Core Data: Support server side save filters.

[epiqueras]

Description

core-data saving had an issue where edits (and dirtyness) would be kept after saving when the server had filters that modified the data. This happened because the client did not recognize the edits as persisted if the API responded with different values to what was sent.

This PR fixes this by considering that the API can change values while saving, and it changes core-data so it will not keep the corresponding edits unless they were made after the request was sent.

How has this been tested?

This filter was added to test the behavior with a post title filter:

function test_filter_post_data( $data, $postarr ) {
	if ( 'error' === $data['post_title'] ) {
		throw new Exception();
	}

	$data['post_title'] .= '_filtered!';
	return $data;
}
add_filter( 'wp_insert_post_data', 'test_filter_post_data', 10, 2 );

It was then verified that saving error keeps everything as is, but saving any other title clears dirty state and sets the current title to whatever you saved with '_filtered!' concatenated at the end.

Types of Changes

New Feature: core-data now plays nice with server side save filters.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

closes #17491

[youknowriad]

Unrelated, but it would be good to clear the "saving error notice" if the next save is successful.

[youknowriad]

Tested and works well

[youknowriad]

LGTM 👍 Thanks for the fixes Enrique

This is probably something that could use an e2e test but not a blocker for this PR. We can work on it later if needed.

[epiqueras]

Unrelated, but it would be good to clear the "saving error notice" if the next save is successful.

I thought it would happen automatically, because they use the same notice ID, is this not expected of the notices package?

This is probably something that could use an e2e test but not a blocker for this PR. We can work on it later if needed.

Yeah.

[youknowriad]

I thought it would happen automatically, because they use the same notice ID, is this not expected of the notices package?

It would but there's no success notices for draft saves :)

[epiqueras]

Should I add an explicit removeNotice here?

[youknowriad]

We could add a remove on successful saves (but probably something in the editor store instead, I'm also fine for it being a separate PR)

[epiqueras]

Sounds good.

[glendaviesnz]

FYI - this fixes an issue we were having with full site editing - Automattic/wp-calypso#36015 - the FSE posts currently have the header and footer template blocks added and stripped server side, and this was causing the edits/dirty state to be retained following a successful save. I am not able to replicate this issue after applying this patch locally 👍

[epiqueras]

@glendaviesnz That's great. I'll merge this as soon as the CI issues are resolved.