Record file mode in the manifest, 0000 unless asked (closes #161)

MFFilePath gains mode (field 304): a file's permission bits, 0777 at
most, or 0000, meaning none recorded. gen and freshen record real modes
only with --include-permissions (ScannerOptions.IncludePermissions); the
builder keeps only mode.Perm(), so setuid, setgid and sticky are never
written. list -l and export show the mode in octal. check reports
MODE_MISMATCH for a recorded mode other than 0000 the file lacks. fetch
refuses a manifest with a mode above 0777 before requesting any file,
sets only the permission bits of each recorded mode on the files it
writes, and downloads again a present file whose mode differs. The decoding-cost bound counts a file
entry at 176 bytes, up from 160.

Model: opus-5-5

mfer/scanner_test.go

@@ -4,6 +4,7 @@ package mfer
 import (
 	"bytes"
 	"context"
+	"os"
 	"testing"
 	"time"
 
@@ -350,6 +351,46 @@ func TestScannerFileEntryFields(t *testing.T) {
 	assert.WithinDuration(t, now, time.Time(entry.Mtime), 2*time.Second)
 }
 
+// A manifest records every mode as 0000 unless the creator asks for
+// permissions; then it records each file's permission bits and never the
+// setuid, setgid or sticky bits.
+func TestScannerRecordsModeOnlyWhenAsked(t *testing.T) {
+	t.Parallel()
+
+	fs := afero.NewMemMapFs()
+	require.NoError(t, afero.WriteFile(fs, "/"+testFile1, []byte("a"), 0o640))
+	require.NoError(t, afero.WriteFile(fs, "/run.sh", []byte("b"), 0o755))
+	require.NoError(t, afero.WriteFile(fs, "/su", []byte("c"), 0o755))
+	require.NoError(t, fs.Chmod("/su", 0o755|os.ModeSetuid|os.ModeSetgid|os.ModeSticky))
+
+	for _, tc := range []struct {
+		includePermissions bool
+		want               map[string]uint32
+	}{
+		{false, map[string]uint32{testFile1: 0, "run.sh": 0, "su": 0}},
+		{true, map[string]uint32{testFile1: 0o640, "run.sh": 0o755, "su": 0o755}},
+	} {
+		s := NewScannerWithOptions(&ScannerOptions{
+			Fs:                 fs,
+			IncludePermissions: tc.includePermissions,
+		})
+		require.NoError(t, s.EnumerateFS(fs, "/", nil))
+
+		var buf bytes.Buffer
+		require.NoError(t, s.ToManifest(context.Background(), &buf, nil))
+
+		m, err := NewManifestFromReader(&buf)
+		require.NoError(t, err)
+
+		got := map[string]uint32{}
+		for _, f := range m.Files() {
+			got[f.GetPath()] = f.GetMode()
+		}
+
+		assert.Equal(t, tc.want, got, "IncludePermissions: %v", tc.includePermissions)
+	}
+}
+
 func TestScannerLargeFileEnumeration(t *testing.T) {
 	t.Parallel()