Bug #80951 [Com]: WeakReferences are collected on scope
Edit report at https://bugs.php.net/bug.php?id=80951&edit=1
ID: 80951
Comment by: alex dot a dot pott at gmail dot com
Reported by: alex dot a dot pott at gmail dot com
Summary: WeakReferences are collected on scope
Status: Not a bug
Type: Bug
Package: *General Issues
Operating System: OS X / Linux
PHP Version: 8.0.3
Block user comment: N
Private report: N
New Comment:
Thanks for the reply nikic.
I've looked at PHP's implementation of WeakMap as well. And it seems to empty when an
object loses scope - at least according to https://php.watch/versions/8.0/weakmap.
With an application like Drupal where a memory cache is used to prevent reloading objects from a
database, if they are used in multiple parts of building a response for a page then, an object will
lose scope once that part of a page is built. Therefore if we converted our memory cache to use a
weakmap we'd end up having to reload an object even though we've not run out of memory.
Looking at implementation examples in the Java world they all require the equivalent of
gc_collect_cycles() to run. See https://www.baeldung.com/java-weakhashmap#weakhashmap-as-an-efficient-memory-cache
for example.
Previous Comments:
------------------------------------------------------------------------
[2021-04-12 16:34:41] nikic@php.net
WeakReferences have no impact on the object lifecycle at all -- that's their whole point. The
object will be destroyed at exactly the same time it would be destroyed if it had no weak
references.
I couldn't find the wording you quoted in the accepted RFC (https://wiki.php.net/rfc/weakrefs).
Apparently it was part of an older RFC (https://wiki.php.net/rfc/weakreferences).
Note that the canonical use-case for weak references is in the form of WeakMap, where a cache value
is associated with a weak object identifier, not the other way around.
------------------------------------------------------------------------
[2021-04-12 16:11:19] alex dot a dot pott at gmail dot com
Description:
------------
In the WeakReference documentation it says: "Weak references allow the programmer to retain a
reference to an object which does not prevent the object from being destroyed. They are useful for
implementing cache like structures."
And in the RFC it says:"The application might want to fetch the same serial number multiple
times. Fetching the same product again and making a new object could have several problems. It might
be slow and expensive."
However the current implementation means thats weak references don't prevent destruction when
an object loses scope. This means that a cache service that persists objects in memory can't
use the WeakReference object as hoped.
Ideally WeakReferences would prevent an object being destroyed until garbage collection is run.
For more information see https://www.drupal.org/project/drupal/issues/3190992#comment-14055154
Test script:
---------------
$obj = new stdClass;
$weakref = WeakReference::create($obj);
var_dump($weakref->get());
unset($obj);
var_dump($weakref->get());
gc_collect_cycles();
var_dump($weakref->get());
Expected result:
----------------
object(stdClass)#1 (0) {
}
object(stdClass)#1 (0) {
}
NULL
Actual result:
--------------
object(stdClass)#1 (0) {
}
NULL
NULL
------------------------------------------------------------------------
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