Method call may leak if releasing EG(This) triggers GC

Description

Method calls may cause their return value to leak if releasing EG(This) triggers GC.

The following code:

<?php

class A {
    public $cycle;
    public function __construct() { $this->cycle = $this; }
}
class B {
    public function get() {
        return new A();
    }
}

$c = new B();
$objs = [];

while (gc_status()['roots']+2 < gc_status()['threshold']) {
    $obj = new stdClass;
    $objs[] = $obj;
}

var_dump($c->get());

Resulted in a memory leak:

Script:  'test.php'
Zend/zend_objects.c(189) :  Freeing 0x00007ffff7a5c840 (56 bytes), script=test.php
=== Total 1 memory leaks detected ===

Here is what is happening:

• After returning from get(), $c is released, which triggers GC
  • A is removed from buffer, and is not collected because it's referenced by the call stack
• After returning from var_dump(), zend_vm_stack_free_args() releases A with zval_ptr_dtor_nogc(), so A is not added to the GC buffer
• At this point nothing references A but itself, and A is not in the GC buffer, so it leaks

I'm not sure how to fix this appart from switching to zval_ptr_dtor().

PHP Version

master

Operating System

No response

Reproduce the leak with the test.php example, then inspect the linked zend_vm_def.h call sites around lines 2866 and 4019 and the leak report from Zend/zend_objects.c. Confirm the fix prevents the returned cyclic object from remaining uncollected and that the example no longer reports a memory leak.