mcp: re-enable race test after fixing data races (#851)

1. (non-test) Don't queue notifications if there are no connected
sessions.
2. Change `reconnectInitialDelay` to be atomic, as it is modified in
tests.
3. Protect map access with mutex in `TestServerTransportCleanup`.

.github/workflows/test.yml

@@ -18,7 +18,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@4b73464bb391d4059bd26b0524d20df3927bd417 # v6.3.0
         with:
-          go-version: "^1.25"
+          go-version: "^1.26"
       - name: Check formatting
         run: |
           unformatted=$(gofmt -l .)
@@ -52,8 +52,6 @@ jobs:
         run: go test -v ./...
 
   race-test:
-    # Temporarily disable until fixes are prepared.
-    if: false
     runs-on: ubuntu-latest
     steps:
       - name: Check out code
@@ -61,6 +59,6 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@4b73464bb391d4059bd26b0524d20df3927bd417 # v6.3.0
         with:
-          go-version: "1.25"
+          go-version: "1.26"
       - name: Test with -race
         run: go test -v -race ./...

mcp/server.go

@@ -628,6 +628,14 @@ func (s *Server) changeAndNotify(notification string, change func() bool) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	if change() && s.shouldSendListChangedNotification(notification) {
+		if len(s.sessions) == 0 {
+			if t := s.pendingNotifications[notification]; t != nil {
+				t.Stop()
+				s.pendingNotifications[notification] = nil
+			}
+			return
+		}
+
 		// Reset the outstanding delayed call, if any.
 		if t := s.pendingNotifications[notification]; t == nil {
 			s.pendingNotifications[notification] = time.AfterFunc(notificationDelay, func() { s.notifySessions(notification) })

mcp/streamable.go

@@ -26,6 +26,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/modelcontextprotocol/go-sdk/auth"
@@ -1515,9 +1516,13 @@ var (
 	// reconnectInitialDelay is the base delay for the first reconnect attempt.
 	//
 	// Mutable for testing.
-	reconnectInitialDelay = 1 * time.Second
+	reconnectInitialDelay atomic.Int64
 )
 
+func init() {
+	reconnectInitialDelay.Store(int64(1 * time.Second))
+}
+
 // Connect implements the [Transport] interface.
 //
 // The resulting [Connection] writes messages via POST requests to the
@@ -2196,7 +2201,7 @@ func calculateReconnectDelay(attempt int) time.Duration {
 		return 0
 	}
 	// Calculate the exponential backoff using the grow factor.
-	backoffDuration := time.Duration(float64(reconnectInitialDelay) * math.Pow(reconnectGrowFactor, float64(attempt-1)))
+	backoffDuration := time.Duration(float64(reconnectInitialDelay.Load()) * math.Pow(reconnectGrowFactor, float64(attempt-1)))
 	// Cap the backoffDuration at maxDelay.
 	backoffDuration = min(backoffDuration, reconnectMaxDelay)
 

mcp/streamable_client_test.go

@@ -446,10 +446,10 @@ func TestStreamableClientResumption_Cancelled(t *testing.T) {
 	//
 	// TODO(#680): experiment with instead using synctest.
 	const tick = 10 * time.Millisecond
-	defer func(delay time.Duration) {
-		reconnectInitialDelay = delay
-	}(reconnectInitialDelay)
-	reconnectInitialDelay = 2 * tick
+	defer func(delay int64) {
+		reconnectInitialDelay.Store(delay)
+	}(reconnectInitialDelay.Load())
+	reconnectInitialDelay.Store(int64(2 * tick))
 
 	// The setup: terminate a request stream and make the resumed request hang
 	// indefinitely. CallTool should still exit when its context is canceled.
@@ -702,10 +702,10 @@ func TestStreamableClientTransientErrors(t *testing.T) {
 func TestStreamableClientRetryWithoutProgress(t *testing.T) {
 	// Speed up reconnection delays for testing.
 	const tick = 10 * time.Millisecond
-	defer func(delay time.Duration) {
-		reconnectInitialDelay = delay
-	}(reconnectInitialDelay)
-	reconnectInitialDelay = tick
+	defer func(delay int64) {
+		reconnectInitialDelay.Store(delay)
+	}(reconnectInitialDelay.Load())
+	reconnectInitialDelay.Store(int64(tick))
 
 	// Use the fakeStreamableServer pattern like other tests to avoid race conditions.
 	ctx := context.Background()

mcp/streamable_test.go

@@ -626,7 +626,10 @@ func TestServerTransportCleanup(t *testing.T) {
 
 	handler := NewStreamableHTTPHandler(func(*http.Request) *Server { return server }, nil)
 	handler.onTransportDeletion = func(sessionID string) {
-		chans[sessionID] <- struct{}{}
+		mu.Lock()
+		ch := chans[sessionID]
+		mu.Unlock()
+		ch <- struct{}{}
 	}
 
 	httpServer := httptest.NewServer(mustNotPanic(t, handler))
@@ -658,7 +661,11 @@ func TestServerTransportCleanup(t *testing.T) {
 		t.Cleanup(func() { _ = clientSession.Close() })
 	}
 
-	for _, ch := range chans {
+	mu.Lock()
+	channels := slices.Collect(maps.Values(chans))
+	mu.Unlock()
+
+	for _, ch := range channels {
 		select {
 		case <-ctx.Done():
 			t.Errorf("did not capture transport deletion event from all session in 10 seconds")