index: deprecate legacy index format

internal/feature/registry.go

@@ -6,10 +6,12 @@ var Flag = New()
 // flag names are written in kebab-case
 const (
 	ExampleFeature       FlagName = "example-feature"
+	DeprecateLegacyIndex FlagName = "deprecate-legacy-index"
 )
 
 func init() {
 	Flag.SetFlags(map[FlagName]FlagDesc{
 		ExampleFeature:       {Type: Alpha, Description: "just for testing"},
+		DeprecateLegacyIndex: {Type: Beta, Description: "disable support for index format used by restic 0.1.0. Use `restic repair index` to update the index if necessary."},
 	})
 }

internal/index/index.go

@@ -3,12 +3,14 @@ package index
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 	"io"
 	"sync"
 	"time"
 
 	"github.com/restic/restic/internal/crypto"
 	"github.com/restic/restic/internal/errors"
+	"github.com/restic/restic/internal/feature"
 	"github.com/restic/restic/internal/restic"
 
 	"github.com/restic/restic/internal/debug"
@@ -515,6 +517,10 @@ func DecodeIndex(buf []byte, id restic.ID) (idx *Index, oldFormat bool, err erro
 		debug.Log("Error %v", err)
 
 		if isErrOldIndex(err) {
+			if feature.Flag.Enabled(feature.DeprecateLegacyIndex) {
+				return nil, false, fmt.Errorf("index seems to use the legacy format. update it using `restic repair index`")
+			}
+
 			debug.Log("index is probably old format, trying that")
 			idx, err = decodeOldIndex(buf)
 			return idx, err == nil, err

internal/index/index_test.go

@@ -8,6 +8,7 @@ import (
 	"sync"
 	"testing"
 
+	"github.com/restic/restic/internal/feature"
 	"github.com/restic/restic/internal/index"
 	"github.com/restic/restic/internal/restic"
 	rtest "github.com/restic/restic/internal/test"
@@ -427,6 +428,8 @@ func BenchmarkEncodeIndex(b *testing.B) {
 }
 
 func TestIndexUnserializeOld(t *testing.T) {
+	defer feature.TestSetFlag(t, feature.Flag, feature.DeprecateLegacyIndex, false)()
+
 	idx, oldFormat, err := index.DecodeIndex(docOldExample, restic.NewRandomID())
 	rtest.OK(t, err)
 	rtest.Assert(t, oldFormat, "old index format recognized as new format")