/**
 * @page tevent_queue_tutorial The tevent_queue tutorial
 *
 * @section Introduction
 *
 * A tevent_queue is used to queue up async requests that must be
 * serialized. For example writing buffers into a socket must be
 * serialized. Writing a large lump of data into a socket can require
 * multiple write(2) or send(2) system calls. If more than one async
 * request is outstanding to write large buffers into a socket, every
 * request must individually be completed before the next one begins,
 * even if multiple syscalls are required.
 *
 * To do this, every socket gets assigned a tevent_queue struct.
 *
 * Creating a serialized async request follows the usual convention to
 * return a tevent_req structure with an embedded state structure. To
 * serialize the work the requests is about to so, instead of directly
 * starting or doing that work, tevent_queue_add must be called. When it
 * is time for the serialized async request to do its work, the trigger
 * callback function tevent_queue_add was given is called. In the example
 * of writing to a socket, the trigger is called when the write request
 * can begin accessing the socket.
 *
 * How does this engine work behind the scenes? When the queue is empty,
 * tevent_queue_add schedules an immediate call to the trigger
 * callback. The trigger callback starts its work, likely by starting
 * other async subrequests. While these async subrequests are working,
 * more requests can accumulate in the queue by tevent_queue_add. While
 * there is no function to explicitly trigger the next waiter in line, it
 * still works: When the active request in the queue is done, it will be
 * destroyed by talloc_free. Talloc_free of an serialized async request
 * that had been added to a queue will trigger the next request in the
 * queue via a talloc destructor attached to a child of the serialized
 * request. This way the queue will be kept busy when an async request
 * finishes.
 *
 * @section Example
 *
 * @code
 *      Metze: Please add a code example here.
 * @endcode
 */