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Haskell Playground

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# Haskell Playground

There's lots still to do, see
[the issue list](https://github.com/haskell/play-haskell/issues) (as well as
[TODO.txt](https://github.com/haskell/play-haskell/blob/master/TODO.txt) for some further ideas).
If you want to contribute, perhaps connect with me (either via an issue or on
[IRC](https://wiki.haskell.org/IRC_channel)) before writing lots of code.

## GHCup target platform

Because the GHCup installation from the host machine will be used as-is in the
containers of the workers, and because said containers run Ubuntu, the desired
GHC versions must be installed as follows with `ghcup`:

ghcup -p x86_64-deb10-linux install ghc 8.10.7

This ensures that the GHCs will work in the Ubuntu container. Note that
currently (2022-08), Ubuntu GHCs seem to work fine on Arch Linux, for example.

(This seems to be necessary with certain GHC versions only, and it's probably related to [GHC #22268](https://gitlab.haskell.org/ghc/ghc/-/issues/22268).)

## Installation

### System setup: Ubuntu

Note: the worker needs bubblewrap 0.7.0, which is present in the Ubuntu repos
only from Ubuntu 23.04. If you're running an older version of Ubuntu (or
another system without version 0.7.0), build
[bubblewrap](https://github.com/containers/bubblewrap) yourself.

```bash
# Note: earlyoom is only advised if you're deploying; not necessary on your local machine
sudo apt update && sudo apt install earlyoom bubblewrap make npm jq
# Change "-r 60" to "-r 3600" in /etc/default/earlyoom (less spammy logs)
sudo systemctl restart earlyoom

sudo apt install build-essential curl libffi-dev libffi7 libgmp-dev libgmp10 libncurses-dev libncurses5 libtinfo5 pkg-config
# Install ghcup: https://www.haskell.org/ghcup/ (skip HLS and stack)
# Open a new terminal to get ghcup in PATH
```

### System setup: Arch Linux

```bash
# Note: earlyoom is only advised if you're deploying; not necessary on your local machine
sudo pacman -Syu earlyoom bubblewrap make npm jq
sudo systemctl enable --now earlyoom

sudo pacman -S base-devel
# Install ghcup: https://www.haskell.org/ghcup/ (skip HLS and stack)
# Open a new terminal to get ghcup in PATH
# MAKE SURE TO INSTALL GHC FOR TARGET x86_64-deb10-linux
# See above in the 'GHCup target platform' section
```

### Building the applications

```bash
git clone https://github.com/haskell/play-haskell --recurse-submodules
cd play-haskell
```

To build the server (that hosts the website but doesn't run any user code):
```bash
cd play-haskell-server
make
cabal build
```

To build the worker (that the server will connect to, and that runs user code (in a sandbox)):
```bash
cd play-haskell-worker
make  # Or equivalently:
      #   make chroot ; make bwrap-files/systemd-run-shim ; make builders
      # 'make chroot' is interactive, select en_US.UTF-8.
      # 'make builders' takes a long time because it builds all packages that should
      #   be available on the playground with all GHCs you have installed with GHCup.
      #   If you want just a few of those GHCs to work in the playground, manually run:
      #     bwrap-files/mkbuildscript.sh 9.2.5
      #     bwrap-files/mkbuildscript.sh 8.10.7
      #   etc., once for each version you want to be available. Change available
      # packages in bwrap-files/mkbuildscript.sh .
cabal build
```

Optionally, if the machine you're building on does not have enough RAM, do this and use `cabal build -j1`:

```bash
sudo fallocate -l 4G /swapfile
sudo chmod go-rw /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile
```

During development:

- `make frontend-dependencies` in `play-haskell-server/` whenever [play-haskell-server/static/package.json](https://github.com/haskell/play-haskell/blob/master/play-haskell-server/static/package.json) changes
- `make frontend` in `play-haskell-server/` whenever some `*.ts` file for the frontend changes
- `make reload-pages` in `play-haskell-server/` (or re-run `cabal run play-haskell-server`) to reload mustache pages (or send `SIGUSR1`)

### Running the applications

To run the server: (all in `play-haskell-server/`)
1. Put a good password in `adminpass.txt` (filename does not matter, but adjust below)
2. `cabal run gen-secret-key -- secretkey.txt` (filename does not matter, but adjust below)
    - Remember the public key that it prints, say `$SERVER_PUBKEY`
3. Optional: put one or more lines of the form `{http://}hostname{:port} $WORKER1_PUBKEY` in `preload-workers.txt`, where the `{}` parts are optional (if not given, uses https on port 443); that `$WORKER1_PUBKEY` is produced by the steps to run a worker, see below
4. `cabal run play-haskell-server -- --adminpassfile adminpass.txt --secretkey secretkey.txt --preloadworkers preload-workers.txt` (omit the `--preloadworkers` if you didn't do that step)
5. See `cabal run play-haskell-server -- --help` for more info on options; the server listens with http on port 8123 by default

To run a worker: (all in `play-haskell-worker/`)
1. `cabal run gen-secret-key -- secretkey.txt` (filename does not matter, but adjust below)
    - Remember the public key that it prints, say `$WORKER1_PUBKEY`
2. Put one or more `$SERVER_PUBKEY` in `trustedkeys.txt` (filename does not matter, but adjust below)
2. `cabal run play-haskell-worker -- --secretkey secretkey.txt --trustedkeys trustedkeys.txt +RTS -N`
3. See `cabal run play-haskell-worker -- --help` for more info on options; the worker listens with http on port 8124 by default, use a reverse proxy ssl terminator to get https

### Server admin interface

On `example.com/admin` the server exposes a simple admin interface through which you can add and remove workers. If you don't want to add workers this way, use the `--preloadworkers` flag described above.

## Storage

Pasted snippets are stored in an SQLite database in the file `pastes.db`.

## API

The playground web client uses a simple API to submit jobs to the server; this API can also be used by others.
Be aware that the server imposes simple IP-based rate-limiting on job requests; you might get back [429](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/429) responses if you submit too many jobs.

The API is as follows: `POST` to `/submit` on the playground server with the following JSON content:
```jsonc
{
  "code": "main :: IO ()\nmain = print 42",
  "version": "9.2.4",  // or "8.10.7", or "9.6.0.20230210", etc.
  "opt": "O1",  // "O0", "O1" or "O2"
  "output": "run"  // "run", "core" or "asm"
}
```

The response will be a JSON object (with `Content-Type: text/json`) that has one of the following forms:
```jsonc
{"err": "... an error message ..."}
```
or
```jsonc
{
  "ec": 0,  // exit code; 0 on success
  "ghcout": "",  // stderr output from GHC; GHC warnings/errors end up here
  "sout": "42\n",  // stdout from the program, empty if it didn't get to run
  "serr": "",  // stderr from the program, empty if it didn't get to run
  "timesecs": 0.595881831  // time taken to run the job on the worker, excludes server queueing time
}
```

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