Fix a crash in res_ais.

This patch resolves a crash observed in a load testing environment that
involved the use of the res_ais module.  I observed some crashes where
the event delivery callback would get called, but the length parameter
incidcating how much data there was to read was 0.  The code assumed
(with good reason I would think) that if this callback got called, there
was an event available to read.  However, if the rare case that there's
nothing there, catch it and return instead of blowing up.

More specifically, the change always ensure that the size of the received
event in the cluster is always big enough to be a real ast_event.

Review: https://reviewboard.asterisk.org/r/1423/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@335497 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/event.h

@@ -743,6 +743,13 @@ void *ast_event_iterator_get_ie_raw(struct ast_event_iterator *iterator);
  */
 uint16_t ast_event_iterator_get_ie_raw_payload_len(struct ast_event_iterator *iterator);
 
+/*!
+ * \brief Get the minimum length of an ast_event.
+ *
+ * \return minimum amount of memory that will be consumed by any ast_event.
+ */
+size_t ast_event_minimum_length(void);
+
 #if defined(__cplusplus) || defined(c_plusplus)
 }
 #endif

main/event.c

@@ -1769,3 +1769,8 @@ int ast_event_init(void)
 
 	return 0;
 }
+
+size_t ast_event_minimum_length(void)
+{
+	return sizeof(struct ast_event);
+}

res/ais/evt.c

@@ -135,6 +135,13 @@ void evt_event_deliver_cb(SaEvtSubscriptionIdT sub_id,
 		return;
 	}
 
+	if (event_datalen < ast_event_minimum_length()) {
+		ast_debug(1, "Ignoring event that's too small. %u < %u\n",
+			(unsigned int) event_datalen,
+			(unsigned int) ast_event_minimum_length());
+		return;
+	}
+
 	ais_res = saEvtEventDataGet(event_handle, event, &len);
 	if (ais_res != SA_AIS_OK) {
 		ast_log(LOG_ERROR, "Error retrieving event payload: %s\n",