Forum post not visible to students when forum is separate groups and post is sent to all participants

Test setup On the 27 and 28 branches only - before applying the patch (or with it reverted): Create a course with 1 group in it Create three forums in the course - one in each group mode Post some discussions in each forum: No groups - just post Separate groups - one post to 'All participants', one post to your group Visible groups - one post to 'All participants', one post to your group (you should have a total of five posts) Reply to each discussion Inspect the forum_discussions table in the database. You should see a groupid of 0 is present. As a student in the group: Confirm that you cannot see the discussions written to 'All participants' Testing instructions Run the Moodle upgrade Verify that the forum_discussions table no longer has groupid of 0. These should now all be -1 Post and reply in all forums and groups again Verify that the forum_discussions table has no records with a groupid of 0 As a student in the group: Confirm that you can see every discussion Note: For 29/master changes, you need to take a 28 site from last week's stable, and perform the upgrade. You will need bad data before each upgrade - e.g. 28 -> 29; and then a separate 28 -> master