Looks like the links produced here aren't working because at some point in the process (after format_rss_channel passes $args to format_xml_elements) the ampersands get encoded. The resulting URL is no good.

Any suggestions for overcoming the encoded ampersand problem?

You should probably use drupal_get_query_parameters to get the parameters instead of the manual parsing you are doing here. You can see how its used in query_processor.inc. I would recommend using the Drupal url function to make the links.

@jonathangreen I'm not sure that applies here... This is what I get from drupal_get_query_parameters:

Array
(
    [type] => dismax
    [islandora_solr_search_navigation] => 0
    [solr_profile] => rss
)

I've fixed the link problem -- of course I realized these should have been value instead of attributes, so we're good to go.

Crap, never mind - it's still escaping. I'm sort of at a loss here..

@jonathangreen OK, all is well. Apparently you can add encoded => TRUE to the list of array attributes. Ready to test if you don't mind.

@jonathangreen OK, I finally understood what you were talking about... got the 'page' parameter from drupal_get_query_parameters and used that to parse the page numbers.

@jonathangreen Still not sure I can do anything about the $feed_url though -- I haven't found a Drupal function that does the job.

@jonathangreen I think everything works now! Any further items for the review?

@jonathangreen Added two more needed pieces: no "next" link if there is no next page, and remove the "page" attribute for the "previous" link if the previous page is the first page.

All right! I had to learn a lot to make this work, but it's sorted out... All of your comments are addressed, @jonathangreen, I think.

Anything left, @jonathangreen?

Off the top of my head I see a few debugging statements left there. Sorry its been a really busy week for me, but I'm going to try to take another pass at this next week.

Off the top of my head I see a few debugging statements left there

Ack, you're right. I'm too impatient with my pushes! Those have been removed, thanks.

Pinging @jonathangreen Anything left or is this good to go?

So I think the code looks okay, but I don't think it's a valid RSS document anymore, which is probably a bad thing.

RFC-5005 has some details about adding pagination to RSS:
https://tools.ietf.org/html/rfc5005#page-12

I think it's probably reasonable to follow their suggestions about using the atom namespace for pagination. I think we should also have an option to return all results, perhaps setting the RSS page limit to 0. In that case we should follow their suggestion of not displaying these extra links.

Another issue I noticed while testing this is that the & character breaks XML parsing, so we probably need to make sure that is escaped, so things properly validate. I would like to see the output from this validate with the W3C validator (or at least not throw any additional warnings).

@DiegoPino do you have any comments on how to do the pagination for RSS? I think you had mentioned you did at the last committers call.

@jonathangreen i referenced the same document you just did https://tools.ietf.org/html/rfc5005#section-3, there is nothing that says rss/rss2 does not allow it but yes, all examples are for the Atom namespaces.
Wordpress allows pagination for RSS and some APIs enable it, independently of if Atom or pure RSS
https://mibe.github.io/FeedWriter/classes/FeedWriter.Feed.html#method_setPagination

I feel like the RSS examples from the RFC using the atom namespace within RSS make sense to me:

   <?xml version="1.0"?>
   <rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom"
    xmlns:fh="http://purl.org/syndication/history/1.0">
    <channel>
     <title>Liftoff News</title>
     <link>http://liftoff.example.net/</link>
     <description>Liftoff to Space Exploration.</description>
     <lastBuildDate>Fri, 30 May 2003 11:06:42 GMT</lastBuildDate>
     <fh:archive/>
     <atom:link rel="current"
      href="http://liftoff.example.net/index.rss"/>
     <atom:link rel="prev-archive"
      href="http://liftoff.example.net/2003/04/index.rss"/>

     <item>
      <title>Upcoming Eclipse</title>
      <link>http://liftoff.example.net/2003/05/30/eclipse</link>
      <description>Sky watchers in Europe, Asia, and parts of
      Alaska and Canada will experience a partial eclipse of the Sun
      on Saturday, May 31st.</description>
      <pubDate>Fri, 30 May 2003 11:06:42 GMT</pubDate>
      <guid>http://liftoff.example.net/2003/05/30/eclipse</guid>
     </item>
     <item>
      <title>The Engine That Does More</title>
      <link>http://liftoff.example.net/2003/05/27/vasmir</link>
      <description>Before man travels to Mars, NASA hopes to
      design new engines that will let us fly through the Solar
      System more quickly.  The proposed VASIMR engine would do
      that.</description>
      <pubDate>Tue, 27 May 2003 08:37:32 GMT</pubDate>
      <guid>http://liftoff.example.net/2003/05/27/vasmir</guid>
     </item>
    </channel>
   </rss>

@jonathangreen If I move the link from 'value' into 'attributes->href', I end up encoding all of the parameters. Adding 'encoded' => TRUE only seems to work for the text in 'value' and not in 'href'.

That is...

    $result['args'][] = array(
        'key' => 'link',
        'attributes' => array(
          'rel' => 'next',
          'href' => $link_next,
        'encoded' => TRUE,
      );
    }

gets me <link rel="next" href="http://localhost:8000/islandora/search/test?page=1&amp;type=dismax&amp;islandora_solr_search_navigation=0&amp;solr_profile=rss 1" />

Any idea if it's possible to get rid of the &amp conversion?

& needs to be encoded to & or its not valid XML anymore.
http://xml.silmaril.ie/specials.html

Yes, so that's the problem... this is supposed to be a link, and &amp in the link won't resolve properly.

I'm going to dismiss my review and step back from this one and let someone with RSS experience review it. I don't think we should be emitting invalid XML with & in it. I would have assumed RSS readers would do the sensible thing and unescape the URL before using it, but if they don't I don't know what the solution is here.

@jonathangreen Looking at the Example RSS from the WC3 XML 2.0 Specs a link can have a & character but not completely sure if there are limits within the <link> tag. Just a thought. I'm not really familiar with it but just giving my 3 seconds of research.