Revert "Merge pull request #13938 from NixOS/import-thunk"

[xokdvium]

Motivation

This has multiple dangling pointer issues that lead to segfaults in e.g.:

nix eval --expr '(builtins.getFlake "github:nixos/nixpkgs/25.05")' --impure

Context

This reverts commit ad17572, reversing changes made to d314750.

---

Add 👍 to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.

[edolstra]

This has multiple dangling pointer issues that lead to segfaults in e.g.:

Can you be more specific?

nix eval --expr '(builtins.getFlake "github:nixos/nixpkgs/25.05")' --impure

I wasn't able to verify this since this command OOMs after consuming 40+ GB.

[xokdvium]

Can you be more specific?

Left comments on the original PR.

I wasn't able to verify this since this command OOMs after consuming 40+ GB.

In my machine it segfaulted in ExprParseFile::eval because path reference was dangling. You can also run ASAN and the issue will become apparent.

[edolstra]

So it turns out that this was caused by #13930 not being merged yet. The "failed" value type ensures the evalFile() post-condition that the ExprParseFile thunk is finished, so there is no reference to it anymore. But on current master, we don't have that yet, so in case of an exception forcing the thunk, the value can revert to a thunk pointing to ExprParseFile. That then segfaults if the same file is evaluated again.