core/rawdb: no need to run truncateFile for readonly mode (ethereum#2…

…8145)

Avoid truncating files, if ancients are opened in readonly mode. With this change, we return error instead of trying (and failing)  to repair

core/rawdb/freezer_table.go

@@ -212,6 +212,9 @@ func (t *freezerTable) repair() error {
 	}
 	// Ensure the index is a multiple of indexEntrySize bytes
 	if overflow := stat.Size() % indexEntrySize; overflow != 0 {
+		if t.readonly {
+			return fmt.Errorf("index file(path: %s, name: %s) size is not a multiple of %d", t.path, t.name, indexEntrySize)
+		}
 		truncateFreezerFile(t.index, stat.Size()-overflow) // New file can't trigger this path
 	}
 	// Retrieve the file sizes and prepare for truncation
@@ -270,6 +273,9 @@ func (t *freezerTable) repair() error {
 	// Keep truncating both files until they come in sync
 	contentExp = int64(lastIndex.offset)
 	for contentExp != contentSize {
+		if t.readonly {
+			return fmt.Errorf("freezer table(path: %s, name: %s, num: %d) is corrupted", t.path, t.name, lastIndex.filenum)
+		}
 		verbose = true
 		// Truncate the head file to the last offset pointer
 		if contentExp < contentSize {