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New gc_status() fields, and WeakMap cycle collection

php.internals

Arnaud.lb

3 years ago
Hi everyone, I'm considering merging the following two PRs: https://github.com/php/php-src/pull/11523: Exposes the time spent in the cycle collector via new gc_status() fields. https://github.com/php/php-src/pull/10932: Allows the GC to collect certain types of cycles involving WeakMaps. The second one introduces a minor BC break. If there are no objections, I plan to merge these PRs prior to the feature freeze. Best regards, Arnaud

Larry Garfield

3 years ago
On Fri, Jul 7, 2023, at 1:09 PM, Arnaud Le Blanc wrote:
> Hi everyone, > > I'm considering merging the following two PRs: > > https://github.com/php/php-src/pull/11523: Exposes the time spent in > the cycle collector via new gc_status() fields. > https://github.com/php/php-src/pull/10932: Allows the GC to collect > certain types of cycles involving WeakMaps. > > The second one introduces a minor BC break. > > If there are no objections, I plan to merge these PRs prior to the > feature freeze. > > Best regards, > Arnaud
For the second, doesn't that translate to "WeakMaps are currently not actually Weak, and thus don't work at all?" I don't quite know how that would be a BC break to fix... --Larry Garfield

Arnaud.lb

3 years ago
I agree that we may not consider this a BC break, but I'm asking in case people rely on the current behavior or if the change is more controversial than I thought. To be fair they work as specified in the RFC, and are actually weak as they do not explicitly retain their keys, but they need explicit support from the GC to be able to collect some types of cycles. WeakMap implementations based on WeakReferences tend to have this limitation. For instance, the Java documentation for WeakHashMap has this warning: "The value objects in a WeakHashMap are held by ordinary strong references. Thus care should be taken to ensure that value objects do not strongly refer to their own keys, either directly or indirectly, since that will prevent the keys from being discarded". On Fri, Jul 7, 2023 at 4:03 PM Larry Garfield <larry@garfieldtech.com> wrote: