Nonsensical infinite loop when tracing runCommand result

[nh2]

Issue description

I just spent over 3 hours tracing down / minimising an infinite loop that just made nix hang.

Steps to reproduce

Create and deploy a nixops deployment with contents:

{
  machine1 = { pkgs, ... }: {

    systemd.services.bla = {
      preStart = builtins.trace (pkgs.runCommand "a" {} "b") "c";
    };

  };
}

nixops deploying this will hang forever.

Moving the trace further out the following way succeeds and traces something:

{
  machine1 = { pkgs, ... }: {

    systemd.services.bla = builtins.trace (pkgs.runCommand "a" {} "b") {
      preStart = "c";
    };

  };
}

Moving it even further out the following way produces error: infinite recursion encountered

{
  machine1 = { pkgs, ... }: builtins.trace (pkgs.runCommand "a" {} "b") {

    systemd.services.bla = {
      preStart = "c";
    };

  };
}

What the hell is going on?

How can we avoid it to save people debugging time when they accidentally write this?

I'd also be thankful for reproductions by other people, and in particular whether this works with normal configuration.nix and without nixops.

[nh2]

Interestingly, this does not hang:

{
  machine1 = { pkgs, ... }: {

    systemd.services.bla = {
      unitConfig.ExecStart = builtins.trace (pkgs.runCommand "a" {} "b") "c";
    };

  };
}

What is special about preStart (or script, which also hangs) that makes it hang for that?

It suggests that pkgs itself, or pkgs.runCommand, somehow depends on the preStart value, but I don't get why that would be (or why it would make sense to be this way).

[stale]

Thank you for your contributions.

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