Need some guidance on getting Async to work ...

[jcogs-design]

Hi everyone.
Hopefully someone can see what I'm doing wrong here, and let me know how to get this to work...
The following code works if run on system where isSupported() returns false, but fails on systems where isSupported() returns true.

                $pool->add(function () use ($url, $context) {
                    // Request the file
                    $output = @file_get_contents($url, false, $context);
                    if($output) {
                        $output = trim($output);
                    }
                    return $output;
                })->then(function ($output) use ($addon, &$version_info) {
                    if (!$output) {
                        return false;
                    }
                    // Clean up data returned
                    return json_decode($output, true));
                })->catch(function (Throwable $exception) {
                    // Handle exception
                    return false;
                });
                $pool->wait();

Putting a debugger on this indicates that the $pool->add() step is run, but the ->then() and ->catch() steps are never called when async is supported. When not supported, the steps in add() and then() are executed serially.
Separately I have confirmed that the @file_get_contents() step returns data with the values of $url and $context provided.

Thanks in advance for any help anyone can offer.