DRTVWR-474: Make LLEventMailDrop pass all saved events to listener.

Previously, LLEventMailDrop would send only the first queued event to a
newly-connected listener. If you wanted to flush all queued events, you'd have
to "pump" the queue by repeatedly disconnecting and reconnecting -- with no
good way to know when you'd caught up.

The new behavior makes LLEventMailDrop resemble a multi-valued future: a
rendezvous between producer and consumer that, once connected, pushes values
rather than requiring them to be pulled (as with a simple queue) -- regardless
of the relative order in which post() and listen() are called.

1 files changed, 13 insertions, 7 deletions

diff --git a/indra/llcommon/llevents.h b/indra/llcommon/llevents.h
index 1d51c660ed..5d60c63810 100644
--- a/indra/llcommon/llevents.h
+++ b/indra/llcommon/llevents.h
@@ -650,15 +650,21 @@ public:
  *   LLEventMailDrop
  *****************************************************************************/
 /**
- * LLEventMailDrop is a specialization of LLEventStream. Events are posted normally, 
- * however if no listeners return that they have handled the event it is placed in 
- * a queue. Subsequent attaching listeners will receive stored events from the queue 
- * until a listener indicates that the event has been handled.  In order to receive 
- * multiple events from a mail drop the listener must disconnect and reconnect.
+ * LLEventMailDrop is a specialization of LLEventStream. Events are posted
+ * normally, however if no listener returns that it has handled the event
+ * (returns true), it is placed in a queue. Subsequent attaching listeners
+ * will receive stored events from the queue until some listener indicates
+ * that the event has been handled.
+ *
+ * LLEventMailDrop completely decouples the timing of post() calls from
+ * listen() calls: every event posted to an LLEventMailDrop is eventually seen
+ * by all listeners, until some listener consumes it. The caveat is that each
+ * event *must* eventually reach a listener that will consume it, else the
+ * queue will grow to arbitrary length.
  * 
  * @NOTE: When using an LLEventMailDrop (or LLEventQueue) with a LLEventTimeout or
- * LLEventFilter attaching the filter downstream using Timeout's constructor will
- * cause the MailDrop to discharge any of it's stored events. The timeout should 
+ * LLEventFilter attaching the filter downstream, using Timeout's constructor will
+ * cause the MailDrop to discharge any of its stored events. The timeout should 
  * instead be connected upstream using its listen() method.  
  * See llcoro::suspendUntilEventOnWithTimeout() for an example.
  */