infinite recusion. Likely to do with line 73 and 39.

infinite.nix

let

  ppkgs_files = (builtins.fetchTarball {
    # Descriptive name to make the store path easier to identify
    name = "nixos-unstable-2020-07-03";
    # Commit hash for nixos-unstable as of 2018-09-12
    url =
      "https://github.com/nixos/nixpkgs/archive/9a3e34ae60130c63b93809eecf555b0d796f001c.tar.gz";
    # Hash obtained using `nix-prefetch-url --unpack <url>`
    sha256 = "0azkaa2q4pp62icfs56s1hsmzpyzqafaw24v365n7qv9xffbk044";
  });

  buildVm = conf: (import "${ppkgs_files}/nixos" { configuration = conf; }).vm;

  ppkgs = import "${ppkgs_files}" { };

  myOverlay = self: super:
    let writeJson = d: super.writeText "app-config.json" (builtins.toJSON d);
    in rec {
      inherit writeJson;
      buildScheduledTask =
        { name, scriptContents, timeoutSec ? "3600", restartSec ? "10" }:
        let
          service = {

            description = name;

            startAt = "*:10";
            script = "${runScript}/bin/runner";

            serviceConfig = {
              RuntimeMaxSec = timeoutSec;
              RestartSec = restartSec;
            };
          };

          runScript = super.writeShellScriptBin "runner" scriptContents;

          nixosService = {
            systemd = { services = { "${name}" = service; }; };
          };

        in {
          service = nixosService;
          scriptContents = runScript;
          serviceContents = writeJson nixosService;
        };

    };

in {

  infiniteMachine = buildVm ({ config, pkgs, ... }:
    let
      xx = pkgs.writeShellScriptBin "runStuff" ''
        set +e

        echo ${pkgs.writeText "app-config.json" (builtins.toJSON { ok = 42; })}
        echo ${pkgs.writeJson yz.serviceContents}

      '';

      yz = pkgs.buildScheduledTask {
        name = "testService2";
        scriptContents = "echo doStuff";
      };

    in ({

      environment.systemPackages = [ xx ];

      nixpkgs.overlays = [ myOverlay ];
    } // yz.service));

  okMachine = buildVm ({ config, pkgs, ... }:
    let
      xx = pkgs.writeShellScriptBin "runStuff" ''
        set +e

        echo ${pkgs.writeText "app-config.json" (builtins.toJSON { ok = 42; })}
        echo ${pkgs.writeJson yz.serviceContents}

      '';

      yz = pkgs.buildScheduledTask {
        name = "testService2";
        scriptContents = "echo doStuff";
      };

    in {

      environment.systemPackages = [ xx ];

      nixpkgs.overlays = [ myOverlay ];
    });

}

nix-build vms.nix -A okMachine is fine, nix-build vms.nix -A infiniteMachine crashes with an infinite recursion error.

The issue seems to be the merging (//) of the results of a function that build a service that comes from an overlay.