migrations: Fix S3 backend detection

internal/backend/limiter/limiter_backend.go

@@ -46,6 +46,8 @@ func (r rateLimitedBackend) Load(ctx context.Context, h restic.Handle, length in
 	})
 }
 
+func (r rateLimitedBackend) Unwrap() restic.Backend { return r.Backend }
+
 type limitedReader struct {
 	io.Reader
 	writerTo io.WriterTo

internal/backend/logger/log.go

@@ -75,3 +75,5 @@ func (be *Backend) Close() error {
 	debug.Log("  close err %v", err)
 	return err
 }
+
+func (be *Backend) Unwrap() restic.Backend { return be.Backend }

internal/backend/retry/backend_retry.go

@@ -191,3 +191,7 @@ func (be *Backend) List(ctx context.Context, t restic.FileType, fn func(restic.F
 
 	return err
 }
+
+func (be *Backend) Unwrap() restic.Backend {
+	return be.Backend
+}

internal/backend/sema/backend.go

@@ -85,3 +85,7 @@ func (be *SemaphoreBackend) Remove(ctx context.Context, h restic.Handle) error {
 
 	return be.Backend.Remove(ctx, h)
 }
+
+func (be *SemaphoreBackend) Unwrap() restic.Backend {
+	return be.Backend
+}

internal/cache/backend.go

@@ -211,3 +211,7 @@ func (b *Backend) Stat(ctx context.Context, h restic.Handle) (restic.FileInfo, e
 func (b *Backend) IsNotExist(err error) bool {
 	return b.Backend.IsNotExist(err)
 }
+
+func (b *Backend) Unwrap() restic.Backend {
+	return b.Backend
+}

internal/migrations/s3_layout.go

@@ -8,7 +8,6 @@ import (
 
 	"github.com/restic/restic/internal/backend/layout"
 	"github.com/restic/restic/internal/backend/s3"
-	"github.com/restic/restic/internal/cache"
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/errors"
 	"github.com/restic/restic/internal/restic"
@@ -22,24 +21,26 @@ func init() {
 // "default" layout.
 type S3Layout struct{}
 
-func toS3Backend(repo restic.Repository) *s3.Backend {
+func toS3Backend(b restic.Backend) *s3.Backend {
-	b := repo.Backend()
+	for b != nil {
-	// unwrap cache
+		if be, ok := b.(*s3.Backend); ok {
-	if be, ok := b.(*cache.Backend); ok {
+			return be
-		b = be.Backend
 		}
 
-	be, ok := b.(*s3.Backend)
+		if be, ok := b.(restic.BackendUnwrapper); ok {
-	if !ok {
+			b = be.Unwrap()
+		} else {
+			// not the backend we're looking for
+			break
+		}
+	}
 	debug.Log("backend is not s3")
 	return nil
 }
-	return be
-}
 
 // Check tests whether the migration can be applied.
 func (m *S3Layout) Check(ctx context.Context, repo restic.Repository) (bool, string, error) {
-	be := toS3Backend(repo)
+	be := toS3Backend(repo.Backend())
 	if be == nil {
 		debug.Log("backend is not s3")
 		return false, "backend is not s3", nil
@@ -91,7 +92,7 @@ func (m *S3Layout) moveFiles(ctx context.Context, be *s3.Backend, l layout.Layou
 
 // Apply runs the migration.
 func (m *S3Layout) Apply(ctx context.Context, repo restic.Repository) error {
-	be := toS3Backend(repo)
+	be := toS3Backend(repo.Backend())
 	if be == nil {
 		debug.Log("backend is not s3")
 		return errors.New("backend is not s3")

internal/migrations/s3_layout_test.go

@@ -0,0 +1,27 @@
+package migrations
+
+import (
+	"testing"
+
+	"github.com/restic/restic/internal/backend/mock"
+	"github.com/restic/restic/internal/backend/s3"
+	"github.com/restic/restic/internal/cache"
+	"github.com/restic/restic/internal/test"
+)
+
+func TestS3UnwrapBackend(t *testing.T) {
+	// toS3Backend(b restic.Backend) *s3.Backend
+
+	m := mock.NewBackend()
+	test.Assert(t, toS3Backend(m) == nil, "mock backend is not an s3 backend")
+
+	// uninitialized fake backend for testing
+	s3 := &s3.Backend{}
+	test.Assert(t, toS3Backend(s3) == s3, "s3 was not returned")
+
+	c := &cache.Backend{Backend: s3}
+	test.Assert(t, toS3Backend(c) == s3, "failed to unwrap s3 backend")
+
+	c.Backend = m
+	test.Assert(t, toS3Backend(c) == nil, "a wrapped mock backend is not an s3 backend")
+}

internal/restic/backend.go

@@ -70,6 +70,11 @@ type Backend interface {
 	Delete(ctx context.Context) error
 }
 
+type BackendUnwrapper interface {
+	// Unwrap returns the underlying backend or nil if there is none.
+	Unwrap() Backend
+}
+
 // FileInfo is contains information about a file in the backend.
 type FileInfo struct {
 	Size int64