GuzzleHttp Asynchronous Request Exception

Question

I can't figure out how I can throw an exception from Guzzle future response handler. Here's my code:

<?php
require 'vendor/autoload.php';

$client = new \GuzzleHttp\Client();

$req = $client->createRequest('GET', 'http://www.google.com', array(
    'future' => true,
));
echo "Sending request\n";
$response = $client->send($req);

try {
    $response->then(function ($data) {
        echo "Response is received\n";
        throw new Exception('Test');
    })->then(function () {
        // success handler
    }, function (Exception $exception) {
        echo "Error handler invoked\n";
        throw $exception;
    });
} catch (Exception $e) {
    echo "Exception catched\n";
}
echo "Finish\n";

The catch block is never reached in this case.

Answer 1

You are working with promises when using asynchronous Guzzle requests. Using the then() function off of a FutureResponse will create a promise that is fulfilled or rejected when the request completes. If an error occurs while sending, the promise is rejected, which means the second callback provided to the then function is invoked. When a request completes successfully, it is resolved, and the first callback provided to the then function is invoked. When an exception is thrown in any of the promise functions, the exception is caught inside of the promise and forwarded to the next error handler in the chain. In your example, if the request succeeds, then you throw an exception which will trigger the error callback. Throwing an exception in the error callback will either forward the exception to the next error callback in the promise chain, or silently eat the error (in your case, there are no further error callbacks to trigger).

The React Promises library that is used by Guzzle has more documentation on resolution and rejection forwarding of promises: https://github.com/reactphp/promise#how-promise-forwarding-works. The author of this library is looking into adding a done() function that can be used as a terminal promise handler that actually throws unhandled exceptions.