v2/pkg/engine/datasource/graphql_datasource/graphql_datasource.go (1)

1803-1808: Potential data race on lazy pool initialization.

getPrintKitPool() performs a non-atomic check-then-write on f.printKitPool. If Planner() is called concurrently when the pool is uninitialized, multiple pools could be created with one being lost.

If concurrent factory usage is expected, consider using sync.Once for initialization:

♻️ Thread-safe lazy initialization

 type Factory[T Configuration] struct {
 	executionContext   context.Context
 	httpClient         *http.Client
 	grpcClient         grpc.ClientConnInterface
 	grpcClientProvider func() grpc.ClientConnInterface
 	subscriptionClient GraphQLSubscriptionClient
 	printKitPool       *sync.Pool
+	printKitPoolOnce   sync.Once
+	relaxNullability   bool
 }

 func (f *Factory[T]) EnableSubgraphFieldSelectionMergingNullabilityRelaxation() {
-	f.printKitPool = newPrintKitPool(astvalidation.WithRelaxFieldSelectionMergingNullability())
+	f.relaxNullability = true
 }

 func (f *Factory[T]) getPrintKitPool() *sync.Pool {
-	if f.printKitPool == nil {
-		f.printKitPool = newPrintKitPool()
-	}
+	f.printKitPoolOnce.Do(func() {
+		if f.relaxNullability {
+			f.printKitPool = newPrintKitPool(astvalidation.WithRelaxFieldSelectionMergingNullability())
+		} else {
+			f.printKitPool = newPrintKitPool()
+		}
+	})
 	return f.printKitPool
 }

v2/pkg/astvalidation/operation_validation_test.go (1)

1080-1131: Add a relaxed‑mode overlap guard test.
The new cases cover non‑overlapping concrete types, but they don’t assert that FieldSelectionMerging(true) still rejects potentially overlapping types (e.g., interface vs object). Adding a variant will lock that behavior in.

🧪 Suggested test addition

+					t.Run("still rejects differing return type nullability when overlap is possible (relaxed)", func(t *testing.T) {
+						runWithDefinition(t, boxDefinition, `
+								{
+									someBox {
+										... on NonNullStringBox1 {
+											scalar
+										}
+										... on StringBox {
+											scalar
+										}
+									}
+								}`, FieldSelectionMerging(true), Invalid,
+							withValidationErrors(`fields 'scalar' conflict because they return conflicting types 'String!' and 'String'`))
+					})

v2/pkg/engine/datasource/graphql_datasource/graphql_datasource.go (1)

1723-1725: Avoid reconfiguring the pool after initialization.

EnableSubgraphFieldSelectionMergingNullabilityRelaxation() replaces the pool without synchronization; if called after any planner creation, you can end up with mixed validation behavior across planners and potential data races. Consider storing a flag and applying options only at first initialization.

♻️ Suggested refactor to make configuration one-time

 type Factory[T Configuration] struct {
 	executionContext   context.Context
 	httpClient         *http.Client
 	grpcClient         grpc.ClientConnInterface
 	grpcClientProvider func() grpc.ClientConnInterface
 	subscriptionClient GraphQLSubscriptionClient
 	printKitPool       *sync.Pool
 	printKitPoolOnce   sync.Once
+	relaxFieldSelectionMergingNullability bool
 }
 
 func (f *Factory[T]) EnableSubgraphFieldSelectionMergingNullabilityRelaxation() {
-	f.printKitPool = newPrintKitPool(astvalidation.WithRelaxFieldSelectionMergingNullability())
+	f.relaxFieldSelectionMergingNullability = true
 }
 
 func (f *Factory[T]) getPrintKitPool() *sync.Pool {
 	f.printKitPoolOnce.Do(func() {
-		if f.printKitPool == nil {
-			f.printKitPool = newPrintKitPool()
-		}
+		opts := []astvalidation.Option{}
+		if f.relaxFieldSelectionMergingNullability {
+			opts = append(opts, astvalidation.WithRelaxFieldSelectionMergingNullability())
+		}
+		f.printKitPool = newPrintKitPool(opts...)
 	})
 	return f.printKitPool
 }

Also applies to: 1796-1811

v2/pkg/engine/datasource/graphql_datasource/graphql_datasource.go (1)

1797-1814: Consider adding a safeguard against out-of-order calls.

The sync.Once pattern means that if getPrintKitPool() (or Planner()) is called before EnableSubgraphFieldSelectionMergingNullabilityRelaxation(), the relaxation flag will be ignored silently. The comment documents the requirement, but a defensive check could prevent subtle bugs.

💡 Optional: Add a check or panic for out-of-order calls

 func (f *Factory[T]) EnableSubgraphFieldSelectionMergingNullabilityRelaxation() {
+	// Ensure this is called before the pool is initialized
+	if f.printKitPool != nil {
+		panic("EnableSubgraphFieldSelectionMergingNullabilityRelaxation must be called before Planner() or getPrintKitPool()")
+	}
 	f.relaxFieldMergingNullability = true
 }