restore: do not delete target if it is a file

This is implicitly achieved by creating the target directory as the
first step, which will fail if a file already exist in its place.

internal/restorer/restorer.go

@@ -347,6 +347,14 @@ func (res *Restorer) RestoreTo(ctx context.Context, dst string) error {
 		}
 	}
 
+	if !res.opts.DryRun {
+		// ensure that the target directory exists and is actually a directory
+		// Using ensureDir is too aggressive here as it also removes unexpected files
+		if err := fs.MkdirAll(dst, 0700); err != nil {
+			return fmt.Errorf("cannot create target directory: %w", err)
+		}
+	}
+
 	idx := NewHardlinkIndex[string]()
 	filerestorer := newFileRestorer(dst, res.repo.LoadBlobsFromPack, res.repo.LookupBlob,
 		res.repo.Connections(), res.opts.Sparse, res.opts.Delete, res.opts.Progress)

internal/restorer/restorer_test.go

@@ -1372,3 +1372,25 @@ func TestRestoreDelete(t *testing.T) {
 		})
 	}
 }
+
+func TestRestoreToFile(t *testing.T) {
+	snapshot := Snapshot{
+		Nodes: map[string]Node{
+			"foo": File{Data: "content: foo\n"},
+		},
+	}
+
+	repo := repository.TestRepository(t)
+	tempdir := filepath.Join(rtest.TempDir(t), "target")
+
+	// create a file in the place of the target directory
+	rtest.OK(t, os.WriteFile(tempdir, []byte{}, 0o700))
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	sn, _ := saveSnapshot(t, repo, snapshot, noopGetGenericAttributes)
+	res := NewRestorer(repo, sn, Options{})
+	err := res.RestoreTo(ctx, tempdir)
+	rtest.Assert(t, strings.Contains(err.Error(), "cannot create target directory"), "unexpected error %v", err)
+}