Cache memory leak when using PHP 7.4.x

I am currently investigation a memory leak with PHP 7.4.x. I upgraded from 7.2.26 to 7.4.10 and subsequently htmlpurifier from 4.10 to 4.13 as 4.10 is not PHP 7.4 compatible. Since then I have a huge issue with memory leaks, in my application a couple dozen of calls can leak ~512MB. I am still investigating the root cause, but I hope somebody has an idea what might happen. I will try to strip down my application to the minimum code required for reproducing the issue.

Things I found out so far:

• PHP 7.4.x is affected, I tested 7.4.0 and 7.4.10. 7.3.22 and 7.2.26 do not have this issue
• HTML-Purifier 4.12 and 4.13 have this issue, 4.10 is not PHP 7.4 compatible and 4.11 either had this issue or was not 7.4. compatible, but I tested it.
• With Cache.DefinitionImpl set to null no memory leaking occurs
• The cache directory does not grow.
• Running php mem profiler shows that the unserialize statement cause the most memory allocations. I am running a lot of de/serialization and have no issues there.
• It always happens at the same location.

Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 20480 bytes) in /home/someproject/libs/htmlpurifier-4.12.0-lite/library/HTMLPurifier/DefinitionCache/Serializer.php on line 73
...
   45.9637  516876080  16. HTMLPurifier->purify(string(7), ???) /home/someproject/Classes/Util/HTMLSanitizer.php:164
   45.9637  516879240  17. HTMLPurifier_Generator->__construct(class HTMLPurifier_HTML5Config, class HTMLPurifier_Context) /home/someproject/libs/htmlpurifier-4.12.0-lite/library/HTMLPurifier.php:158
   45.9637  516879240  18. HTMLPurifier_HTML5Config->getHTMLDefinition(???, ???) /home/someproject/libs/htmlpurifier-4.12.0-lite/library/HTMLPurifier/Generator.php:74
   45.9637  516879240  19. HTMLPurifier_HTML5Config->getDefinition(string(4), false, false) /home/someproject/libs/htmlpurifier-4.12.0-lite/library/HTMLPurifier/Config.php:415
   45.9637  516879240  20. HTMLPurifier_HTML5Config->getDefinition(string(4), true, true) /home/someproject/libs/htmlpurifier-html5-master/library/HTMLPurifier/HTML5Config.php:86
   45.9637  516879240  21. HTMLPurifier_DefinitionCache_Decorator_Cleanup->get(class HTMLPurifier_HTML5Config) /home/someproject/libs/htmlpurifier-4.12.0-lite/library/HTMLPurifier/Config.php:579
   45.9637  516879240  22. HTMLPurifier_DefinitionCache_Decorator_Cleanup->get(class HTMLPurifier_HTML5Config) /home/someproject/libs/htmlpurifier-4.12.0-lite/library/HTMLPurifier/DefinitionCache/Decorator/Cleanup.php:70
   45.9637  516879240  23. HTMLPurifier_DefinitionCache_Serializer->get(class HTMLPurifier_HTML5Config) /home/someproject/libs/htmlpurifier-4.12.0-lite/library/HTMLPurifier/DefinitionCache/Decorator.php:81
   45.9640  517026992  24. unserialize(string(132328)) /home/someproject/libs/htmlpurifier-4.12.0-lite/library/HTMLPurifier/DefinitionCache/Serializer.php:73

config:

"AutoFormat.AutoParagraph" => true,
"AutoFormat.Linkify" => true,
"AutoFormat.RemoveEmpty" => true, 
"AutoFormat.RemoveSpansWithoutAttributes" => true,
"Core.RemoveProcessingInstructions" => true,
"URI.AllowedSchemes" => array (
    'http' => true,
    'https' => true,
    'mailto' => true
),
"URI.DefaultScheme" => "https",
"Output.TidyFormat" => true,
"HTML.ForbiddenAttributes" => array("class", "@data-community-tooltip"),
"HTML.ForbiddenElements" => [ "iframe", "form", "button", "input", "body", "html", "frameset", "head", "meta", "script", "style" ],
"Attr.ForbiddenClasses" => array("bb_ul", "bb_tag"),
"Core.CollectErrors" => true,
"Cache.SerializerPath" => "/some/path"

More info will follow.

Start with HTMLPurifier/DefinitionCache/Serializer.php at line 73 and the Cache.DefinitionImpl and Cache.SerializerPath settings. Reproduce the leak across the PHP and HTML-Purifier versions listed, focusing on repeated unserialize calls in the DefinitionCache path. Done means the cause is isolated with a minimal reproduction and the memory behavior is verified after the change.