Reusable Blocks: Preload user permissions

[aduth]

Previously: #12378

This pull request seeks to add the blocks permissions REST API request as a preloaded path, for two reasons:

• We use preload data to avoid UI "flickering". Presently, without these changes, UI such as the "Add to Reusable Blocks" button in a block settings menu appears only after a brief delay, which is undesirable.
• As described at Testing: Skip unreliable end-to-end tests #15059 (comment) , the immediate unavailability of permissions data may be a culprit in end-to-end test instability

It appears this was already added in #12378 and exists in WordPress 5.2 (WordPress/wordpress-develop@79a3abc), but should remain in the Gutenberg plugin until WordPress 5.2 is the minimum supported version. It could be considered as erroneously removed via #13569.

Testing Instructions:

Verify that when clicking the block settings menu for the first time in a page session, the "Add to Reusable Blocks" is shown immediately if the user has permissions to undertake this operation (i.e. there is no delay in its appearance).

Ensure unit tests pass:

npm run test-php

[aduth]

Rebased in mind of the merge of #15059, and unskipped the affected test described in #15059 (comment) to reflect the stability improved by these changes.

[aduth]

For future consideration: The implementation of gutenberg_extend_block_editor_preload_paths could be dramatically simplified and less error-prone on considering duplicates once we can officially drop support for PHP 5.2.

diff --git a/lib/client-assets.php b/lib/client-assets.php
index d9fcc8105..cde1ae0fb 100644
--- a/lib/client-assets.php
+++ b/lib/client-assets.php
@@ -582,10 +582,8 @@ function gutenberg_extend_block_editor_preload_paths( $preload_paths ) {
 	 * This is present in WordPress 5.2 and should be removed from Gutenberg
 	 * once WordPress 5.2 is the minimum supported version.
 	 */
-	if ( ! in_array( array( '/wp/v2/blocks', 'OPTIONS' ), $preload_paths ) ) {
-		$preload_paths[] = array( '/wp/v2/blocks', 'OPTIONS' );
-	}
+	$preload_paths[] = array( '/wp/v2/blocks', 'OPTIONS' );
 
-	return $preload_paths;
+	return array_unique( $preload_paths, SORT_REGULAR );
 }
 add_filter( 'block_editor_preload_paths', 'gutenberg_extend_block_editor_preload_paths' );

The function array_unique() does not have a parameter "sort_flags" in PHP version 5.2.8 or earlier (PHPCompatibility.PHP.NewFunctionParameters.array_unique_sort_flagsFound)

• https://www.php.net/manual/en/function.array-unique.php
• https://make.wordpress.org/core/2018/12/08/updating-the-minimum-php-version/

[gziolo]

I can confirm it is preloaded properly and I can see the menu item immediately 👍