Pass meta_id when retrieving multiple metas

[GregLone]

Hello.

In some (edge?) cases we would need a unique identifier when we retrieve metas. For example, get_post_custom_values( '_yolo', $post->ID ) will return:

Array
(
    [0] => foo
    [1] => bar
)

while the following, with the meta_id as array key, would be helpful imho:

Array
(
    [1022] => foo
    [1029] => bar
)

The modification is trivial in update_meta_cache(), and some other functions would only need something like metas[0] => reset( metas ).

[sc0ttkclark]

I like the idea behind this, based on your purpose.

[GregLone]

In the same time, all the *_metadata_by_mid() would benefit of the cache.

[boonebgorges]

Replying to GregLone:

In the same time, all the *_metadata_by_mid() would benefit of the cache.

IMO, this makes the proposal worth pursuing. It'd be nice to see improved unit tests around cache invalidation for update_metadata(), update_metadata_by_id(), and their delete_ equivalents as part of this improvement.

I'm somewhat concerned about backward compatibility. In the same way that core sometimes references $metas[0], it's possible that there are plugins doing the same thing. Does anyone have a good idea of how to gauge this impact? My google-fu is failing me :)