NOTE: this isn't a theoretical case but a real issue I had in a production environment.
Current filesystem cache write implementation leads to cache collision under certain conditions.
I'll try to explain the conditions that lead to it and the reasoning behind each change step by step.

1. Once initialized, Symfony/Component/Cache/Traits/FilesystemCommonTrait::$tmp never changes. It means that if process is forked, both processes now use the same tmp file path to write to the cache. You don't really need that much concurrency to end up with a collision this way.
   The first fix iteration that comes to mind is removing the if condition here:
   

   symfony/src/Symfony/Component/Cache/Traits/FilesystemCommonTrait.php

   Lines 95 to 97 in cce5a42

   	if (null === $this->tmp) {
   	$this->tmp = $this->directory.uniqid('', true);
   	}

   
   That would work better, but there's still a possibility of a race condition.

2. As you know uniqid() is based on timestamp in microseconds. With enough luck and concurrency you may get exactly the same value from it in different processes/threads. The more_entropy argument is supposed to help with that, however it doesn't work very well if it had been used before forking.
   Check out this example:

<?php

echo uniqid('', true).PHP_EOL;
$pid = pcntl_fork();
if ($pid == -1) {
    die('could not fork');
} elseif ($pid) {
    // we are the parent
    echo uniqid('', true).PHP_EOL;
    pcntl_wait($status); //Protect against Zombie children
} else {
    // we are the child
    echo uniqid('', true).PHP_EOL;
}

Output:

5ffc343b65a718.78746310
5ffc343b65dab3.19705677
5ffc343b7e6493.19705677

As you can see the 2nd and the 3rd line have the same "more entropy" value. The problem is that under the hood it's using an LCG algorithm. Once its constants are initialized, they never change and the sequence of generated numbers is pre-determined.
Thus uniqid() was replaced with bin2hex(random_bytes(40)).

3. The change in the last step should be sufficient for the majority of use cases, however the statefulness of this code (i. e. $this->tmp) still leads to problems when used with coroutines (Swoole) and presumably - threads (I have no experience with multi-threading solutions in PHP, maybe they do some magic around it to prevent this kind of issue).
   As a result, $tmp property was replaced with a local variable.