source: vendor/google/apiclient/src/AuthHandler/Guzzle6AuthHandler.php

<?php

namespace Google\AuthHandler;

use Google\Auth\CredentialsLoader;
use Google\Auth\FetchAuthTokenCache;
use Google\Auth\HttpHandler\HttpHandlerFactory;
use Google\Auth\Middleware\AuthTokenMiddleware;
use Google\Auth\Middleware\ScopedAccessTokenMiddleware;
use Google\Auth\Middleware\SimpleMiddleware;
use GuzzleHttp\Client;
use GuzzleHttp\ClientInterface;
use Psr\Cache\CacheItemPoolInterface;

/**
 * This supports Guzzle 6
 */
class Guzzle6AuthHandler
{
    protected $cache;
    protected $cacheConfig;

    public function __construct(?CacheItemPoolInterface $cache = null, array $cacheConfig = [])
    {
        $this->cache = $cache;
        $this->cacheConfig = $cacheConfig;
    }

    public function attachCredentials(
        ClientInterface $http,
        CredentialsLoader $credentials,
        ?callable $tokenCallback = null
    ) {
        // use the provided cache
        if ($this->cache) {
            $credentials = new FetchAuthTokenCache(
                $credentials,
                $this->cacheConfig,
                $this->cache
            );
        }

        return $this->attachCredentialsCache($http, $credentials, $tokenCallback);
    }

    public function attachCredentialsCache(
        ClientInterface $http,
        FetchAuthTokenCache $credentials,
        ?callable $tokenCallback = null
    ) {
        // if we end up needing to make an HTTP request to retrieve credentials, we
        // can use our existing one, but we need to throw exceptions so the error
        // bubbles up.
        $authHttp = $this->createAuthHttp($http);
        $authHttpHandler = HttpHandlerFactory::build($authHttp);
        $middleware = new AuthTokenMiddleware(
            $credentials,
            $authHttpHandler,
            $tokenCallback
        );

        $config = $http->getConfig();
        $config['handler']->remove('google_auth');
        $config['handler']->push($middleware, 'google_auth');
        $config['auth'] = 'google_auth';
        $http = new Client($config);

        return $http;
    }

    public function attachToken(ClientInterface $http, array $token, array $scopes)
    {
        $tokenFunc = function ($scopes) use ($token) {
            return $token['access_token'];
        };

        // Derive a cache prefix from the token, to ensure setting a new token
        // results in a cache-miss.
        // Note: Supplying a custom "prefix" will bust this behavior.
        $cacheConfig = $this->cacheConfig;
        if (!isset($cacheConfig['prefix']) && isset($token['access_token'])) {
            $cacheConfig['prefix'] = substr(sha1($token['access_token']), -10);
        }

        $middleware = new ScopedAccessTokenMiddleware(
            $tokenFunc,
            $scopes,
            $cacheConfig,
            $this->cache
        );

        $config = $http->getConfig();
        $config['handler']->remove('google_auth');
        $config['handler']->push($middleware, 'google_auth');
        $config['auth'] = 'scoped';
        $http = new Client($config);

        return $http;
    }

    public function attachKey(ClientInterface $http, $key)
    {
        $middleware = new SimpleMiddleware(['key' => $key]);

        $config = $http->getConfig();
        $config['handler']->remove('google_auth');
        $config['handler']->push($middleware, 'google_auth');
        $config['auth'] = 'simple';
        $http = new Client($config);

        return $http;
    }

    private function createAuthHttp(ClientInterface $http)
    {
        return new Client(['http_errors' => true] + $http->getConfig());
    }
}