[wp-trac] [WordPress Trac] #36126: lastBuildDate in comment feed may not be empty

[WordPress Trac]

#36126: lastBuildDate in comment feed may not be empty
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 Reporter:  Cybr               |       Owner:  stevenkword
     Type:  defect (bug)       |      Status:  assigned
 Priority:  normal             |   Milestone:  Awaiting Review
Component:  Feeds              |     Version:  4.4.2
 Severity:  normal             |  Resolution:
 Keywords:  reporter-feedback  |     Focuses:
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Changes (by Cybr):

 * keywords:   => reporter-feedback


Comment:

 Hi @stevenkword,

 Thanks for looking into this.

 It's quite complex, and if you feel like it, please read on. Otherwise, go
 to the tl;dr section below.

 It shouldn't return empty, but it will:
 This is because the `$wp_query->is_comment_feed()` is true on the comment
 feed, so it will return early with a `max()` statement, which then will
 return `null` when no comments are available in the posts.

 When I take a look at the function `get_last_build_date_feed()`, I can see
 it can make `max()` return false on two occasions.
 https://core.trac.wordpress.org/browser/trunk/src/wp-
 includes/feed.php?rev=33224 (line 111 and 116).

 When listening to these `max()` rules:
 1. When comparing two different types, the first one found will be used
 (which can be null).
 2. Comparing an empty Array will always return false.

 And since `max()` is used twice, it has the possibility to return false
 twice:
 1. Having no posts will result in an empty array (and thus false).
 2. Having no comments or posts will result in null on the SQL query, which
 is then used as the type setter.

 `mysqltodate()` will return false if the 2nd parameter is empty. And when
 converting this to a PHP string on echo: `''`.

 '''tl;dr:'''
 Taking all these variables into account, I believe this bug is achieved by
 either removing all approved comments, or having no posts available.

 Removing all the approved comments is the easiest way to test this bug.

 Having the lastBuildDate output being put into a variable and checking if
 it's there before outputting that line will solve this issue.
 Please be aware that `if ( ! empty( $var ) )` is three times as slow as
 just `if ( $var )`, since we're dealing with scalar types (which we are).

 I hope this clears things up!

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Ticket URL: <https://core.trac.wordpress.org/ticket/36126#comment:4>
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