Strengthen composition and import selection tests

internal/engine/composition_fuzz_test.go

@@ -43,9 +43,10 @@ func FuzzCompositionAndOverlay(f *testing.F) {
 			if differences := compareConfigurationValues(composed.root, reversed.root); len(differences) != 0 {
 				t.Fatalf("composition order changed effective values: %#v", differences)
 			}
-			composed.root.Mapping = append(composed.root.Mapping, document.Entry{Key: "fuzz-detachment"})
-			if !reflect.DeepEqual(left, leftBefore) || !reflect.DeepEqual(right, rightBefore) {
-				t.Fatal("composition result aliases an input document")
+			if mutateExistingDocumentDescendant(composed.root) {
+				if !reflect.DeepEqual(left, leftBefore) || !reflect.DeepEqual(right, rightBefore) {
+					t.Fatal("composition result aliases an input document")
+				}
 			}
 		}
 
@@ -60,9 +61,41 @@ func FuzzCompositionAndOverlay(f *testing.F) {
 		if !reflect.DeepEqual(left, leftBefore) || !reflect.DeepEqual(right, rightBefore) {
 			t.Fatal("overlay mutated an input document")
 		}
-		overlaid.root.Mapping = append(overlaid.root.Mapping, document.Entry{Key: "fuzz-detachment"})
-		if !reflect.DeepEqual(left, leftBefore) || !reflect.DeepEqual(right, rightBefore) {
-			t.Fatal("overlay result aliases an input document")
+		if mutateExistingDocumentDescendant(overlaid.root) {
+			if !reflect.DeepEqual(left, leftBefore) || !reflect.DeepEqual(right, rightBefore) {
+				t.Fatal("overlay result aliases an input document")
+			}
 		}
 	})
 }
+
+func mutateExistingDocumentDescendant(node *document.Node) bool {
+	if node == nil {
+		return false
+	}
+	// Empty containers have no existing element for the alias probe.
+	switch node.Kind {
+	case document.Mapping:
+		if len(node.Mapping) == 0 {
+			return false
+		}
+		entry := &node.Mapping[0]
+		if mutateExistingDocumentDescendant(entry.Value) {
+			return true
+		}
+		entry.Key += "-fuzz-detachment"
+		return true
+	case document.Sequence:
+		if len(node.Sequence) == 0 {
+			return false
+		}
+		if mutateExistingDocumentDescendant(node.Sequence[0]) {
+			return true
+		}
+		node.Sequence[0].Text += "fuzz-detachment"
+		return true
+	default:
+		node.Text += "fuzz-detachment"
+		return true
+	}
+}