CreateDestinationFolderException: mkdir race condition

[AlexeyKosov]

When a browser sends many requests for image conversion concurrently (e.g. an image gallery is loaded) and the destination directory does not exist, every request tries to create the directory, which ends up with CreateDestinationFolderException because of a race condition.

So instead of

if (! is_dir($dir) && ! mkdir($dir, 0777, true)) {
    throw new CreateDestinationFolderException();
}

the following should be used:

if (! is_dir($dir) && ! mkdir($dir, 0777, true) && ! is_dir($dir)) {
    throw new CreateDestinationFolderException();
}

(code is simplified)

More info about this issue:

https://www.google.com/search?q=php+mkdir+race+condition
https://bugs.php.net/bug.php?id=35326
https://stackoverflow.com/questions/44322783/is-is-dir-unreliable-or-are-there-race-conditions-that-can-be-mitigated-here/57939677#57939677
https://app.bountysource.com/issues/49427500-mkdir-race-condition-enhancement
symfony/symfony#11626