migrations: add temporary hack for s3_layout

The migration will be removed after the next restic release anyways.
Thus, there's no need for a clean implementation.

internal/checker/checker.go

@@ -256,8 +256,10 @@ func isS3Legacy(b backend.Backend) bool {
 func (c *Checker) Packs(ctx context.Context, errChan chan<- error) {
 	defer close(errChan)
 
-	if isS3Legacy(c.repo.Backend()) {
-		errChan <- ErrLegacyLayout
+	if r, ok := c.repo.(*repository.Repository); ok {
+		if isS3Legacy(repository.AsS3Backend(r)) {
+			errChan <- ErrLegacyLayout
+		}
 	}
 
 	debug.Log("checking for %d packs", len(c.packs))

internal/migrations/s3_layout.go

@@ -11,6 +11,7 @@ import (
 	"github.com/restic/restic/internal/backend/s3"
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/errors"
+	"github.com/restic/restic/internal/repository"
 	"github.com/restic/restic/internal/restic"
 )
 
@@ -24,7 +25,7 @@ type S3Layout struct{}
 
 // Check tests whether the migration can be applied.
 func (m *S3Layout) Check(_ context.Context, repo restic.Repository) (bool, string, error) {
-	be := backend.AsBackend[*s3.Backend](repo.Backend())
+	be := repository.AsS3Backend(repo.(*repository.Repository))
 	if be == nil {
 		debug.Log("backend is not s3")
 		return false, "backend is not s3", nil
@@ -76,7 +77,7 @@ 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 := backend.AsBackend[*s3.Backend](repo.Backend())
+	be := repository.AsS3Backend(repo.(*repository.Repository))
 	if be == nil {
 		debug.Log("backend is not s3")
 		return errors.New("backend is not s3")

internal/repository/s3_backend.go

@@ -0,0 +1,12 @@
+package repository
+
+import (
+	"github.com/restic/restic/internal/backend"
+	"github.com/restic/restic/internal/backend/s3"
+)
+
+// AsS3Backend extracts the S3 backend from a repository
+// TODO remove me once restic 0.17 was released
+func AsS3Backend(repo *Repository) *s3.Backend {
+	return backend.AsBackend[*s3.Backend](repo.be)
+}