addAttribute() helper function overwrites blocktype attributes settings

[elliotcondon]

Describe the bug
When registering a block type that supports "anchor", the attributes.anchor setting is being overwritten by a helper function addAttribute().

This helper function is designed to work with a static block type that returns an element from the save() function but is causing issues with dynamic block types that return null.

As an example, here is a dynamic block type that allows "anchor" and expects the anchor value to be serialized within the block HTML JSON.

// Register with WP.
var result = wp.blocks.registerBlockType( 'test/my-block', {
	title: 'Test block',
	attributes: {
		anchor: {
			type: 'string'
		}
	}
	supports: {
		anchor: true
	}
	save: function(){
		return null;
	}
});

Even though the anchor attribute has been defined for the block type, the result will actually look like this because the attributes.anchor setting has been overwritten by the addAttribute() helper function.

{
	title: 'Test block',
	attributes: {
		anchor: {
			type: 'string',
			source: 'attribute',
			attribute: 'id',
			selector: '*',
		}
	}
	supports: {
		anchor: true
	}
	save: function(){
		return null;
	}
}

Expected behavior
If the attributes.anchor setting has already been specified when registering a block type, it should not be overwritten by the helper function.

Additional info
You can use the following line of code to remove the helper function, which allows dynamic block type to save "anchor" data, but obviously causes issues with core, and is not a valid solution:

wp.hooks.removeFilter( 'blocks.registerBlockType', 'core/anchor/attribute' );

[sarahquigley]

+1 on this. Causing big issues on our projects.

[okadots]

+1 definitely need to resolve this

[gziolo]

It looks like something that needs to be fixed. We can follow what already exists for supports: align:

https://github.com/WordPress/gutenberg/blob/master/packages/block-editor/src/hooks/align.js#L78-L81

[gziolo]

It should be fixed with #15959.