[6.x] Optimize eager loading memory handling

[netpok]

Fixes #30114

This pull request fixes a memory leak issue reported in the mentioned issue, I think this may be a bug in PHP itself so I will report to them too.

The problem is that the Eloquent builder is creating closures for the relations, these relations are stored in the eagerLoads property of the class because these closures are containing a reference to the builder the GC thinks the class is still referenced. By refactoring these closures to static ones this reference does not get stored so the GC can correctly clean up the builder earlier.

edit: Turns out this just delays the GC, it gets detected later when memory usage hits ~24MB. Updated the title to optimize.

These methods does not use the $this reference so there is no other change needed.

[taai]

Hey, @netpok, from your PR I learned something new today – about static closures. Thank you! So now all closures should get checked and converted to static closures, if they don't use $this. For example, at the same file, in the hydrate method is a closure that does not use $this and can be converted to a static closure. Am I right?

[netpok]

Yes that can be replaced too.

Although that's not that big of a problem, here the main issue was that these calbacks were stored in the same class, so the gc viewed the class as it still had a reference. There are $this cycles in the gc to check this but that is not executed that often because of the performance penalty it causes.

So I'm not sure the performance boost worth the effort. But other classes may have this kind of issue too.