Fix #81585: cached_chunks are not counted to real_size on shutdown

[cmb69]

The amount of allocated system memory is kept in real_size, including
the allocated cached_chunks. Thus, we need to keep the proper count
at the end of the shutdown.

[dstogov]

I think, it's right now.

[pvandommelen]

@cmb69 @dstogov I believe this change has had an (unforeseen?) side effect which can be relevant in some scenarios. Within a single php worker process, a previous request will now more directly impact the amount of memory available in the following request.

I believe the new behaviour is more correct, as the memory reported now more accurately reports the actual consumed memory from the OS. Our code can also be adapted to handle this new behaviour, probably by taking the currently available memory into account. So there is probably nothing actionable (except maybe more general improvements to the memory manager), but this reply can maybe be useful as additional documentation.

In our case we call ini_set('memory_limit', ...) on every request, where we expect each request's memory usage to start below the default memory limit. Changing the memory limit makes a check against the currently allocated memory (since #7040). This wasn't a problem before the change of this MR, but with this change it will randomly fail when a previous request happened to consume a lot of memory which wasn't entirely collected.

The following script illustrates the issue. Works with apache's mod_php + mpm_prefork (making sure only 1 worker runs), but is more easily tested with php's internal webserver (php -S).

header('Content-Type:text/plain');
ini_set('display_errors', 1);

echo sprintf("\nmemory usage: %.3f MB", memory_get_usage(true) / 1024 / 1024);
ini_set('memory_limit', '64M');

ini_set('memory_limit', '1G');
for ($i = 0; $i < 100; ++$i) {
	// create an object which isn't immediately garbage collected by making it reference itself
	$a = new \StdClass();
	$a->a = $a;
	// have it consume memory. Behaviour does not appear to trigger when this string is 2 MB or larger,
	// possibly because it can be more easily be collected when it exceeds ZEND_MM_CHUNK_SIZE?
	$a->b = str_repeat('a', 1024*1024*1);
}
echo sprintf("\nmemory usage: %.3f MB", memory_get_usage(true) / 1024 / 1024);

gc_collect_cycles();
echo sprintf("\nmemory usage: %.3f MB", memory_get_usage(true) / 1024 / 1024);

PHP 8.0.14, the memory reported goes down for each executed request (which is incorrect):

memory usage: 2.000 MB
memory usage: 200.000 MB
memory usage: 4.000 MB

memory usage: 2.000 MB
memory usage: 198.000 MB
memory usage: 100.000 MB

memory usage: 2.000 MB
memory usage: 100.000 MB
memory usage: 52.000 MB

memory usage: 2.000 MB
memory usage: 50.000 MB
memory usage: 26.000 MB

memory usage: 2.000 MB
memory usage: 14.000 MB
memory usage: 10.000 MB

...

memory usage: 2.000 MB
memory usage: 2.000 MB
memory usage: 2.000 MB

PHP 8.0.15. Memory reset at start will fail when the memory of the previous request is not collected.

memory usage: 2.000 MB
memory usage: 200.000 MB
memory usage: 4.000 MB

memory usage: 4.000 MB
memory usage: 200.000 MB
memory usage: 102.000 MB

memory usage: 102.000 MB<br />
<b>Warning</b>:  Failed to set memory limit to 67108864 bytes (Current memory usage is 106954752 bytes) in <b>/home/peter/projects/objectenregister/memtest.php</b> on line <b>6</b><br />
memory usage: 200.000 MB
memory usage: 152.000 MB

memory usage: 152.000 MB<br />
<b>Warning</b>:  Failed to set memory limit to 67108864 bytes (Current memory usage is 159383552 bytes) in <b>/home/peter/projects/objectenregister/memtest.php</b> on line <b>6</b><br />
memory usage: 200.000 MB
memory usage: 176.000 MB

...

memory usage: 200.000 MB<br />
<b>Warning</b>:  Failed to set memory limit to 67108864 bytes (Current memory usage is 209715200 bytes) in <b>/home/peter/projects/objectenregister/memtest.php</b> on line <b>6</b><br />
memory usage: 200.000 MB
memory usage: 200.000 MB

[dstogov]

I hope #8053 should fix the problem