mxqd: Implement round-robin for equal users

Move a user, for whom a job was started, to the end of the user list.
This way the server will do round-robin between users who have otherwise
equal precedence based on running_global_slot_count.

mxqd.c

@@ -1491,6 +1491,23 @@ static int could_potentially_start_job_for_user(struct mxq_user_list *user) {
     return 0;
 }
 
+static void move_user_to_end(struct mxq_server *server, struct mxq_user_list *user) {
+    struct mxq_user_list **ptr;
+
+    if (!user->next)
+        return;
+
+    ptr = &server->users;
+    while (*ptr != user)
+        ptr = &(*ptr)->next;
+    *ptr = user->next;
+    ptr = &(user->next->next);
+    while (*ptr)
+        ptr = &(*ptr)->next;
+    *ptr = user;
+    user->next = NULL;
+}
+
 long start_user_with_least_running_global_slot_count(struct mxq_server *server)
 {
     struct mxq_user_list *ulist;
@@ -1524,8 +1541,12 @@ long start_user_with_least_running_global_slot_count(struct mxq_server *server)
             return -1;
 
         slots_started = start_user(ulist, slots_free, df_scratch);
-        if (slots_started)
+        if (slots_started) {
+            /* move user to end of list so that we get a round-robin with with
+             * other users which sort to the same precedence. */
+            move_user_to_end(server, ulist);
             return slots_started;
+        }
 
         if (waiting)
             continue;