Ensure main query loop is entered for singular content in block themes

src/wp-includes/block-template.php

@@ -210,12 +210,12 @@ function _block_template_render_title_tag() {
  *
  * @global string   $_wp_current_template_content
  * @global WP_Embed $wp_embed
+ * @global WP_Query $wp_query
  *
  * @return string Block template markup.
  */
 function get_the_block_template_html() {
-	global $_wp_current_template_content;
-	global $wp_embed;
+	global $_wp_current_template_content, $wp_embed, $wp_query;
 
 	if ( ! $_wp_current_template_content ) {
 		if ( is_user_logged_in() ) {
@@ -228,7 +228,30 @@ function get_the_block_template_html() {
 	$content = $wp_embed->autoembed( $content );
 	$content = shortcode_unautop( $content );
 	$content = do_shortcode( $content );
-	$content = do_blocks( $content );
+
+	/*
+	 * Most block themes omit the `core/query` and `core/post-template` blocks in their singular content templates.
+	 * While this technically still works since singular content templates are always for only one post, it results in
+	 * the main query loop never being entered which causes bugs in core and the plugin ecosystem.
+	 *
+	 * The workaround below ensures that the loop is started even for those singular templates. The while loop will by
+	 * definition only go through a single iteration, i.e. `do_blocks()` is only called once. Additional safeguard
+	 * checks are included to ensure the main query loop has not been tampered with and really only encompasses a
+	 * single post.
+	 *
+	 * Even if the block template contained a `core/query` and `core/post-template` block referencing the main query
+	 * loop, it would not cause errors since it would use a cloned instance and go through the same loop of a single
+	 * post, within the actual main query loop.
+	 */
+	if ( is_singular() && 1 === $wp_query->post_count && have_posts() ) {
+		while ( have_posts() ) {
+			the_post();
+			$content = do_blocks( $content );
+		}
+	} else {
+		$content = do_blocks( $content );
+	}
+
 	$content = wptexturize( $content );
 	$content = convert_smilies( $content );
 	$content = wp_filter_content_tags( $content, 'template' );

tests/phpunit/tests/block-template.php

@@ -184,4 +184,118 @@ public function test_template_remains_unchanged_if_templates_array_is_empty() {
 		$resolved_template_path = locate_block_template( '', 'search', array() );
 		$this->assertSame( '', $resolved_template_path );
 	}
+
+	/**
+	 * Tests that `get_the_block_template_html()` wraps block parsing into the query loop when on a singular template.
+	 *
+	 * This is necessary since block themes do not include the necessary blocks to trigger the main query loop, and
+	 * since there is only a single post in the main query loop in such cases anyway.
+	 *
+	 * @ticket 58154
+	 * @covers ::get_the_block_template_html
+	 */
+	public function test_get_the_block_template_html_enforces_singular_query_loop() {
+		global $_wp_current_template_content, $wp_query, $wp_the_query;
+
+		// Register test block to log `in_the_loop()` results.
+		$in_the_loop_logs = array();
+		$this->register_in_the_loop_logger_block( $in_the_loop_logs );
+
+		// Set main query to single post.
+		$post_id      = self::factory()->post->create( array( 'post_title' => 'A single post' ) );
+		$wp_query     = new WP_Query( array( 'p' => $post_id ) );
+		$wp_the_query = $wp_query;
+
+		// Use block template that just renders post title and the above test block.
+		$_wp_current_template_content = '<!-- wp:post-title /--><!-- wp:test/in-the-loop-logger /-->';
+
+		$expected  = '<div class="wp-site-blocks">';
+		$expected .= '<h2 class="wp-block-post-title">A single post</h2>';
+		$expected .= '</div>';
+
+		$output = get_the_block_template_html();
+		$this->unregister_in_the_loop_logger_block();
+		$this->assertSame( $expected, $output, 'Unexpected block template output' );
+		$this->assertSame( array( true ), $in_the_loop_logs, 'Main query loop was not triggered' );
+	}
+
+	/**
+	 * Tests that `get_the_block_template_html()` does not start the main query loop generally.
+	 *
+	 * @ticket 58154
+	 * @covers ::get_the_block_template_html
+	 */
+	public function test_get_the_block_template_html_does_not_generally_enforce_loop() {
+		global $_wp_current_template_content, $wp_query, $wp_the_query;
+
+		// Register test block to log `in_the_loop()` results.
+		$in_the_loop_logs = array();
+		$this->register_in_the_loop_logger_block( $in_the_loop_logs );
+
+		// Set main query to a general post query (i.e. not for a specific post).
+		$post_id      = self::factory()->post->create(
+			array(
+				'post_title'   => 'A single post',
+				'post_content' => 'The content.',
+			)
+		);
+		$wp_query     = new WP_Query(
+			array(
+				'post_type'   => 'post',
+				'post_status' => 'publish',
+			)
+		);
+		$wp_the_query = $wp_query;
+
+		/*
+		 * Use block template that renders the above test block, followed by a main query loop.
+		 * `get_the_block_template_html()` should not start the loop, but the `core/query` and `core/post-template`
+		 * blocks should.
+		 */
+		$_wp_current_template_content  = '<!-- wp:test/in-the-loop-logger /-->';
+		$_wp_current_template_content .= '<!-- wp:query {"query":{"inherit":true}} -->';
+		$_wp_current_template_content .= '<!-- wp:post-template -->';
+		$_wp_current_template_content .= '<!-- wp:post-title /-->';
+		$_wp_current_template_content .= '<!-- wp:post-content /--><!-- wp:test/in-the-loop-logger /-->';
+		$_wp_current_template_content .= '<!-- /wp:post-template -->';
+		$_wp_current_template_content .= '<!-- /wp:query -->';
+
+		$expected  = '<div class="wp-site-blocks">';
+		$expected .= '<ul class="wp-block-post-template is-layout-flow wp-block-post-template-is-layout-flow wp-block-query-is-layout-flow">';
+		$expected .= '<li class="wp-block-post post-' . $post_id . ' post type-post status-publish format-standard hentry category-uncategorized">';
+		$expected .= '<h2 class="wp-block-post-title">A single post</h2>';
+		$expected .= '<div class="entry-content wp-block-post-content is-layout-flow wp-block-post-content-is-layout-flow">' . wpautop( 'The content.' ) . '</div>';
+		$expected .= '</li>';
+		$expected .= '</ul>';
+		$expected .= '</div>';
+
+		$output = get_the_block_template_html();
+		$this->unregister_in_the_loop_logger_block();
+		$this->assertSame( $expected, $output, 'Unexpected block template output' );
+		$this->assertSame( array( false, true ), $in_the_loop_logs, 'Main query loop was triggered incorrectly' );
+	}
+
+	/**
+	 * Registers a test block to log `in_the_loop()` results.
+	 *
+	 * @param array $in_the_loop_logs Array to log function results in. Passed by reference.
+	 */
+	private function register_in_the_loop_logger_block( array &$in_the_loop_logs ) {
+		register_block_type(
+			'test/in-the-loop-logger',
+			array(
+				'render_callback' => function() use ( &$in_the_loop_logs ) {
+					$in_the_loop_logs[] = in_the_loop();
+					return '';
+				},
+			)
+		);
+	}
+
+	/**
+	 * Unregisters the test block registered by the `register_in_the_loop_logger_block()` method.
+	 */
+	private function unregister_in_the_loop_logger_block() {
+		unregister_block_type( 'test/in-the-loop-logger' );
+	}
 }