migrations: move logic of upgrade_repo_v2 to repository package

The migration modifies repository internals and thus should live within
the repository package.

internal/repository/upgrade_repo_test.go

@@ -0,0 +1,82 @@
+package repository
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"sync"
+	"testing"
+
+	"github.com/restic/restic/internal/backend"
+	"github.com/restic/restic/internal/errors"
+	rtest "github.com/restic/restic/internal/test"
+)
+
+func TestUpgradeRepoV2(t *testing.T) {
+	repo := TestRepositoryWithVersion(t, 1)
+	if repo.Config().Version != 1 {
+		t.Fatal("test repo has wrong version")
+	}
+
+	err := UpgradeRepo(context.Background(), repo.(*Repository))
+	rtest.OK(t, err)
+}
+
+type failBackend struct {
+	backend.Backend
+
+	mu                        sync.Mutex
+	ConfigFileSavesUntilError uint
+}
+
+func (be *failBackend) Save(ctx context.Context, h backend.Handle, rd backend.RewindReader) error {
+	if h.Type != backend.ConfigFile {
+		return be.Backend.Save(ctx, h, rd)
+	}
+
+	be.mu.Lock()
+	if be.ConfigFileSavesUntilError == 0 {
+		be.mu.Unlock()
+		return errors.New("failure induced for testing")
+	}
+
+	be.ConfigFileSavesUntilError--
+	be.mu.Unlock()
+
+	return be.Backend.Save(ctx, h, rd)
+}
+
+func TestUpgradeRepoV2Failure(t *testing.T) {
+	be := TestBackend(t)
+
+	// wrap backend so that it fails upgrading the config after the initial write
+	be = &failBackend{
+		ConfigFileSavesUntilError: 1,
+		Backend:                   be,
+	}
+
+	repo := TestRepositoryWithBackend(t, be, 1, Options{})
+	if repo.Config().Version != 1 {
+		t.Fatal("test repo has wrong version")
+	}
+
+	err := UpgradeRepo(context.Background(), repo.(*Repository))
+	if err == nil {
+		t.Fatal("expected error returned from Apply(), got nil")
+	}
+
+	upgradeErr := err.(*upgradeRepoV2Error)
+	if upgradeErr.UploadNewConfigError == nil {
+		t.Fatal("expected upload error, got nil")
+	}
+
+	if upgradeErr.ReuploadOldConfigError == nil {
+		t.Fatal("expected reupload error, got nil")
+	}
+
+	if upgradeErr.BackupFilePath == "" {
+		t.Fatal("no backup file path found")
+	}
+	rtest.OK(t, os.Remove(upgradeErr.BackupFilePath))
+	rtest.OK(t, os.Remove(filepath.Dir(upgradeErr.BackupFilePath)))
+}