Bug: MemcachedHandler::decrement() produces the opposite sign of File/Redis/Predis for a non-existent key #10510

PHP Version

8.2

CodeIgniter4 Version

4.7.5-dev

CodeIgniter4 Installation Method

Git

Which operating systems have you tested for this bug?

macOS

Which server did you use?

cli-server (PHP built-in webserver)

Environment

development

Database

No response

What happened?

Description

MemcachedHandler::decrement() behaves inconsistently with the other cache
handlers (File, Redis, Predis) when the key being decremented does not
yet exist. This is already flagged in the code with a // FIXME comment,
but not resolved:

https://github.com/codeigniter4/CodeIgniter4/blob/develop/system/Cache/Handlers/MemcachedHandler.php#L179

public function decrement(string $key, int $offset = 1): false|int
{
    if (! $this->config['raw']) {
        return false;
    }

    $key = static::validateKey($key, $this->prefix);

    // FIXME: third parameter isn't other handler actions.

    return $this->memcached->decrement($key, $offset, $offset, 60);
}

How to see it

$cache = \Config\Services::cache(); // memcached handler, 'raw' => true

$cache->decrement('brand_new_key', 5);
// -> 5   (positive!)

Now try the same thing with File, Redis, or Predis instead — they all give:

$cache->decrement('brand_new_key', 5);
// -> -5

What I'd expect

If I decrement a key by 5, I'd expect a negative number back when it starts from nothing — like every other handler does. Memcached gives the opposite sign, which feels like a bug you could easily not notice until you switch cache drivers between dev and prod and your numbers suddenly flip.

Why this happens

I checked the PHP docs for Memcached::decrement() — the 3rd argument (initial_value) is used as-is if the key doesn't exist yet, it's not something the offset gets subtracted from. So passing $offset there just plants the key at +$offset instead of decrementing from zero.

(increment() doesn't have this problem, by the way — it's just a lucky coincidence that 0 + $offset and passing $offset as the initial value land on the same number.)