Allow dynamic blocks to create loops

Stores and restores the global post object around dynamic block callbacks. This allows dynamic blocks to create new `WP_Query` instances and set up its post data, without changing the currently edited post.

See #7427, #8035.

lib/blocks.php

@@ -151,11 +151,14 @@ function gutenberg_render_block( $block ) {
  * Parses dynamic blocks out of `post_content` and re-renders them.
  *
  * @since 0.1.0
+ * @global WP_Post $post The post to edit.
  *
  * @param  string $content Post content.
  * @return string          Updated post content.
  */
 function do_blocks( $content ) {
+	global $post;
+
 	$rendered_content      = '';
 	$dynamic_block_pattern = get_dynamic_blocks_regex();
 
@@ -213,8 +216,15 @@ function do_blocks( $content ) {
 			$content       = substr( $content, $end_offset + strlen( $end_tag ) );
 		}
 
+		/*
+		 * Back up global post, to restore after render callback.
+		 * Allows callbacks to run new WP_Query instances without breaking the global post.
+		 */
+		$global_post = $post;
+
 		// Replace dynamic block with server-rendered output.
 		$rendered_content .= $block_type->render( $attributes, $inner_content );
+		$post = $global_post;
 	}
 
 	// Append remaining unmatched content.

phpunit/class-dynamic-blocks-render-test.php

@@ -38,6 +38,33 @@ function render_dummy_block_numeric() {
 		return 10;
 	}
 
+	/**
+	 * Dummy block rendering function, creating a new WP_Query instance.
+	 *
+	 * @return string Block output.
+	 */
+	function render_dummy_block_wp_query() {
+		$content = '';
+		$recent  = new WP_Query( array(
+			'numberposts'      => 10,
+			'orderby'          => 'ID',
+			'order'            => 'DESC',
+			'post_type'        => 'post',
+			'post_status'      => 'draft, publish, future, pending, private',
+			'suppress_filters' => true,
+		) );
+
+		while ( $recent->have_posts() ) {
+			$recent->the_post();
+
+			$content .= get_the_title();
+		}
+
+		wp_reset_postdata();
+
+		return $content;
+	}
+
 	/**
 	 * Tear down.
 	 */
@@ -87,6 +114,27 @@ function test_dynamic_block_rendering() {
 		);
 	}
 
+	/**
+	 * Tests that do_blocks() maintains the global $post variable when dynamic
+	 * blocks create new WP_Query instances in their callbacks.
+	 *
+	 * @covers ::do_blocks
+	 */
+	function test_global_post_persistence() {
+		global $post;
+
+		register_block_type( 'core/dummy', array(
+			'render_callback' => array( $this, 'render_dummy_block_wp_query' ),
+		) );
+
+		$posts = self::factory()->post->create_many( 5 );
+		$global_post = $post = get_post( end( $posts ) );
+
+		do_blocks( '<!-- wp:core/dummy /-->' );
+
+		$this->assertEquals( $global_post, $post );
+	}
+
 	/**
 	 * Test dynamic blocks return string value from render, even if render
 	 * callback does not.