mcp: Ensure keepalive goroutine is started exactly once

mcp/server.go

@@ -602,9 +602,6 @@ func (ss *ServerSession) initialized(ctx context.Context, params *InitializedPar
 		// params are non-nil.
 		params = new(InitializedParams)
 	}
-	if ss.server.opts.KeepAlive > 0 {
-		ss.startKeepalive(ss.server.opts.KeepAlive)
-	}
 	var wasInit, wasInitd bool
 	ss.updateState(func(state *ServerSessionState) {
 		wasInit = state.InitializeParams != nil
@@ -620,6 +617,9 @@ func (ss *ServerSession) initialized(ctx context.Context, params *InitializedPar
 	if wasInitd {
 		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))
 	}

mcp/server_test.go

@@ -9,6 +9,7 @@ import (
 	"log"
 	"slices"
 	"testing"
+	"time"
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/jsonschema-go/jsonschema"
@@ -371,3 +372,51 @@ func TestServerCapabilities(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")
+	}
+}