Remove ability to add a template part in the post editor

[annezazu]

Description

Remove ability to add a template part in the post editor, including copy and pasting. This was previous in place #37157 but seems to now be allowed to add once more. I can replicate this with 6.1 beta 1.

Step-by-step reproduction instructions

1. Use GB 14.1.1
2. Open post editor
3. Start to add a template part block however you'd like.
4. Notice that you can.
5. Deactivate Gutenberg
6. Open post editor
7. Try to add a template part block and notice you can't

Separately, notice that you can copy/paste a template part block from the template editor into a post editor, with or without Gutenberg installed, causing multi entity saving to appear in the post editor.

Screenshots, screen recording, code snippet

template.parts.mov

Environment info

• WordPress 6.1 beta 1
• Gutenberg 14.1.1 both activate and deactivated
• TT2

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[ramonjd]

I only got so far as to realize that rootClientId returns null at this point in the template part block init():

gutenberg/packages/block-library/src/template-part/index.js

Line 67 in 4f7c19e

rootClientId,

So the tests are failing and the filter will return true (allow the block to be inserted)

I can't see much that might have changed in the meantime since #37157 🤔 cc @adamziel who might know straight away

[Mamaduka]

I think the filter stopped working after #42258 got merged.

Testing

• ✅ The commit before merging the filter is working - 9620bae
• ❌ The merge commit the filter stops working - 4f7c19e

It might be a good idea to audit other blocks that are using side-effects - navigation-link, query and post-terms.

@gziolo, any ideas why the filters might stop working?

[gziolo]

@Mamaduka, I guess it means that the filter should get applied globally rather than only when the Template Part gets registered. It might be related to the order of execution, too.

[adamziel]

I can't see much that might have changed in the meantime since #37157 🤔 cc @adamziel who might know straight away

Unfortunately I don't :( @Mamaduka and @gziolo seem to be on the right track, though. It sounds like canInsertBlockTypeUnmemoized receives null as rootClientId on some occasions. It would be nice to have some assertions in place, e.g. assert( rootClientId ) – that way we'd find such problems right away.

[Mamaduka]

I guess it means that the filter should get applied globally rather than only when the Template Part gets registered. It might be related to the order of execution, too.

That might be the case for filter side effects. Would it be okay to add filters directly in the init.js file but outside the init method?

[gziolo]

I guess it means that the filter should get applied globally rather than only when the Template Part gets registered. It might be related to the order of execution, too.

That might be the case for filter side effects. Would it be okay to add filters directly in the init.js file but outside the init method?

init.js exists only for 3-rd party usage when devs want to init individual blocks. In the case of WordPress core, we call init from `index.js.

I think I identified what has changed:

The filter from the block library got applied first, and then it would get overridden by the following version in post editor:

gutenberg/packages/edit-post/src/index.js

Lines 83 to 97 in 7829913

	// Prevent adding template part in the post editor.
	// Only add the filter when the post editor is initialized, not imported.
	addFilter(
	'blockEditor.__unstableCanInsertBlockType',
	'removeTemplatePartsFromInserter',
	( can, blockType ) => {
	if (
	! select( editPostStore ).isEditingTemplate() &&
	blockType.name === 'core/template-part'
	) {
	return false;
	}
	return can;
	}
	);

Now, the filter registered with the block goes after. The fix should be as simple as moving the code above after registerCoreBlocks call in the file.

[Mamaduka]

The filter from the block library got applied first, and then it would get overridden by the following version in post editor

@gziolo, I'm not sure if that's the case.

Even if I move the edit post filter after the registerCoreBlocks call or comment it out, I can still insert the Template Parts block.