[Nix-dev] [***SPAM***] ‘nix-instantiate --xml’ and self-referential attrsets

[Ludovic Courtès]

Hello!

Consider this example:

--8<---------------cut here---------------start------------->8---
let x = derivation {
      name = "x"; builder = ./foo.sh; system = builtins.currentSystem;
    };
    y = { b = x; };
in
  x // y
--8<---------------cut here---------------end--------------->8---

Running ‘nix-instantiate --strict --eval-only --xml’ yields this:

--8<---------------cut here---------------start------------->8---
<?xml version='1.0' encoding='utf-8'?>
<expr>
  <derivation drvPath="/nix/store/b1pmg2fhxnlnxy0z9hfglgyk66wpa2ar-x.drv" outPath="/nix/store/fw6flnp42lqn9i62aknpch29sadds5pi-x">
    <attr column="11" line="2" name="b" path="/home/ludo/src/nixpkgs/maintainers/scripts/gnu/self-ref.nix">
      <derivation drvPath="/nix/store/b1pmg2fhxnlnxy0z9hfglgyk66wpa2ar-x.drv" outPath="/nix/store/fw6flnp42lqn9i62aknpch29sadds5pi-x">
        <repeated />
      </derivation>
    </attr>

    [...]

  </derivation>
</expr>
--8<---------------cut here---------------end--------------->8---

When parsing this with SAX, the ‘repeated’ element is seen before the
first occurrence of the derivation, which is a problem: the program that
parses it doesn’t have anything to repeat yet.

I believe this patch fixes it:

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OK to apply, along with a test?

(For the record, I just encountered this problem for the first time with
‘buildInternalPythonModule’ in the Python 2.7 expression.)

Thanks,
Ludo’.