The override that builds the custom python for integrations-core was overriding python, but pythonPackages was still being inherited from a call to datadog-integrations-core {}, causing service.datadog-agent.extraIntegrations to be ignored.

Motivation for this change

services.datadog-agent.extraIntegrations failed to include varnish related files from datadogs integration-core repo when I tried to include varnish.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

There isn't an integrated nixos test for datadog, but you can run the daemon and validate scripts without a functional API key or network access (assuming the checks don't need external network access). I used this snippet to validate my varnish change succeeded.

import <nixpkgs/nixos/tests/make-test.nix> ({ pkgs, ...} : {
  name = "datadog";

  nodes = {
    machine =
      { ... }:
      {
        imports = [ ../modules/profiles/minimal.nix ];
        services.datadog-agent.enable = true;
        users.groups.varnish.members = [ "datadog" ];
        services.datadog-agent.apiKeyFile = "/dev/null";
        services.datadog-agent.extraIntegrations = {
          varnish = (ps: [ ]);
        };
        services.datadog-agent.checks = {
          varnish = {
            init_config = {};
            instances = [{
              varnishstat = "${pkgs.varnish}/bin/varnishstat";
            }];
          };
        };
        services.varnish.enable = true;
        services.varnish.config = ''
          vcl 4.0;
          backend foo {
            .host = "127.0.0.1";
            .port = "8080";
          }
        '';
      };
    };

    testScript = ''
      startAll;
      $machine->waitForUnit("varnish.service");
      $machine->succeed("sleep 30");
      $machine->fail("journalctl -b -u datadog-agent | grep -q 'Error running check varnish'");
    '';
  })