Resolve candidate paths in traversal order

internal/validation/destination_test.go

@@ -174,3 +174,59 @@ func TestResolveDestinationIgnoresNeighboringFiles(t *testing.T) {
 		t.Fatalf("neighbor changed resolution: absent=%#v present=%#v", gotWithoutNeighbor, gotWithNeighbor)
 	}
 }
+
+func TestResolveDestinationPreservesParentTraversalOrder(t *testing.T) {
+	repository := newTreeRepository(t)
+	for _, name := range []string{"README.md", "docs/file.md", "nested/file.md", "nested/inner/item.txt"} {
+		writeTreeFile(t, repository, name, []byte("captured\n"))
+	}
+	for name, target := range map[string]string{
+		"docs/link":        "../nested/inner",
+		"docs/parent-link": "link/../file.md",
+		"escape":           "../neighbor",
+	} {
+		if err := os.Symlink(target, filepath.Join(repository, filepath.FromSlash(name))); err != nil {
+			t.Fatal(err)
+		}
+	}
+	gitTree(t, repository, "add", "--all")
+	gitTree(t, repository, "commit", "-m", "capture parent traversal")
+	tree, err := ReadCandidateTree(t.Context(), repository, strings.TrimSpace(gitTree(t, repository, "rev-parse", "HEAD")))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, test := range []struct {
+		destination string
+		kind        DestinationKind
+		resolved    string
+	}{
+		{"docs/link/../file.md", DestinationEntry, "nested/file.md"},
+		{"docs/parent-link", DestinationEntry, "nested/file.md"},
+		{"escape/../README.md", DestinationOutsideCheckout, ""},
+		{"missing/../README.md", DestinationMissing, ""},
+		{"README.md/../README.md", DestinationMissing, ""},
+		{"README.md/.", DestinationMissing, ""},
+	} {
+		t.Run(test.destination, func(t *testing.T) {
+			got, err := tree.ResolveDestination(t.Context(), "README.md", test.destination)
+			if err != nil || got.Kind != test.kind || (test.resolved != "" && got.Path != test.resolved) {
+				t.Fatalf("resolution = %#v, %v; want %s %q", got, err, test.kind, test.resolved)
+			}
+		})
+	}
+	before, err := tree.ResolveDestination(t.Context(), "README.md", "escape/../README.md")
+	if err != nil {
+		t.Fatal(err)
+	}
+	neighbor := filepath.Join(filepath.Dir(repository), "neighbor")
+	if err := os.MkdirAll(neighbor, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(neighbor, "file.md"), []byte("neighbor"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	after, err := tree.ResolveDestination(t.Context(), "README.md", "escape/../README.md")
+	if err != nil || before != after {
+		t.Fatalf("neighbor changed resolution: %#v -> %#v, %v", before, after, err)
+	}
+}