Going to '/admin' while using storecodes in url and a different adminhtml url will throw exception

[koenner01]

If you are using an adminhtml url that is different from 'admin' and set the 'Add Store Code to Urls' to 'Yes', going to '/admin' will throw an exception

Preconditions

1. PHP7.0
2. M2.1.9 CE (we also tested 2.1.7 CE and 2.2.0 CE)

Steps to reproduce

1. use a different url than 'admin' for the adminhtml
2. set 'Stores' > 'Configuration' > 'General' > 'Web' > 'Add Store Code to Urls' to 'Yes'
3. go to '/admin' url

Expected result

1. Page should go to the no-route or at least show 404

Actual result

1. Page tries to go to the homepage with the current store set to 'admin'

We did some debugging already and found that in \Magento\Store\App\Request\PathInfoProcessor the current store is being set to 'admin'.

public function process(\Magento\Framework\App\RequestInterface $request, $pathInfo)
{
    $pathParts = explode('/', ltrim($pathInfo, '/'), 2);
    $storeCode = $pathParts[0];
    try {
        /** @var \Magento\Store\Api\Data\StoreInterface $store */
        $store = $this->storeManager->getStore($storeCode);
    } catch (NoSuchEntityException $e) {
        return $pathInfo;
    }
    if ($store->isUseStoreInUrl()) {
        if (!$request->isDirectAccessFrontendName($storeCode)) {
            $this->storeManager->setCurrentStore($storeCode);
            $pathInfo = '/' . (isset($pathParts[1]) ? $pathParts[1] : '');
            return $pathInfo;
        } elseif (!empty($storeCode)) {
            $request->setActionName('noroute');
            return $pathInfo;
        }
        return $pathInfo;
    }
    return $pathInfo;
}

'admin' will be treated as a valid storecode because the store table in the database contains this value for the store with id 0

[koenner01]

This was already reported in #9947 but closed because of 'inability to reproduce'

[magento-engcom-team]

@koenner01, thank you for your report.
We've created internal ticket(s) MAGETWO-80591 to track progress on the issue.

[sylvainraye]

I'm working on it #SQUASHTOBERFEST