) {
 	notifications := make(chan string)
 	server := NewServer(testImpl, nil)
 
-	httpServer := httptest.NewServer(NewStreamableHTTPHandler(func(*http.Request) *Server { return server }, nil))
+	httpServer := httptest.NewServer(mustNotPanic(t, NewStreamableHTTPHandler(func(*http.Request) *Server { return server }, nil)))
 	defer httpServer.Close()
 
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
@@ -857,7 +860,7 @@ func TestStreamableServerTransport(t *testing.T) {
 }
 
 func testStreamableHandler(t *testing.T, handler http.Handler, requests []streamableRequest) {
-	httpServer := httptest.NewServer(handler)
+	httpServer := httptest.NewServer(mustNotPanic(t, handler))
 	defer httpServer.Close()
 
 	// blocks records request blocks by jsonrpc. ID.
@@ -1247,7 +1250,7 @@ func TestStreamableStateless(t *testing.T) {
 
 	testClientCompatibility := func(t *testing.T, handler http.Handler) {
 		ctx := context.Background()
-		httpServer := httptest.NewServer(handler)
+		httpServer := httptest.NewServer(mustNotPanic(t, handler))
 		defer httpServer.Close()
 		cs, err := NewClient(testImpl, nil).Connect(ctx, &StreamableClientTransport{Endpoint: httpServer.URL}, nil)
 		if err != nil {
@@ -1332,7 +1335,7 @@ func TestTokenInfo(t *testing.T) {
 		}, nil
 	}
 	handler := auth.RequireBearerToken(verifier, nil)(streamHandler)
-	httpServer := httptest.NewServer(handler)
+	httpServer := httptest.NewServer(mustNotPanic(t, handler))
 	defer httpServer.Close()
 
 	transport := &StreamableClientTransport{Endpoint: httpServer.URL}
@@ -1366,7 +1369,7 @@ func TestStreamableGET(t *testing.T) {
 	server := NewServer(testImpl, nil)
 
 	handler := NewStreamableHTTPHandler(func(req *http.Request) *Server { return server }, nil)
-	httpServer := httptest.NewServer(handler)
+	httpServer := httptest.NewServer(mustNotPanic(t, handler))
 	defer httpServer.Close()
 
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
@@ -1442,7 +1445,7 @@ func TestStreamableClientContextPropagation(t *testing.T) {
 	defer cancel()
 	ctx2 := context.WithValue(ctx, testKey, testValue)
 
-	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+	server := httptest.NewServer(mustNotPanic(t, http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
 		switch req.Method {
 		case "POST":
 			w.Header().Set("Content-Type", "application/json")
@@ -1455,7 +1458,7 @@ func TestStreamableClientContextPropagation(t *testing.T) {
 		case "DELETE":
 			w.WriteHeader(http.StatusNoContent)
 		}
-	}))
+	})))
 	defer server.Close()
 
 	transport := &StreamableClientTransport{Endpoint: server.URL}
@@ -1486,3 +1489,19 @@ func TestStreamableClientContextPropagation(t *testing.T) {
 	}
 
 }
+
+// mustNotPanic is a helper to enforce that test handlers do not panic (see
+// issue #556).
+func mustNotPanic(t *testing.T, h http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		defer func() {
+			if r := recover(); r != nil {
+				buf := make([]byte, 1<<20)
+				n := runtime.Stack(buf, false)
+				fmt.Fprintf(os.Stderr, "handler panic: %v\n\n%s", r, buf[:n])
+				t.Errorf("handler panicked: %v", r)
+			}
+		}()
+		h.ServeHTTP(w, req)
+	})
+}