Separate underlay acquisition from composition

internal/engine/load_request_test.go

@@ -73,6 +73,50 @@ items: [{id: b}]
 	}
 }
 
+func TestUnderlayGroupAcquisitionRetainsScopesUntilComposition(t *testing.T) {
+	filesystem := NewFilesystem("handoff", fstest.MapFS{
+		"main.yml":  &fstest.MapFile{Data: []byte("configkit: {imports: [{path: child.yml}]}\nmain: retained\n")},
+		"child.yml": &fstest.MapFile{Data: []byte("child: retained\n")},
+	})
+	accounting := &importAccounting{}
+	group := loadGroup{inputs: []loadInput{{source: filesystem.File("main.yml"), imports: true}}}
+	acquired, diagnostics := acquireUnderlayGroupScopes(group, 2, nil, accounting, nil, sequencePolicy{})
+	if len(diagnostics) != 0 {
+		t.Fatalf("acquire underlay group: %#v", diagnostics)
+	}
+	if acquired == nil || acquired.index != 2 || len(acquired.scopes) != 1 || len(acquired.scopes[0].contributions) != 2 {
+		t.Fatalf("acquisition handoff lost its source scope: %#v", acquired)
+	}
+	if len(acquired.sources) != 2 || acquired.sources[0].UnderlayIndex != 2 || acquired.sources[1].ID != "source-2" {
+		t.Fatalf("acquired source evidence = %#v", acquired.sources)
+	}
+	if acquired.inputBytes != int64(len("configkit: {imports: [{path: child.yml}]}\nmain: retained\n")+len("child: retained\n")) {
+		t.Fatalf("group input byte accounting = %d", acquired.inputBytes)
+	}
+
+	composition, diagnostics := composeUnderlayGroup(acquired, nil, sequencePolicy{}, collectionPolicyScope{})
+	if len(diagnostics) != 0 {
+		t.Fatalf("compose acquired group: %#v", diagnostics)
+	}
+	for key, want := range map[string]string{"main": "retained", "child": "retained"} {
+		value, present := documentAtPath(composition.root, Keys(key))
+		if !present || value.Text != want {
+			t.Errorf("composed %s = %#v, %v; want %q", key, value, present, want)
+		}
+	}
+	if composition.inputBytes != acquired.inputBytes || len(composition.acquisition) != 1 || len(composition.acquisition[0].contributions) != 2 {
+		t.Fatalf("composed group handoff = %#v", composition)
+	}
+
+	empty, diagnostics := acquireUnderlayGroupScopes(loadGroup{}, 3, nil, accounting, nil, sequencePolicy{})
+	if len(diagnostics) != 0 || len(empty.scopes) != 0 {
+		t.Fatalf("empty group acquisition = %#v, %#v", empty, diagnostics)
+	}
+	if composition, diagnostics := composeUnderlayGroup(empty, nil, sequencePolicy{}, collectionPolicyScope{}); len(diagnostics) != 0 || composition != nil {
+		t.Fatalf("empty group composition = %#v, %#v", composition, diagnostics)
+	}
+}
+
 func TestUnderlayNamespaceAnchorsUseInputPositionsAndFilesystemIdentity(t *testing.T) {
 	firstNamespace := NewFilesystem("same-label", fstest.MapFS{
 		"root.yml":  &fstest.MapFile{Data: []byte("items: [{id: first}]\n")},