Strengthen composition and import selection tests

docs/roadmap/implementation.md

@@ -605,6 +605,14 @@ Focused check: `go test ./internal/engine -run 'Import|Directive|Directory|Glob|
 Findings: **F10, F11**.
 Owners: `internal/engine/composition_fuzz_test.go`, `import_selection_test.go`.
 
+Status: **complete** (F10, F11). Prior completed commits: `2736ddf`, `deb9aa5`,
+`ae3408a`, `ea27f17`, `08a8c25`, `877c497`, `8aba43d`, `7bf2f0c`,
+`0f9ecf1`, `119b42c`, `247a12b`, `4fcff5f`, `12d350a`, `08f2a8f`,
+`826062a`, `e4a5ecb`, `3122e2f`, `6f6130f`. Validation passed:
+`env GOTOOLCHAIN=local GOPROXY=off GOTELEMETRY=off go test ./internal/engine -run 'FuzzCompositionAndOverlay|SelectNamedImports' -count=1`
+and `git diff --check`. The bounded composition campaign remains scheduled for
+Stage 33.
+
 In `FuzzCompositionAndOverlay`, mutate an existing retained descendant or in-length
 element before comparing both original inputs with their saved values. Handle
 empty containers without assuming append affects shared storage. Keep existing

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
+	}
+}

internal/engine/import_selection_test.go

@@ -161,8 +161,21 @@ func TestSelectNamedImportsChecksNormalizedPathsLexically(t *testing.T) {
 	if len(diagnostics) != 0 || len(selection.candidates) != 2 {
 		t.Fatalf("selection, diagnostics = %#v, %#v", selection, diagnostics)
 	}
-	if strings.Join(filesystem.statPaths, ",") != "a.yaml,a.yaml,z.yaml,z.yaml" {
-		t.Fatalf("target stat order = %#v", filesystem.statPaths)
+	if selection.candidates[0].path != "a.yaml" || selection.candidates[1].path != "z.yaml" {
+		t.Fatalf("candidate order = %#v, want a.yaml then z.yaml", selection.candidates)
+	}
+
+	failingFilesystem := &statOnlyImportFilesystem{statErrors: map[string]error{
+		"a.yaml": fs.ErrPermission,
+		"z.yaml": fs.ErrPermission,
+	}}
+	failingAccess := newNativeAccess(failingFilesystem, ".", &namespaceToken{})
+	_, diagnostics = selectNamedImportCandidates(failingAccess, ".", specs[0], resolvedImportPolicy{})
+	if len(diagnostics) != 1 || diagnostics[0].Code != CodeImportInvalid || diagnostics[0].Path.String() != "/configkit/imports/0/path/1" {
+		t.Fatalf("first lexical failure = %#v, want /configkit/imports/0/path/1", diagnostics)
+	}
+	if diagnostics[0].Primary == nil || diagnostics[0].Primary.SourceID != "source-1" {
+		t.Fatalf("first lexical failure location = %#v", diagnostics[0].Primary)
 	}
 }
 
@@ -226,7 +239,6 @@ type statOnlyImportFilesystem struct {
 	statErrors map[string]error
 	openCalls  int
 	statCalls  int
-	statPaths  []string
 }
 
 func (filesystem *statOnlyImportFilesystem) Open(name string) (fs.File, error) {
@@ -236,7 +248,6 @@ func (filesystem *statOnlyImportFilesystem) Open(name string) (fs.File, error) {
 
 func (filesystem *statOnlyImportFilesystem) Stat(name string) (fs.FileInfo, error) {
 	filesystem.statCalls++
-	filesystem.statPaths = append(filesystem.statPaths, name)
 	if err := filesystem.statErrors[name]; err != nil {
 		return nil, err
 	}