add a filterable list of feed namespaces

[pfefferle]

Trac ticket: https://core.trac.wordpress.org/ticket/65785

The rss2_ns and atom_ns actions are echo-only, so two plugins that add the same namespace produce a duplicate attribute, and the whole feed becomes invalid XML.

This adds get_feed_namespaces() and feed_namespaces() (named after feed_content_type()) and a wp_feed_namespaces filter. Namespaces are keyed by their prefix, so the same namespace can not be printed twice. The default namespaces of the templates moved into the filtered list, otherwise a plugin could still duplicate them. The *_ns actions stay untouched for backwards compatibility.

What it does not solve: a plugin that still echoes a namespace via the old actions can produce a duplicate, like before. That would need output buffering around the actions, and I did not want to go there.

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[westonruter]

Suggested change

	* @param string $type Type of feed. Possible values include 'rss2', 'rss2-comments',
	* 'rdf', 'atom', and 'atom-comments'.
	* @return string[] Array of namespace URIs, keyed by their prefix.
	*/
	function get_feed_namespaces( $type ) {
	* @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 {

[westonruter]

Not sure if the 'rss2'|'rss2-comments'|'rdf'|'atom'|'atom-comments' enum is warranted, if other feed types are intended to be supported.

[westonruter]

Otherwise, a non-falsy-string would be a good narrowed type.

[pfefferle]

🤔 good point, but I would assume that modern feed formats are no longer XML, so I am not sure if we need a "generic" fallback!?

[pfefferle]

But I have no strong opinion on that! I am fine with having a general namespace support that could be adopted by third party implementers too!

[westonruter]

Suggested change

	printf( "xmlns:%s=\"%s\"\n\t", $prefix, esc_attr( $uri ) );
	printf( "xmlns:%s=\"%s\"\n\t", $prefix, esc_url( $uri ) );

[pfefferle]

I applied your suggestion, but I am not sure it works for every value. XML namespaces are IRIs, so they can also be URNs for example (urn:schemas-microsoft-com:office:office), so we may have to stick with esc_attr or have more specific check(s)!?

[pfefferle]

Should WordPress have an esc_iri? Maybe based on src/wp-includes/SimplePie/src/IRI.php???