MDL-65574 forum: discussion_list vault uses post table for author

Update the discussion_list vault to use the first post record from
the posts table join to get the author of the first post rather
than relying on it's own "userid" column which can be incorrect in
the case of split discussions.

mod/forum/classes/local/vaults/discussion_list.php

@@ -137,9 +137,9 @@ protected function generate_get_records_sql(string $wheresql = null, ?string $so
         $issortbyreplies = in_array($sortsql, $sortkeys);
 
         $tables = $thistable->get_from_sql();
-        $tables .= ' JOIN {user} fa ON fa.id = ' . $alias . '.userid';
-        $tables .= ' JOIN {user} la ON la.id = ' . $alias . '.usermodified';
         $tables .= ' JOIN ' . $posttable->get_from_sql() . ' ON fp.id = ' . $alias . '.firstpost';
+        $tables .= ' JOIN {user} fa ON fa.id = fp.userid';
+        $tables .= ' JOIN {user} la ON la.id = ' . $alias . '.usermodified';
         $tables .= $favsql;
 
         if ($issortbyreplies) {

mod/forum/tests/behat/split_forum_discussion.feature

@@ -9,13 +9,15 @@ Feature: Forum discussions can be split
       | username | firstname | lastname | email |
       | teacher1 | Teacher | 1 | teacher1@example.com |
       | student1 | Student | 1 | student1@example.com |
+      | student2 | Student | 2 | student2@example.com |
     And the following "courses" exist:
       | fullname | shortname | category |
       | Science 101 | C1 | 0 |
     And the following "course enrolments" exist:
       | user | course | role |
       | teacher1 | C1 | editingteacher |
       | student1 | C1 | student |
+      | student2 | C1 | student |
     And I log in as "teacher1"
     And I am on "Science 101" course homepage with editing mode on
     And I add a "Forum" to section "1" and I fill the form with:
@@ -31,6 +33,16 @@ Feature: Forum discussions can be split
     And I reply "Photosynethis discussion" post from "Study discussions" forum with:
       | Message | Can anyone tell me which number is the mass number in the periodic table? |
     And I log out
+    And I log in as "student2"
+    And I am on "Science 101" course homepage
+    And I follow "Study discussions"
+    And I follow "Photosynethis discussion"
+    And I click on "Reply" "link" in the "//div[contains(concat(' ', normalize-space(@class), ' '), ' forumpost ')][contains(., 'Can anyone tell me which number is the mass number in the periodic table?')]" "xpath_element"
+    And I wait to be redirected
+    And I set the following fields to these values:
+      | Message | I would also like to know this |
+    And I press "Post to forum"
+    And I log out
 
   Scenario: Split a forum discussion
     Given I log in as "teacher1"
@@ -44,4 +56,9 @@ Feature: Forum discussions can be split
     Then I should see "Mass number in periodic table"
     And I follow "Study discussions"
     And I should see "Teacher 1" in the "Photosynethis" "table_row"
+    # Confirm that the last post author has been updated.
+    And I should not see "Student 2" in the "Photosynethis" "table_row"
+    # Confirm that the corrent author has been shown for the new split discussion.
     And I should see "Student 1" in the "Mass number in periodic table" "table_row"
+    # Confirm that the last post author has been updated for the new discussion.
+    And I should see "Student 2" in the "Mass number in periodic table" "table_row"