Because you want to close the file in two separate contexts (once you are done and when there was an error), you could also declare it as var file: ?File and, when closing, check if it is valid:
errdefer { if (file) |f| { f.close(); f = null; }
// do stuff
...
// eagerly close the file:
if (file) |f| { f.close(); f = null; }
// do more stuff, file is now closed
...