If an error occurs in one feed, the other feeds will be ignored. As I understand it, SimplePie will continue processing even if an error occurs and merge successful feeds:

https://github.com/t-hamano/wordpress-develop/blob/b47e1c7c5985fff02153cd89cbba912aa85b8a02/src/wp-includes/SimplePie/src/SimplePie.php#L1704-L1713

I haven't tested it, but how about an approach like this:

foreach ( (array) $url as $feed_url ) {
	$simplepie_instance = clone $feed;
	$simplepie_instance->set_feed_url( $feed_url );
	$simplepie_instance->init();
	$simplepie_instance->set_output_encoding( get_bloginfo( 'charset' ) );

	if ( ! $simplepie_instance->error() ) {
		$feeds[] = $simplepie_instance;
	} else {
		$errors[] = $simplepie_instance->error();
	}
}

if ( empty( $feeds ) ) {
	return new WP_Error( 'simplepie-error', $errors );
}

We may also need unit tests to ensure that if an error occurs, the rest of the feed still gets processed.

What is the 6.8 behavior for when a feed encounters an error? We should try to replicate that

The code doesn't preserve or apply the feed options (cache duration, sanitization settings, etc.) from the original $feed object to the cloned $simplepie_instance that processes individual feeds. This means each individual feed in the array might not respect the options set via wp_feed_options action or other configuration methods.

The cloned instance should inherit all the configuration from the parent feed object to ensure consistent behavior.

When one feed in the array fails to fetch or has an error, the function returns immediately with a WP_Error, abandoning the processing of remaining feeds. This means if you request 5 feeds and the 2nd one fails, you won't get any data from feeds 1, 3, 4, or 5.

Consider whether this is the desired behavior or if it would be better to continue processing remaining feeds and either aggregate errors or return partial results with a warning about failed feeds.

The same $simplepie_instance object is being reused across multiple iterations without proper reinitialization. This means that after the first feed is processed, the instance retains state from the previous feed which could lead to incorrect data or unexpected behavior. Each feed URL should either get a fresh SimplePie instance, or the instance should be properly reset between iterations.

Consider creating a new SimplePie instance for each feed URL in the loop instead of cloning once and reusing.

Directly assigning to $feed->data['items'] bypasses SimplePie's internal state management and could cause issues with other SimplePie methods that depend on proper initialization. The SimplePie object wasn't initialized with init() before this assignment, and calling init() on line 865 after data assignment may not work as expected since the object wasn't properly set up with a feed URL.

Instead of manually manipulating the data property, consider using SimplePie's merge_items() method to create a properly merged feed or ensure the feed object is properly initialized with the merged data.

The condition count( $url ) <= 1 will cause arrays with exactly one URL to be processed differently (extracting the single URL), but arrays with 0 URLs will fall through to the multi-feed handling logic. This could lead to attempting to process an empty array of feeds, which may not be the intended behavior.

Consider also checking for empty arrays and handling them appropriately, or adding validation earlier in the function to ensure the URL parameter is valid.

The wp_feed_options action is called with the original $feed object and the $url parameter before the URL array is processed. However, for multiple feeds, this hook is only called once with the array of URLs, not once per feed. This means plugins or themes that hook into this to configure feed options won't have a chance to configure each individual feed instance that's created in the loop.

Consider whether the hook should be called for each individual feed URL when processing multiple feeds, or clearly document this behavior change.

The test validates that the content of the first two items is identical by passing the same feed URL twice. However, this doesn't verify that both feed URLs were actually processed independently. If there's a bug in the implementation that only processes the first feed and ignores the rest, this test would still pass.

Consider using two different feed URLs (or mock responses) to verify that items from both feeds are actually included in the merged result, and that the feeds are processed independently.

Nit: At this point, this is already known to be an array.

The second call should be returned cached. I think it might make sense to use a different url (maybe just the atom url?) since this response is mocked anyways.

What is the 6.8 behavior for when a feed encounters an error? We should try to replicate that