How I managed to wire devshell, postgres, direnv and VSCode

[yajo]

As I explained in #170 (comment), starting services when starting a nix-shell is kind of messy. Mostly for background services, such as Postgres. That was a conceptual failure for development IMHO.

I tried with devshell-files and its s6-based services backend, but also didn't work out fine. It had a different purpose of what I wanted.

Well, here's how I did it.

This is how I build my shell (of course, pkgs has the usual devshell overlay, etc.):

pkgs.devshell.mkShell {
  imports = [
    # Includes postgres command, env and initial db setup
    "${pkgs.devshell.extraModulesDir}/services/postgres.nix"
  ];
  # Packages not displayed on menu
  devshell.packages = with pkgs; [
    # Reaps services when exiting parent process (like your IDE)
    tini

    # Service helpers
    (writeScriptBin "service-postgres" ''
      #!/usr/bin/env bash
      exec tini -sp SIGTERM -- postgres
    '')
  ];
}

The writeScriptBin part could have been replaced by devshell-files alias, but I felt like it was adding too much dependency for such a simple task. However, if you're using it, it's more ergonomic.

An important part however is tini. It makes sure that, when exiting the IDE, all postgres subprocesses are properly reaped. Close the IDE, all is gone! Cool 😎

Now I add a task for the IDE (VSCode in my case):

// .vscode/tasks.json
{
  "version": "2.0.0",
  "tasks": [
    {
      "label": "Postgres",
      // This is the important part: how it relates to nix-shell
      "type": "shell",
      "command": "service-postgres",
      "options": {
        "shell": {
          "executable": "nix-shell",
          "args": ["--run"]
        }
      },
      // The rest is just required boilerplate to make it work fine
      "isBackground": true,
      "problemMatcher": [
        {
          "pattern": [
            {
              "regexp": "^([^\\\\s].*)\\\\((\\\\d+,\\\\d+)\\\\):\\\\s*(.*)$",
              "file": 1,
              "location": 2,
              "message": 3
            }
          ],
          "background": {
            "activeOnStart": true,
            "beginsPattern": "starting PostgreSQL",
            "endsPattern": "database system is ready to accept connections"
          }
        }
      ],
      "presentation": {
        "reveal": "silent"
      }
    },
  ]
}

Now I want Postgres to autostart when I start debugging a Python file, so I create this launch configuration:

// .vscode/launch.json
{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Python: Archivo actual",
      "type": "python",
      "request": "launch",
      "program": "${file}",
      "console": "integratedTerminal",
      "justMyCode": true,
      "subProcess": true,
      "env": {"PGHOST": "${workspaceFolder}/.data/postgres"},
      "preLaunchTask": "Postgres"
    }
  ]
}

Now all I have to do is to press F5 and:

1. nix-shell gets all required dependencies.
2. Postgres gets a database if it's the 1st launch, thanks to this startup service:

   devshell/extra/services/postgres.nix

   Lines 49 to 58 in d97df53

   	devshell.startup.setup-postgres.text = ''
   	if [[ ! -d "$PGDATA" ]]; then
   	initdb ${concatStringsSep " " cfg.initdbArgs}
   	cat >> "$PGDATA/postgresql.conf" <<EOF
   	listen_addresses = '''
   	unix_socket_directories = '$PGHOST'
   	EOF
   	echo "CREATE DATABASE ''${USER:-$(id -nu)};" | postgres --single -E postgres
   	fi
   	'';

3. Postgres starts.
4. VSCode detects it's ready to listen to connections.
5. VSCode starts debugging my python file, connected to Postgres.

I'm quite happy with it! ❤️