MDL-57677 mod_forum: Fix wrong user displayed as the last post's author

In the recent issue MDL-56225, we started to record the current user as
the usermodified in the forum_discussions table when updating a forum
post. It made sense but it was a mistake.

Even if the current user really modifies the discussion by updating the
post, the field usermodified has actually been always interpreted and
displayed as the last post' author. Not as the last user who touched the
discussion.

This patch reverts that particular change to the previous behaviour and
adds explicit unit test for it.

mod/forum/lib.php

@@ -4516,7 +4516,7 @@ function forum_update_post($newpost, $mform, &$message) {
 
     // Last post modified tracking.
     $discussion->timemodified = $post->modified;
-    $discussion->usermodified = $USER->id;
+    $discussion->usermodified = $post->userid;
 
     if (!$post->parent) {   // Post is a discussion starter - update discussion title and times too
         $discussion->name      = $post->subject;

mod/forum/tests/lib_test.php

@@ -940,4 +940,56 @@ public function test_mod_forum_rating_can_see_item_ratings() {
 
     }
 
+    /**
+     * Test that {@link forum_update_post()} keeps correct forum_discussions usermodified.
+     */
+    public function test_forum_update_post_keeps_discussions_usermodified() {
+        global $DB;
+
+        $this->resetAfterTest();
+
+        // Let there be light.
+        $teacher = self::getDataGenerator()->create_user();
+        $student = self::getDataGenerator()->create_user();
+        $course = self::getDataGenerator()->create_course();
+
+        $forum = self::getDataGenerator()->create_module('forum', (object)[
+            'course' => $course->id,
+        ]);
+
+        $generator = self::getDataGenerator()->get_plugin_generator('mod_forum');
+
+        // Let the teacher start a discussion.
+        $discussion = $generator->create_discussion((object)[
+            'course' => $course->id,
+            'userid' => $teacher->id,
+            'forum' => $forum->id,
+        ]);
+
+        // On this freshly created discussion, the teacher is the author of the last post.
+        $this->assertEquals($teacher->id, $DB->get_field('forum_discussions', 'usermodified', ['id' => $discussion->id]));
+
+        // Let the student reply to the teacher's post.
+        $reply = $generator->create_post((object)[
+            'course' => $course->id,
+            'userid' => $student->id,
+            'forum' => $forum->id,
+            'discussion' => $discussion->id,
+            'parent' => $discussion->firstpost,
+        ]);
+        // The student should now be the last post's author.
+        $this->assertEquals($student->id, $DB->get_field('forum_discussions', 'usermodified', ['id' => $discussion->id]));
+
+        // Let the teacher edit the student's reply.
+        $this->setUser($teacher->id);
+        $newpost = (object)[
+            'id' => $reply->id,
+            'itemid' => 0,
+            'subject' => 'Amended subject',
+        ];
+        forum_update_post($newpost, null, $message);
+
+        // The student should be still the last post's author.
+        $this->assertEquals($student->id, $DB->get_field('forum_discussions', 'usermodified', ['id' => $discussion->id]));
+    }
 }