Block Hooks: Skip parsing and re-serializing post content in the_content when no blocks are hooked

[sboisvert]

Pages with a lot of block content are slow because, since 6.8, every the_content call parses and re-serializes the whole post even when no block is hooked. Since [60173] (6.8.1), the temporary hooked_block_types filter also makes the has_filter( 'hooked_block_types' ) check in apply_block_hooks_to_content() always true on this path, so the per-block Block Hooks run on top of that.

Timing apply_block_hooks_to_content_from_post_object() on ~1 MB of block markup with nothing hooked (PHP 8.4): 34-35 ms per call on trunk, 0.4 ms with this change.

This change returns the content unchanged at the top of apply_block_hooks_to_content_from_post_object(), before the temporary filter is added, when all of these are true:

• get_hooked_blocks() is empty and has_filter( 'hooked_block_types' ) is false;
• has_block( 'core/template-part', $content ) is false;
• $ignored_hooked_blocks_at_root is null (the REST API passes an empty array, so REST requests still get the full parse; see #65008).

Rendered output is unchanged, except for content that begins with a stray closing block delimiter. Since 6.8, the function wraps the post content in a temporary wrapper block before parsing, and that wrapper absorbs a leading stray closer, so the blocks after it render. With this change, such content renders as it did before 6.8.

Adds test_apply_block_hooks_to_content_from_post_object_returns_content_unchanged_if_no_hooked_blocks to applyBlockHooksToContentFromPostObject.php. Run with php vendor/bin/phpunit --group block-hooks.

Trac ticket: https://core.trac.wordpress.org/ticket/66236

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[westonruter]

You can use the MockAction class for this instead. It has a method that gets the call count.