Not all sitemaps providers can be filtered

[Chouby]

For the context: to allow multilingual sitemaps with Polylang, and after
this comment, I implemented a sitemaps provider decorator replacing the existing providers with the filter wp_sitemaps_register_providers. See ​https://github.com/polylang/polylang/blob/2.8-beta1/modules/sitemaps/sitemaps.php#L152-L163

However I realized that if another provider is added directly, for example with wp_register_sitemap(), it is not seen by the filter wp_sitemaps_register_providers and it looks like there is no alternative to filter it.

So I suggest to add a filter allowing to filter all registered sitemap providers.

[swissspidy]

Thanks for your report! Very useful feedback as always.

You're right that folks can (and should) use wp_register_sitemap() to register sitemaps, and that will not pass through the wp_sitemaps_register_providers filter.

The filter doc block is a bit misleading: "Filters the list of registered sitemap providers."

It would be more accurate to say "Filters the list of built-in sitemap providers before they are registered.". Not to mention the doc block for the class method which could be improved...

Anyway, I'd consider not being able to unregister/filter sitemaps added via wp_register_sitemap a bug, and we should fix it.

[davidbaumwald]

@swissspidy Is this still in the cards for 5.5?

[swissspidy]

@davidbaumwald If we can come up with a solution, yes

[pbiron]

50660.2.diff​ looks good to me

[swissspidy]

In 48543:

Sitemaps: Replace wp_sitemaps_register_providers filter with more suitable wp_sitemaps_add_provider filter.

The previous filter failed the goal of allowing developers to filter all providers before they are registered, since it only filtered the built-in ones.

The more specific wp_sitemaps_add_provider filter enables exactly that, as it filters every sitemap provider right before it is added to the sitemaps registry.

Props pbiron, pfefferle, Chouby, swissspidy.
Fixes #50660.