freetdm: add asynchronous signal notification

libs/freetdm/src/include/private/ftdm_core.h

@@ -466,6 +466,7 @@ struct ftdm_span {
 	ftdm_state_map_t *state_map;
 	ftdm_caller_data_t default_caller_data;
 	ftdm_queue_t *pendingchans;
+	ftdm_queue_t *pendingsignals;
 	struct ftdm_span *next;
 };
 
@@ -579,6 +580,8 @@ FT_DECLARE(ftdm_status_t) ftdm_span_next_event(ftdm_span_t *span, ftdm_event_t *
  */
 FT_DECLARE(ftdm_status_t) ftdm_channel_queue_dtmf(ftdm_channel_t *ftdmchan, const char *dtmf);
 
+/* dequeue pending signals and notify the user via the span signal callback */
+FT_DECLARE(ftdm_status_t) ftdm_span_trigger_signals(const ftdm_span_t *span);
 
 /*!
   \brief Assert condition

libs/freetdm/src/include/private/ftdm_types.h

@@ -176,6 +176,10 @@ typedef enum {
 	FTDM_SPAN_USE_CHAN_QUEUE = (1 << 6),
 	FTDM_SPAN_SUGGEST_CHAN_ID = (1 << 7),
 	FTDM_SPAN_USE_AV_RATE = (1 << 8),
+	/* If you use this flag, you MUST call ftdm_span_trigger_signals to deliver the user signals
+	 * after having called ftdm_send_span_signal(), which with this flag it will just enqueue the signal
+	 * for later delivery */
+	FTDM_SPAN_USE_SIGNALS_QUEUE = (1 << 9),
 } ftdm_span_flag_t;
 
 /*! \brief Channel supported features */