Canonical: Redirect when front page's paginated states not found.

Perform a canonical redirect for an invalid pagination request of a static front page.

When a site has a static front page assigned and that page has a `<!--nextpage-->` within its content, previously accessing non-existing pages (e.g. `example.com/page/3/`) did not redirect or return a 404 or 301. This changeset resolves that issue by performing a canonical redirect.

Unit tests are also included for this specific use case and to ensure the fix does not affect a blog listing home page.

Follow-up to [47738], [47727], [34492].

Props dd32, audrasjb, chaion07, hellofromTonya, joemcgill, lukecarbis, Mte90, mukesh27, peterwilsoncc, rajinsharwar, SergeyBiryukov. 
Fixes #50163.
See meta#5184.

git-svn-id: https://develop.svn.wordpress.org/trunk@59091 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp.php

@@ -747,14 +747,15 @@ public function handle_404() {
 			$content_found = true;
 
 			if ( is_singular() ) {
-				$post = isset( $wp_query->post ) ? $wp_query->post : null;
-				$next = '<!--nextpage-->';
+				$post    = isset( $wp_query->post ) ? $wp_query->post : null;
+				$next    = '<!--nextpage-->';
+				$page_qv = is_front_page() ? 'paged' : 'page';
 
 				// Check for paged content that exceeds the max number of pages.
-				if ( $post && ! empty( $this->query_vars['page'] ) ) {
+				if ( $post && ! empty( $this->query_vars[ $page_qv ] ) ) {
 					// Check if content is actually intended to be paged.
 					if ( str_contains( $post->post_content, $next ) ) {
-						$page          = trim( $this->query_vars['page'], '/' );
+						$page          = trim( $this->query_vars[ $page_qv ], '/' );
 						$content_found = (int) $page <= ( substr_count( $post->post_content, $next ) + 1 );
 					} else {
 						$content_found = false;

tests/phpunit/tests/canonical/paged.php

@@ -26,4 +26,49 @@ public function test_redirect_canonical_with_nextpage_pagination() {
 		// Non-existing page should redirect to the permalink.
 		$this->assertCanonical( $link . '4/', $link );
 	}
+
+	/**
+	 * Ensures canonical redirects are performed for sites with a static front page.
+	 *
+	 * @ticket 50163
+	 */
+	public function test_redirect_missing_front_page_pagination_canonical() {
+		update_option( 'show_on_front', 'page' );
+
+		$page_id = self::factory()->post->create(
+			array(
+				'post_title'   => 'front-page-1',
+				'post_type'    => 'page',
+				'post_content' => "Front Page 1\n<!--nextpage-->\nPage 2",
+			)
+		);
+
+		update_option( 'page_on_front', $page_id );
+
+		$link = parse_url( get_permalink( $page_id ), PHP_URL_PATH );
+
+		// Valid page numbers should not redirect.
+		$this->assertCanonical( $link, $link, 'The home page is not expected to redirect.' );
+		$this->assertCanonical( $link . 'page/2/', $link . 'page/2/', 'Page 2 exists and is not expected to redirect.' );
+
+		// Invalid page numbers should redirect to the front page.
+		$this->assertCanonical( $link . 'page/3/', $link, 'Page 3 does not exist and is expected to redirect to the home page.' );
+	}
+
+	/**
+	 * Ensures that canonical redirects are not performed for sites with a blog listing home page.
+	 *
+	 * @ticket 50163
+	 */
+	public function test_redirect_missing_front_page_pagination_does_not_affect_posts_canonical() {
+		self::factory()->post->create_many( 3 );
+		update_option( 'posts_per_page', 2 );
+
+		// Valid page numbers should not redirect.
+		$this->assertCanonical( '/', '/', 'Page one of the blog archive should not redirect.' );
+		$this->assertCanonical( '/page/2/', '/page/2/', 'Page 2 of the blog archive exists and is not expected to redirect.' );
+
+		// Neither should invalid page numbers.
+		$this->assertCanonical( '/page/3/', '/page/3/', 'Page 3 of the blog archive is not populated but is not expected to redirect.' );
+	}
 }