cache: Auto-remove invalid files

internal/cache/file.go

@@ -6,6 +6,7 @@ import (
 	"path/filepath"
 
 	"github.com/pkg/errors"
+	"github.com/restic/restic/internal/crypto"
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/fs"
 	"github.com/restic/restic/internal/restic"
@@ -50,6 +51,18 @@ func (c *Cache) Load(h restic.Handle, length int, offset int64) (io.ReadCloser,
 		return nil, errors.Wrap(err, "Open")
 	}
 
+	fi, err := f.Stat()
+	if err != nil {
+		_ = f.Close()
+		return nil, errors.Wrap(err, "Stat")
+	}
+
+	if fi.Size() <= crypto.Extension {
+		_ = f.Close()
+		_ = c.Remove(h)
+		return nil, errors.New("cached file is truncated, removing")
+	}
+
 	if offset > 0 {
 		if _, err = f.Seek(offset, io.SeekStart); err != nil {
 			f.Close()