APCu: Cannot call key() on invalid iterator

Question

I'm working on an older version of ILIAS 5.4 in an DDEV (Docker) development environment. It is a Learning Management System written in PHP. The runtime environment consists of:

• GNU/Linux Debian 10 Buster
• PHP 7.3
• MariaDB 10.3

Currently I stumbled over an error regarding the APCUIterator which I don't know how to handle. Regarding the while-clause, where the key()-method is being called on the iterator

    /**
     * @param bool $complete
     *
     * @return bool
     */
    public function flush($complete = false)
    {
        if ($complete) {
            return apcu_clear_cache();
        }

        $key_prefix = $this->returnKey('');
        $apcu_iterator = new APCUIterator();
        $apcu_iterator->rewind();
        while ($current_key = $apcu_iterator->key()) {
            // "begins with"
            if (substr($current_key, 0, strlen($key_prefix)) === $key_prefix) {
                $this->delete($current_key);
            }
            $apcu_iterator->next();
        }
        return true;
    }

the error message says: Cannot call key() on invalid iterator—Inside the container I can reproduce the error with the following extracted snippet

<?php
$iterator = new APCUIterator();
$iterator->key();
# Nothing else matters

and the resulting error renders like the following screenshot in the browser

My best guess is that there is something missing inside the container. Either a dependency or a configuration. Any suggestions?

EDIT: Here follows the APC configuration related ddev php -i | grep -i apc output.

/etc/php/7.3/cli/conf.d/20-apcu.ini,
/etc/php/7.3/cli/conf.d/25-apcu_bc.ini,
/etc/php/7.3/cli/conf.d/enableapcu.ini,
apc
APC Compatibility => 1.0.5
apcu
APCu Support => Enabled
APCu Debugging => Disabled
apc.coredump_unmap => Off => Off
apc.enable_cli => On => On
apc.enabled => On => On
apc.entries_hint => 4096 => 4096
apc.gc_ttl => 3600 => 3600
apc.mmap_file_mask => no value => no value
apc.preload_path => no value => no value
apc.serializer => php => php
apc.shm_segments => 1 => 1
apc.shm_size => 32M => 32M
apc.slam_defense => Off => Off
apc.smart => 0 => 0
apc.ttl => 0 => 0
apc.use_request_time => Off => Off
igbinary APCu serializer ABI => 0

EDIT 2: Here follows the APC configuration related php -i | grep -i apc output from the native staging system (not Docker) based on Debian with Apache web server.

/etc/php/7.3/cli/conf.d/20-apcu.ini,
apcu
APCu Support => Disabled
APCu Debugging => Disabled
apc.coredump_unmap => Off => Off
apc.enable_cli => Off => Off
apc.enabled => On => On
apc.entries_hint => 4096 => 4096
apc.gc_ttl => 3600 => 3600
apc.mmap_file_mask => no value => no value
apc.preload_path => no value => no value
apc.serializer => php => php
apc.shm_segments => 1 => 1
apc.shm_size => 32M => 32M
apc.slam_defense => Off => Off
apc.smart => 0 => 0
apc.ttl => 0 => 0
apc.use_request_time => On => On

Answer 1

Hit the same issue on $apcu->current().

One way to avoid this is to try $apcu->valid(), which should return true:

$apcu_iterator = new APCUIterator();
if ($apcu_iterator->valid()) {
 // do stuff on iterator
}

It seems that when the iterator didn't found any valid key (either there's nothing stored, or you filtered it and no key matches), it returns an invalid iterator:

$apc = new APCUIterator();
var_dump($apc->getTotalCount());
var_dump($apc->valid());

With keys:

int(5)
bool(true)

Without valid keys:

int(0)
bool(false)

So you can use either ->valid() or check ->getTotalCount() before using the iterator.