Fix destruction of DerivationBuilder implementations

[xokdvium]

Motivation

This unsures that we call the correct virtual functions when destroying a particular
DerivationBuilder.

Usually the order of destructors is in the reverse order of inheritance:

ChrootLinuxDerivationBuilder -> ChrootDerivationBuilder -> DerivationBuilderImpl

autoDelChroot was being destroyed before the DerivationBuilderImpl::killChild was run and it would fail to clean up the chroot directory, since there were still processes writing to it. Note that ChrootLinuxDerivationBuilder::killSandbox was never run in the interrupted case at all, since virtual functions in destructors do not call derived class methods.

I could reproduce the issue with the following derivation:

let
  pkgs = import <nixpkgs> { };
in
pkgs.runCommand "chroot-cleanup-race" { } ''
  mkdir -p $out

  for i in $(seq 1 200); do
    (
      mkfifo $out/fifo$i
      cat $out/fifo$i > /dev/null &

      while true; do
        : > $out/file$i
      done
    ) &
  done

  sleep 0.05
  echo done > $out/main
''

While interrupting it manually when it would hang.

Wrapping the unique pointer in a custom deleter function we can run all of the necessary clean up code consistently and calling the right virtual functions. Ideally we'd have a lint that bans the usage of virtual functions in destructors completely.

Context

---

Add 👍 to pull requests you find important.

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

[internal-nix-ci]

Backport failed for 2.33-maintenance, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin 2.33-maintenance
git worktree add -d .worktree/backport-15072-to-2.33-maintenance origin/2.33-maintenance
cd .worktree/backport-15072-to-2.33-maintenance
git switch --create backport-15072-to-2.33-maintenance
git cherry-pick -x b752c5cb64c2675dc51aef6eb6b97d16a2a477e4