RFR: 8374549: Extend MetaspaceClosure to cover non-MetaspaceObj types

[Ashutosh Mehra]

On Mon, 5 Jan 2026 20:32:46 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> Previously, `MetaspaceClosure` could iterate only `MetaspaceObject`, using its four member functions:
> 
> - `is_read_only_by_default()`
> - `metaspace_pointers_do()`
> - `size()`
> - `type()`
> 
> `PackageEntry`, `ModuleEntry` and `GrowableArray` cannot be subclasses from `MetaspaceObject` due to various constraints.  As a result, they were copied with ad-hoc code.
> 
> This PR updates the templates in `MetaspaceClosure` so that it can iterate any classes  that have the above four functions. This allows new types of data to be uniformly copied into the AOT cache (aka CDS archive) without ad-hoc copiers.
> 
> This PR is necessary for future evolution of AOT as more types of data will be copied into the AOT cache. For example, in Valhalla we have a `GrowableArray` that needs to be copied along with `AdapterHandleEntry`.

Marked as reviewed by asmehra (Committer).

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PR Review: https://git.openjdk.org/jdk/pull/29049#pullrequestreview-3699357150