wp_cache_get returns false instead of string ‘0’

I’ve created PHP script to confirm this issue:

<?php

include_once( __DIR__ . '/public_html/wp-load.php' );
global $wp_object_cache;

$my_key = 'can_compress_scripts';
if ( isset( $argv[1] ) ) {
    $my_key = $argv[1];   
}
 
$my_group = 'options';

echo 'wp_cache_add : ';
var_dump( wp_cache_add( $my_key, '0', $my_group ) );
echo 'wp_cache_get : ';
var_dump( wp_cache_get( $my_key, $my_group, false, $found ) );
echo '  found = ';
var_dump( $found );

$mc = $wp_object_cache->get_mc( $my_group );

echo 'Memcached::get : ';
var_dump( $mc->get( $wp_object_cache->key( $my_key, $my_group ) ) );
echo 'Memcached::getResultCode : ';
var_dump($mc->getResultCode());

Results of the script:

[~]# wp --path=public_html cache flush
Success: The cache was flushed.

[~]# php test-mc.php
wp_cache_add : bool(false)
wp_cache_get : string(1) "0"
  found = bool(true)
Memcached::get : string(1) "0"
Memcached::getResultCode : int(0)

[~]# php test-mc.php
wp_cache_add : bool(false)
wp_cache_get : bool(false)
  found = bool(true)
Memcached::get : string(1) "0"
Memcached::getResultCode : int(0)

First run, the function wp_cache_add updates memcache and local cache ($this->cache property). The function wp_cache_get retrieves value from local cache (‘0’).

Second run, the function wp_cache_add returns false (it’s correct because the key is already cached) and local cache isn’t populated. The function wp_cache_get retrieves value from memcache which is converted to false (which is wrong).

The issue is the consequence of “empty” conditional (the method get in object cache drop-in):

$value = $mc->get( $key );
if ( empty( $value ) || ( is_integer( $value ) && -1 == $value ) ){
    $value = false;
	$found = $mc->getResultCode() !== Memcached::RES_NOTFOUND;
} else {
	$found = true;
}

Empty returns true for '0',”,0,array(),false… So, these values can’t be cached… Also -1 can’t be cached.

The method Memcached::get returns only false if the value isn’t stored:

Returns the value stored in the cache or false otherwise.

Please return false only if it’s “real boolean false” (which often means “not found”). Other drop-ins return proper value. I didn’t notice similar issues with Redis Object Cache or other memcache drop-ins.