False positive "Replier can only add posts at the end of a thread" from non-monotomic time()

[Xon]

Affected version

2.3.6

php's time() function is non-monotomic. This means it can technically go backwards, noticeable on machines which use a timesync service on a very busy forum

Consider the case;

• UserA starts replying
  • finalSetup - Clock N, Post.post_date = N
• Clock progress to N+1
• Another users posts, updating the xf_threadrecord
  • xf_thread.last_post_date = N+1
• Timsync causes the clock to be set to Clock N
• UserA's request reloads the last post information for the xf_threadrecord
  • Post.post_date = N
  • xf_thread.last_post_date = N+1
• Replying fails with an internal server error.

The relevant code:

php
    $time = time();
    if ($threadLatest['last_post_date'] > $post->post_date && $threadLatest['last_post_date'] <= $time)
    {
        $post->set('post_date', $time, ['forceSet' => true]);
    }

    $this->setPostPosition($threadLatest);
`

The problematic code is && $threadLatest['last_post_date'] <= $time


This should handle wonky time-syncs and preserve the post ordering:

$time = max($threadLatest['last_post_date'], time());
if ($threadLatest['last_post_date'] > $post->post_date)
{
    $post->set('post_date', $time, ['forceSet' => true]);
}

Debugging this was tricky as setPostPosition, doesn't report the values and it assumed time() behaved itself