do_blocks(): Free up transient memory leak

There has been a memory inefficiency inside of `do_blocks()` where it
parses the given content then iterates through each top-level block to
render it. Unfortunately each top-level block can considerably add to
the overall memory use involved, and once moving on to the next
top-level block, all of the allocated memory will be retained until the
end of the call to `do_blocks()`.

In this change, each parsed block sub-tree is freed via reset to `null`
after it has been rendered. All top-level blocks are rendered
independently of each other and so this operation is safe — there are no
data dependencies between them.

For a test post of length 2.3 MB containing 138 top-level blocks, this
brought the minimum memory requirement for the PHP process down from
around 450 MB to around 316 MB. This was “around” since the memory
requirements are non-deterministic and some runs will succeed while
other runs will crash for out-of-memory given the same `memory_limit`.

Impact of this change will be most profound when there exist one or
more top-level blocks which allocate a significant amount of memory
which are not the last top-level-block in a post. This means that
this change might even impact small and typical posts, if the right
blocks are near the start of the post.

Developed in #8999
Discussed in https://core.trac.wordpress.org/ticket/63588

Props dmsnell, joemcgill.
Fixes #63588.


git-svn-id: https://develop.svn.wordpress.org/trunk@60316 602fd350-edb4-49c9-b593-d223f7449a82

Author: dmsnell

src/wp-includes/blocks.php

@@ -2403,11 +2403,13 @@ function parse_blocks( $content ) {
  * @return string Updated post content.
  */
 function do_blocks( $content ) {
-	$blocks = parse_blocks( $content );
-	$output = '';
+	$blocks                = parse_blocks( $content );
+	$top_level_block_count = count( $blocks );
+	$output                = '';
 
-	foreach ( $blocks as $block ) {
-		$output .= render_block( $block );
+	for ( $i = 0; $i < $top_level_block_count; $i++ ) {
+		$output .= render_block( $blocks[ $i ] );
+		$blocks[ $i ] = null;
 	}
 
 	// If there are blocks in this content, we shouldn't run wpautop() on it later.