add a filterable list of feed namespaces

src/wp-includes/feed-atom-comments.php

@@ -14,14 +14,18 @@
 <feed
 	xmlns="http://www.w3.org/2005/Atom"
 	xml:lang="<?php bloginfo_rss( 'language' ); ?>"
-	xmlns:thr="http://purl.org/syndication/thread/1.0"
 	<?php
+		feed_namespaces( 'atom-comments' );
+
 		/** This action is documented in wp-includes/feed-atom.php */
 		do_action( 'atom_ns' );
 
 		/**
 		 * Fires inside the feed tag in the Atom comment feed.
 		 *
+		 * Consider using the `wp_feed_namespaces` filter instead, which
+		 * prevents duplicate `xmlns` attributes.
+		 *
 		 * @since 2.8.0
 		 */
 		do_action( 'atom_comments_ns' );

src/wp-includes/feed-atom.php

@@ -20,12 +20,16 @@
 ?>
 <feed
 	xmlns="http://www.w3.org/2005/Atom"
-	xmlns:thr="http://purl.org/syndication/thread/1.0"
 	xml:lang="<?php bloginfo_rss( 'language' ); ?>"
 	<?php
+	feed_namespaces( 'atom' );
+
 	/**
 	 * Fires at end of the Atom feed root to add namespaces.
 	 *
+	 * Consider using the `wp_feed_namespaces` filter instead, which prevents
+	 * duplicate `xmlns` attributes.
+	 *
 	 * @since 2.0.0
 	 */
 	do_action( 'atom_ns' );

src/wp-includes/feed-rdf.php

@@ -15,15 +15,15 @@
 ?>
 <rdf:RDF
 	xmlns="http://purl.org/rss/1.0/"
-	xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
-	xmlns:dc="http://purl.org/dc/elements/1.1/"
-	xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
-	xmlns:admin="http://webns.net/mvcb/"
-	xmlns:content="http://purl.org/rss/1.0/modules/content/"
 	<?php
+	feed_namespaces( 'rdf' );
+
 	/**
 	 * Fires at the end of the feed root to add namespaces.
 	 *
+	 * Consider using the `wp_feed_namespaces` filter instead, which prevents
+	 * duplicate `xmlns` attributes.
+	 *
 	 * @since 2.0.0
 	 */
 	do_action( 'rdf_ns' );

src/wp-includes/feed-rss2-comments.php

@@ -13,11 +13,9 @@
 do_action( 'rss_tag_pre', 'rss2-comments' );
 ?>
 <rss version="2.0"
-	xmlns:content="http://purl.org/rss/1.0/modules/content/"
-	xmlns:dc="http://purl.org/dc/elements/1.1/"
-	xmlns:atom="http://www.w3.org/2005/Atom"
-	xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
 	<?php
+	feed_namespaces( 'rss2-comments' );
+
 	/** This action is documented in wp-includes/feed-rss2.php */
 	do_action( 'rss2_ns' );
 	?>
@@ -26,6 +24,9 @@
 	/**
 	 * Fires at the end of the RSS root to add namespaces.
 	 *
+	 * Consider using the `wp_feed_namespaces` filter instead, which
+	 * prevents duplicate `xmlns` attributes.
+	 *
 	 * @since 2.8.0
 	 */
 	do_action( 'rss2_comments_ns' );

src/wp-includes/feed-rss2.php

@@ -21,16 +21,15 @@
 do_action( 'rss_tag_pre', 'rss2' );
 ?>
 <rss version="2.0"
-	xmlns:content="http://purl.org/rss/1.0/modules/content/"
-	xmlns:wfw="http://wellformedweb.org/CommentAPI/"
-	xmlns:dc="http://purl.org/dc/elements/1.1/"
-	xmlns:atom="http://www.w3.org/2005/Atom"
-	xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
-	xmlns:slash="http://purl.org/rss/1.0/modules/slash/"
 	<?php
+	feed_namespaces( 'rss2' );
+
 	/**
 	 * Fires at the end of the RSS root to add namespaces.
 	 *
+	 * Consider using the `wp_feed_namespaces` filter instead, which prevents
+	 * duplicate `xmlns` attributes.
+	 *
 	 * @since 2.0.0
 	 */
 	do_action( 'rss2_ns' );

src/wp-includes/feed.php

@@ -699,6 +699,126 @@ function self_link() {
 	echo esc_url( apply_filters( 'self_link', get_self_link() ) );
 }
 
+/**
+ * Retrieves the XML namespaces for the root element of a feed.
+ *
+ * Namespaces are keyed by their prefix, so the same prefix cannot be
+ * declared twice. The default namespaces of a feed type cannot be removed,
+ * as the bundled feed templates use them in their static markup.
+ *
+ * @since 7.2.0
+ *
+ * @param string $type Type of feed. Possible values include 'rss2', 'rss2-comments',
+ *                     'rdf', 'atom', and 'atom-comments'.
+ * @return array<string, string> Array of namespace URIs, keyed by their prefix.
+ * @phpstan-param 'rss2'|'rss2-comments'|'rdf'|'atom'|'atom-comments' $type
+ * @phpstan-return array<non-falsy-string, non-falsy-string>
+ */
+function get_feed_namespaces( string $type ): array {
+	$defaults = array();
+
+	switch ( $type ) {
+		case 'rss2':
+			$defaults = array(
+				'content' => 'http://purl.org/rss/1.0/modules/content/',
+				'wfw'     => 'http://wellformedweb.org/CommentAPI/',
+				'dc'      => 'http://purl.org/dc/elements/1.1/',
+				'atom'    => 'http://www.w3.org/2005/Atom',
+				'sy'      => 'http://purl.org/rss/1.0/modules/syndication/',
+				'slash'   => 'http://purl.org/rss/1.0/modules/slash/',
+			);
+			break;
+
+		case 'rss2-comments':
+			$defaults = array(
+				'content' => 'http://purl.org/rss/1.0/modules/content/',
+				'dc'      => 'http://purl.org/dc/elements/1.1/',
+				'atom'    => 'http://www.w3.org/2005/Atom',
+				'sy'      => 'http://purl.org/rss/1.0/modules/syndication/',
+			);
+			break;
+
+		case 'rdf':
+			$defaults = array(
+				'rdf'     => 'http://www.w3.org/1999/02/22-rdf-syntax-ns#',
+				'dc'      => 'http://purl.org/dc/elements/1.1/',
+				'sy'      => 'http://purl.org/rss/1.0/modules/syndication/',
+				'admin'   => 'http://webns.net/mvcb/',
+				'content' => 'http://purl.org/rss/1.0/modules/content/',
+			);
+			break;
+
+		case 'atom':
+		case 'atom-comments':
+			$defaults = array(
+				'thr' => 'http://purl.org/syndication/thread/1.0',
+			);
+			break;
+	}
+
+	/**
+	 * Filters the XML namespaces of a feed's root element.
+	 *
+	 * Namespaces are keyed by their prefix, which makes duplicate `xmlns`
+	 * attributes impossible.
+	 *
+	 * @since 7.2.0
+	 *
+	 * @param array<string, string> $namespaces Array of namespace URIs, keyed by their prefix.
+	 * @param string                $type       Type of feed. Possible values include 'rss2',
+	 *                                          'rss2-comments', 'rdf', 'atom', and 'atom-comments'.
+	 */
+	$namespaces = apply_filters( 'wp_feed_namespaces', $defaults, $type );
+
+	if ( ! is_array( $namespaces ) ) {
+		$namespaces = array();
+	}
+
+	// The bundled feed templates use the default namespaces, so they cannot be removed.
+	$namespaces = array_merge( $namespaces, $defaults );
+
+	$sanitized = array();
+
+	foreach ( $namespaces as $prefix => $uri ) {
+		$prefix = (string) $prefix;
+
+		// Prefixes must be valid XML names, and the `xml` and `xmlns` prefixes are reserved.
+		if ( ! preg_match( '/^[\p{L}_][\p{L}\p{M}\p{N}._\-\x{B7}]*$/u', $prefix )
+			|| in_array( strtolower( $prefix ), array( 'xml', 'xmlns' ), true )
+		) {
+			continue;
+		}
+
+		if ( ! is_string( $uri ) || empty( $uri ) ) {
+			continue;
+		}
+
+		$sanitized[ $prefix ] = $uri;
+	}
+
+	return $sanitized;
+}
+
+/**
+ * Displays the XML namespaces for the root element of a feed.
+ *
+ * Plugins should add namespaces via the `wp_feed_namespaces` filter instead
+ * of the older `{$type}_ns` actions, as two action callbacks printing the
+ * same namespace produce a duplicate attribute, which is a well-formedness
+ * error in XML.
+ *
+ * @since 7.2.0
+ *
+ * @param string $type Type of feed. Possible values include 'rss2', 'rss2-comments',
+ *                     'rdf', 'atom', and 'atom-comments'.
+ * @phpstan-param 'rss2'|'rss2-comments'|'rdf'|'atom'|'atom-comments' $type
+ */
+function feed_namespaces( string $type ): void {
+	foreach ( get_feed_namespaces( $type ) as $prefix => $uri ) {
+		printf( "xmlns:%s=\"%s\"\n\t", $prefix, esc_url( $uri ) );
+	}
+}
+
 /**
  * Gets the UTC time of the most recently modified post from WP_Query.
  *