chore(dev): Add Flox for local environment setup

[Twixes]

Problem

To get started developing PostHog, you have to install an array of dependencies with the correct versions, running a dozen of commands from the Developing locally guide. And then xmlsec ruins your day anyway. What if it took just one command to install all the right components with actual version control of the environment, without relying on janky system-wide deps?

Changes

Flox (brew install flox) is a take on this problem – specifically, a dev environment manager backed by Nix. As opposed to Homebrew, Nix has real versioning (xmlsec issues begone).

This PR adds a Flox manifest for the PostHog local environment, which turns basically the whole "Developing locally" guide into these few steps:

1. brew install flox ruff rustup && rustup-init && rustup default stable – to install the only system-level dependencies needed: flox itself, ruff for Python pre-commit checks, cargo via rustup for Rust checks:

2. flox activate

That's it actually. This gets you a full-fledged environment under .flox/ in the repository directory's, with everything needed to run and develop PostHog. Once done, you'll see instructions for migrations and running the app.
On the first activation you'll also be prompted if you'd like the environment to be activated automatically.

As a bonus, this PR replaces the old bin/start with bin/start-mprocs, by popular demand.

[github-actions]

Size Change: 0 B

Total Size: 1.11 MB

ℹ️ View Unchanged

Filename	Size
frontend/dist/toolbar.js	1.11 MB

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[rafaeelaudibert]

Log message letting us know what we're doing would be helpful, otherwise it might feel like we're stuck

[Twixes]

You still get CH migration logs as migrations progress, so there shouldn't be that feeling – have you seen otherwise?

[rafaeelaudibert]

🎉 (didnt test it myself, looks good)

[mariusandra]

Tried running it, here's the output

marius@Mariuss-MBP-2 ~/P/P/posthog (flox)> flox activate
Flox collects basic usage metrics in order to improve the user experience.

Flox includes a record of the subcommand invoked along with a unique token.
It does not collect any personal information.

The collection of metrics can be disabled in the following ways:

  environment: FLOX_DISABLE_METRICS=true
    user-wide: flox config --set-bool disable_metrics true
  system-wide: update /etc/flox.toml as described in flox(1)

This is a one-time notice.


✅ You are now using the environment 'posthog'.
To stop using this environment, type 'exit'

Using Python 3.11.9 interpreter at: .flox/run/aarch64-darwin.posthog.dev/bin/python3
Creating virtualenv at: .flox/cache/venv
Activate with: source .flox/cache/venv/bin/activate.fish
Python virtual environment path: .flox/cache/venv
Python interpreter path, for your code editor: .flox/cache/venv/bin/python

👉 Use direnv (https://direnv.net) for automatic activation of this environment by your shell.
❓ Would you like direnv to be set up now? (y/n, default: y): 
🔄 Installing direnv using 'brew install direnv'...
==> Fetching dependencies for direnv: bash
==> Fetching bash
==> Fetching direnv
==> Installing dependencies for direnv: bash
==> Installing direnv dependency: bash
🍺  /opt/homebrew/Cellar/bash/5.2.37: 163 files, 12.4MB
==> Installing direnv
🍺  /opt/homebrew/Cellar/direnv/2.35.0: 12 files, 10.5MB
==> Running `brew cleanup direnv`...
Disable this behaviour by setting HOMEBREW_NO_INSTALL_CLEANUP.
Hide these hints with HOMEBREW_NO_ENV_HINTS (see `man brew`).
✅ Installed direnv
✅ Configured ~/.config/direnv/direnv.toml
✅ Injected direnv hook into /Users/marius/.config/fish/config.fish
💫 direnv is now active

IT'S DANGEROUS TO GO ALONE! RUN THIS:
1. bin/migrate - to run all migrations
2. bin/start - to start the entire stack
3. ./manage.py generate_demo_data - to create a user with demo data

fish: Unknown command: '[global]'
~/.config/fish/config.fish (line 18): 
[global]
^~~~~~~^
from sourcing file ~/.config/fish/config.fish
        called during startup
fish: Unknown command: warn_timeout
~/.config/fish/config.fish (line 19): 
warn_timeout = 0 # Ignore timeout from this issue: https://github.com/direnv/direnv/issues/1065 - added by PostHog's Flox activation hook (../posthog/.flox/env/manifest.toml)
^~~~~~~~~~~^
from sourcing file ~/.config/fish/config.fish
        called during startup
Welcome to fish, the friendly interactive shell
Type help for instructions on how to use fish
(venv) flox [posthog] marius@Mariuss-MacBook-Pro-2 ~/P/P/posthog (flox)> 

Not sure what that is. The end of my fish config looks like this:

status is-login; and pyenv init --path | source
status is-interactive; and pyenv init - | source

rvm default

[global]
warn_timeout = 0 # Ignore timeout from this issue: https://github.com/direnv/direnv/issues/1065 - added by PostHog's Flox activation hook (../posthog/.flox/env/manifest.toml)

# Initialize direnv - added by PostHog's Flox activation hook (../posthog/.flox/env/manifest.toml)
direnv hook fish | source

bin/start works in this shell, so I assume it works... though everything worked before as well:

[Twixes]

Oops, you can remove lines [global] and warn_timeout = 0, that was a mishap in my script. Thanks for catching, fixed!

[daibhin]

We should probably post about this in Slack and update the Developing Locally doc. I ran into an issue this morning because I didn't have mprocs installed. Trying to figure out the migration path to flox now

[Twixes]

Oh, right, the switch to bin/start-mprocs has been by popular demand, but I neglected a migration path if you haven't used it before 😅 You can just brew install mprocs! Will put out a PR to make this automatic

[Twixes]

Fix here: #27052

[t0rx]

Out of curiosity, why not also specify ruff and rustup in your Flox environment?