HTTP cache invalidation: sub-resource collections need an extension point

[soyuka]

Problem

PurgeHttpCacheListener (Symfony Doctrine and Laravel Eloquent) cannot invalidate sub-resource collection operations such as /parents/{parentId}/children. Resolving the IRI requires parent uriVariables that the listener does not have when it sees the modified entity, so those collection tags are silently skipped.

Pragmatic solution

Expose a public extension point — symmetric to Serializer\TagCollectorInterface on the response side — that the listener consults to collect extra tags for a given entity:

namespace ApiPlatform\HttpCache;

interface PurgeTagProviderInterface
{
    /**
     * @return iterable<string> additional cache tags to invalidate for $entity
     */
    public function getTagsForResource(object $entity): iterable;
}

Wiring:

• Tagged service (Symfony) / iterator binding (Laravel), aggregated and injected into PurgeHttpCacheListener.
• The listener appends whatever the providers yield to its existing tag set.
• Default ships with no provider — users opt in.

This keeps identifier-finding out of the framework (the user knows how their child invalidates their parents) and lets each project plug exactly the strategy that fits its routing (parent uri_variables, surrogate-key prefixes, class-based tags, etc.).

Scope

• Add the interface in src/HttpCache/.
• Inject iterable<PurgeTagProviderInterface> (optional, default empty) into both Symfony and Laravel PurgeHttpCacheListener.
• Document the seam alongside TagCollectorInterface.