If stdin is read more then once subsequent calls will return an empty string since it's consumed already.

@LnL7 The kosher thing to do here would be to have some state in the eval state to determine if stdin has been read and if so what its contents were.

IMHO this is a really bad idea, reading stdin is very impure and very non-compositional (for example, consider programs like nixos-rebuild or NixOps where an expression might be evaluated multiple times).

If the goal is to pass large strings, it might be better to add some flag for this, e.g. nix-instantiate --arg-from-file foobar /dev/stdin.

I like the idea of --arg-from-file, do I change this to throw an error so it fails everywhere then?

@LnL7 Hm, I don't understand the question, what should throw an error?

I made this to make the behaviour between darwin and linux consistent, so if we don't want this I would prefer to also disable the current behaviour on darwin.

I'm not sure this would be sufficient, e.g. builtins.readFile /dev/fd/0 will still behave the same I presume. Perhaps checking the file type using fstat(0, &st) and only allowing regular files would be more appropriate.

Hmm guess I didn't push my second commit, that's almost what I did 😄.

Maybe it should use S_ISREG instead — I don't think we want to allow FIFOs either?

Using S_ISREG doesn't work for symlinks without resolving them first so I extended the blacklist instead.

Yes it does, as long as you use stat and not lstat.

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