Pagniation links with/without trailing slashes.

src/wp-includes/general-template.php

@@ -4672,9 +4672,26 @@ function paginate_links( $args = '' ) {
 	// Append the format placeholder to the base URL.
 	$pagenum_link = trailingslashit( $url_parts[0] ) . '%_%';
 
+	/*
+	 * Ensures sites not using trailing slashes get links in the form
+	 * `/page/2` rather than `/page/2/`. On these sites, linking to the
+	 * URL with a trailing slash will results in a 301 redirect from the
+	 * incorrect URL to the correctly formattted one. This presents an
+	 * unnecessary performance hit.
+	 */
+	if ( $wp_rewrite->using_permalinks() && ! $wp_rewrite->use_trailing_slashes ) {
+		$pagenum_link = untrailingslashit( $url_parts[0] );
+	} else {
+		$pagenum_link = trailingslashit( $url_parts[0] );
+	}
+	$pagenum_link .= '%_%';
+
 	// URL base depends on permalink settings.
 	$format  = $wp_rewrite->using_index_permalinks() && ! strpos( $pagenum_link, 'index.php' ) ? 'index.php/' : '';
 	$format .= $wp_rewrite->using_permalinks() ? user_trailingslashit( $wp_rewrite->pagination_base . '/%#%', 'paged' ) : '?paged=%#%';
+	if ( $wp_rewrite->using_permalinks() && ! $wp_rewrite->use_trailing_slashes ) {
+		$format = '/' . ltrim( $format, '/' );
+	}
 
 	$defaults = array(
 		'base'               => $pagenum_link, // http://example.com/all_posts.php%_% : %_% is replaced by format (below).

tests/phpunit/tests/general/paginateLinks.php

@@ -9,6 +9,39 @@ class Tests_General_PaginateLinks extends WP_UnitTestCase {
 
 	private $i18n_count = 0;
 
+	/**
+	 * Post IDs created for shared fixtures.
+	 *
+	 * @var int[]
+	 */
+	protected static $post_ids = array();
+
+	/**
+	 * Category ID created for shared fixtures.
+	 *
+	 * @var int
+	 */
+	protected static $category_id = 0;
+
+	/**
+	 * Set up shared fixtures.
+	 *
+	 * @param WP_UnitTest_Factory $factory Factory instance.
+	 */
+	public static function wpSetUpBeforeClass( $factory ) {
+		self::$category_id = $factory->term->create(
+			array(
+				'taxonomy' => 'category',
+				'name'     => 'Categorized',
+			)
+		);
+
+		self::$post_ids = $factory->post->create_many( 10 );
+		foreach ( self::$post_ids as $post_id ) {
+			wp_set_post_categories( $post_id, array( self::$category_id ) );
+		}
+	}
+
 	public function set_up() {
 		parent::set_up();
 
@@ -383,4 +416,127 @@ public function test_custom_base_query_arg_should_be_stripped_from_current_url_b
 		$page_2_url = home_url() . '?foo=2';
 		$this->assertContains( "<a class=\"page-numbers\" href=\"$page_2_url\">2</a>", $links );
 	}
+
+	/**
+	 * Ensures pagination links include trailing slashes when the permalink structure includes them.
+	 *
+	 * @ticket 61393
+	 */
+	public function test_permalinks_with_trailing_slash_produce_links_with_trailing_slashes() {
+		update_option( 'posts_per_page', 2 );
+		$this->set_permalink_structure( '/%postname%/' );
+
+		$this->go_to( '/category/categorized/page/2/' );
+
+		// `current` needs to be passed as it's not picked up from the query vars set by `go_to()` above.
+		$links = paginate_links( array( 'current' => 2 ) );
+
+		$processor   = new WP_HTML_Tag_Processor( $links );
+		$found_links = 0;
+		while ( $processor->next_tag( 'A' ) ) {
+			++$found_links;
+			$href = $processor->get_attribute( 'href' );
+			$this->assertStringEndsWith( '/', $href, "Pagination links should end with a trailing slash, found: $href" );
+		}
+		$this->assertGreaterThan( 0, $found_links, 'There should be pagination links found.' );
+	}
+
+	/**
+	 * Ensures pagination links do not include trailing slashes when the permalink structure doesn't include them.
+	 *
+	 * @ticket 61393
+	 */
+	public function test_permalinks_without_trailing_slash_produce_links_without_trailing_slashes() {
+		update_option( 'posts_per_page', 2 );
+		$this->set_permalink_structure( '/%postname%' );
+
+		$this->go_to( '/category/categorized/page/2' );
+
+		// `current` needs to be passed as it's not picked up from the query vars set by `go_to()` above.
+		$links = paginate_links( array( 'current' => 2 ) );
+
+		$processor   = new WP_HTML_Tag_Processor( $links );
+		$found_links = 0;
+		while ( $processor->next_tag( 'A' ) ) {
+			++$found_links;
+			$href = $processor->get_attribute( 'href' );
+			$this->assertStringEndsNotWith( '/', $href, "Pagination links should end with a trailing slash, found: $href" );
+		}
+		$this->assertGreaterThan( 0, $found_links, 'There should be pagination links found.' );
+	}
+
+	/**
+	 * Ensures the pagination links do not modify query strings (permalinks with trailing slash).
+	 *
+	 * @ticket 61393
+	 * @ticket 63123
+	 *
+	 * @dataProvider data_query_strings
+	 *
+	 * @param string $query_string Query string.
+	 * @param string $unexpected   Unexpected query string.
+	 */
+	public function test_permalinks_with_trailing_slash_do_not_modify_query_strings( string $query_string ) {
+		update_option( 'posts_per_page', 2 );
+		$this->set_permalink_structure( '/%postname%/' );
+
+		$this->go_to( "/page/2/?{$query_string}" );
+
+		// `current` needs to be passed as it's not picked up from the query vars set by `go_to()` above.
+		$links = paginate_links( array( 'current' => 2 ) );
+
+		$processor   = new WP_HTML_Tag_Processor( $links );
+		$found_links = 0;
+		while ( $processor->next_tag( 'A' ) ) {
+			++$found_links;
+			$href = $processor->get_attribute( 'href' );
+			$this->assertStringEndsWith( "/?{$query_string}", $href, "Pagination links should not modify the query string, found: $href" );
+		}
+		$this->assertGreaterThan( 0, $found_links, 'There should be pagination links found.' );
+	}
+
+	/**
+	 * Ensures the pagination links do not modify query strings (permalinks without trailing slash).
+	 *
+	 * @ticket 61393
+	 * @ticket 63123
+	 *
+	 * @dataProvider data_query_strings
+	 *
+	 * @param string $query_string Query string.
+	 * @param string $unexpected   Unexpected query string.
+	 */
+	public function test_permalinks_without_trailing_slash_do_not_modify_query_strings( string $query_string ) {
+		update_option( 'posts_per_page', 2 );
+		$this->set_permalink_structure( '/%postname%' );
+
+		$this->go_to( "/page/2?{$query_string}" );
+
+		// `current` needs to be passed as it's not picked up from the query vars set by `go_to()` above.
+		$links = paginate_links( array( 'current' => 2 ) );
+
+		$processor   = new WP_HTML_Tag_Processor( $links );
+		$found_links = 0;
+		while ( $processor->next_tag( 'A' ) ) {
+			++$found_links;
+			$href = $processor->get_attribute( 'href' );
+			$this->assertStringEndsWith( "?{$query_string}", $href, "Pagination links should not modify the query string, found: $href" );
+			$this->assertStringEndsNotWith( "/?{$query_string}", $href, "Pagination links should not be slashed before the query string, found: $href" );
+		}
+		$this->assertGreaterThan( 0, $found_links, 'There should be pagination links found.' );
+	}
+
+	/**
+	 * Data provider for
+	 *  - test_permalinks_without_trailing_slash_do_not_modify_query_strings
+	 *  - test_permalinks_with_trailing_slash_do_not_modify_query_strings
+	 *
+	 * @return array<string[]> Data provider.
+	 */
+	public function data_query_strings(): array {
+		return array(
+			array( 'foo=bar' ),
+			array( 'foo=bar&pen=pencil' ),
+		);
+	}
 }