YouTube_captions_search_engine/website/websocket.php

<?php

use Ratchet\MessageComponentInterface;
use Ratchet\ConnectionInterface;
use React\EventLoop\LoopInterface;
use React\EventLoop\Timer\Timer;

// Make sure composer dependencies have been installed
require __DIR__ . '/vendor/autoload.php';

class Client
{
    public $id;
    public $timer;
    public $pid;

    public function __construct($id)
    {
        $this->id = $id;
    }

    // `__destruct` can't take arguments.
    public function free($loop)
    {
        $loop->cancelTimer($this->timer);
        // Should in theory verify that the pid wasn't re-assigned.
        posix_kill($this->pid, SIGTERM);
        $clientFilePath = getClientFilePath($this->id);
        if (file_exists($clientFilePath)) {
            $fp = fopen($clientFilePath, 'r+');
            if (flock($fp, LOCK_EX, $WAIT_IF_LOCKED)) {   // acquire an exclusive lock
                unlink($clientFilePath); // delete file
                flock($fp, LOCK_UN);     // release the lock
            } else {
                echo "Couldn't get the lock!";
            }
            fclose($fp);
        }
    }
}

// Need to be passed as a reference to `flock`.
$WAIT_IF_LOCKED = 1;

define('USERS_FOLDER', 'users/');

// Delete users outputs of previous `websocket.php` execution.
// We skip `.`, `..` and `.gitignore`.
foreach (array_slice(scandir(USERS_FOLDER), 3) as $file) {
    unlink(USERS_FOLDER . $file);
}

function getClientFilePath($clientId)
{
    return USERS_FOLDER . "$clientId.txt";
}

// Current implementation may add latency across users.
class MyProcess implements MessageComponentInterface
{
    protected $clients;
    private $loop;
    private $newClientId;
    private $newClientIdSem;

    public function __construct(LoopInterface $loop)
    {
        $this->clients = new \SplObjectStorage();
        $this->loop = $loop;
        $this->newClientId = 0;
        $this->newClientIdSem = sem_get(1, 1);
    }

    private function newClient()
    {
        // If `onOpen` and `onMessage` can't be called at the same time, then this semaphore is useless.
        if (sem_acquire($this->newClientIdSem)) {
            // Note that we don't re-use ids except on `websockets.php` restart, but as the maximal int in PHP is a very great number we are fine for a while (https://www.php.net/manual/en/reserved.constants.php#constant.php-int-max)
            $clientId = $this->newClientId++;
            sem_release($this->newClientIdSem);
            return new Client($clientId);
        } else {
            exit('`newClient` error');
        }
    }

    public function onOpen(ConnectionInterface $conn)
    {
        $client = $this->newClient();
        $this->clients->attach($conn, $client);
    }

    public function onMessage(ConnectionInterface $from, $msg)
    {
        $client = $this->clients->offsetGet($from);
        // If a previous request was received, we execute the new one with another client for simplicity otherwise with current file deletion approach, we can't tell the worker `search.py` that we don't care about its execution anymore.
        if ($client->pid !== null) {
            // As `$this->clients->detach` doesn't call `__destruct` for unknown reason, we clean manually the previous request.
            $client->free($this->loop);
            $client = $this->newClient();
        }
        $clientId = $client->id;
        $clientFilePath = getClientFilePath($clientId);
        // Create the worker output file otherwise it would believe that we don't need this worker anymore.
        file_put_contents($clientFilePath, '');
        // As we are going to use this argument in a shell command, we escape it.
        $msg = escapeshellarg($msg);
        // Start the independent worker.
        // Redirecting `stdout` is mandatory otherwise `exec` is blocking.
        $client->pid = exec("./search.py $clientId $msg > /dev/null & echo $!");
        // `addTimer` doesn't enable us to use independently `$from->send` multiple times with blocking instructions between.
        $client->timer = $this->loop->addPeriodicTimer(1, function () use ($from, $clientId, $clientFilePath, $client) {
            echo "Checking news from $clientId\n";
            // If the worker output file doesn't exist anymore, then it means that the worker have finished its work and acknowledged that `websocket.php` completely read its output.
            if (file_exists($clientFilePath)) {
                // `flock` requires `r`eading permission and we need `w`riting one due to `ftruncate` usage.
                $fp = fopen($clientFilePath, 'r+');
                $read = null;
                if (flock($fp, LOCK_EX, $WAIT_IF_LOCKED)) { // acquire an exclusive lock
                    // We assume that the temporary output is less than 1 MB long.
                    $read = fread($fp, 1_000_000);
                    ftruncate($fp, 0);     // truncate file
                    fflush($fp);           // flush output before releasing the lock
                    flock($fp, LOCK_UN);   // release the lock
                } else {
                    // We `die` instead of `echo`ing to force the developer to investigate the reason.
                    die("Couldn't get the lock!");
                }
                fclose($fp);

                // Assume that empty output doesn't need to me forwarded to the end-user.
                if ($read !== null && $read !== '') {
                    $from->send($read);
                }
            } else {
                // We don't need the periodic timer anymore, as the worker finished its work and acknowledged that `websocket.php` completely read its output.
                $this->loop->cancelTimer($client->timer);
            }
        });
    }

    public function onClose(ConnectionInterface $conn)
    {
        $client = $this->clients->offsetGet($conn);
        $clientId = $client->id;
        $client->free($this->loop);
        echo "$clientId disconnected\n";
        $this->clients->detach($conn);
    }

    public function onError(ConnectionInterface $conn, \Exception $e)
    {
        $conn->close();
        die('`onError`');
    }
}

$loop = \React\EventLoop\Factory::create();

// Run the server application through the WebSocket protocol on port 4430.
// Note that named arguments come with PHP 8 which isn't current Debian one.
$app = new Ratchet\App('crawler.yt.lemnoslife.com', 4430, '127.0.0.1', $loop);
$app->route('/websocket', new MyProcess($loop), array('*'));
$app->run();