Add from library should only display allowed paragraph types

I think that this is happening because the LibraryItem class does not specify any value for default_reference_revision_settings, which means that the DefaultSelection class is used when querying.

Compare with the Paragraph class, which specifies that the ParagraphSelection EntityReferenceSelection plugin class should be used.

So I think the next step will be:

1. define an EntityReferenceSelection plugin for paragraphs library items
2. update the LibraryItem class to use it
3. implement the buildEntityQuery method to limit the selection based on the parent, similar to ParagraphsLibraryItemHasAllowedParagraphsTypeConstraintValidator

I've been working on this to try and get a nice solution, it feels like much better UX.

I think the best solution here would be to set the target bundles in the form alter from a paragraph. In the $context variable we can get the target bundles of the parent field, and then set them on the #selection_settings of field_reusable_paragraph.

This does mean in LibraryItemSelection::buildEntityQuery we can't call parent::buildEntityQuery as it will throw errors. For now in the patch I have just copied relevant code from the parent function, but I guess an alternative would be to store the target bundles in a different field and then use them from there. But it seems clunkier to me.

I have added one patch that has the form_alter built into the paragraphs_library module. I will also add another comment below with a patch that omits the form alter, so people can add their own form alter implementation if they wish.

Here is the patch that omits the form_alter

The last patches were erroneous so here are the patches again but with the fixes in the entity annotation

Having a nightmare here. The last patches didn't have the new custom entity reference selection on.
These two patches should.

@mjmorley I'm confused about the difference between the two patches.

What's the difference, and which one is your preferred option?
Also, it's probably easier to create merge request(s)

I rebased the branch against 8.x-1.x and finished the draft implementation in the MR based on the form alter hook by @mjmorley. I also rewrote LibraryItemSelection, now we can keep the parent::buildEntityQuery($match, $match_operator) call and we don't need to query all paragraph ids of a certain type, which could break for sites with a lot of paragraphs. Now this is actually ready for review, without the need for custom form alters.

The MR should be marked as ready, but it seems like I don't have permission to do so.