Restrict the Task Queue to one task per site each cron run

The task queue can be configured with the number of tasks to be started ('$limit') each time the queue is checked. For efficient operation of a siginficant Aegir installation, it is necessary to have this '$limit" value higher than 1.

The current implementation will select the oldest '$limit' outstanding tasks in the queue and start them running. Now, if there is more than one task in this set for a particular site, then there could be a conflict during task execution. It would be desirable to ensure that the list of '$limit' tasks did not have more than one for any particular site to avoid such potential conflicts.

The tasks to be started are selected by the following SQL statement in function _hosting_get_new_tasks (in file hosting.queues.inc)

$result = db_query("SELECT t.nid FROM {hosting_task} t INNER JOIN {node} n ON t.vid = n.vid WHERE t.task_status = %d ORDER BY n.changed, n.nid ASC LIMIT %d", 0, $limit);

If this statement is changed by the addition of a "GROUP BY" clause, then only one task for each site will appear in the resulting list.

$result = db_query("SELECT t.nid FROM {hosting_task} t INNER JOIN {node} n ON t.vid = n.vid WHERE t.task_status = %d GROUP BY t.rid ORDER BY n.changed, n.nid ASC LIMIT %d", 0, $limit);

I recommend that this change is included in the hosting module.