MT#55283 introduce last PT tracking

Extend PT tracker to not just track the most used PT, but also which PT
has been used last. Use a simple circular array.

Use this new tracking to determine whether a DTX buffer should remain
active. The "most used PT" approach isn't reliable for this, as it leads
to a DTX buffer restarting only after the PT has become the "most used,"
so only after about 10 packets. Meanwhile a newly appearing SSRC would
have a new DTX buffer active immediately. Being strict about which PT
was used last resolves this inconsistency.

Change-Id: If912e4d10e6737a82f6adc0b19d502075aa18a93

daemon/codec.c

@@ -3800,26 +3800,33 @@ static void __dtx_send_later(struct codec_timer *ct) {
 		else if (dtxb->ct.next.tv_sec == 0)
 			shutdown = true;
 		else {
-			shutdown = true; // default if no most used PTs are known
+			shutdown = true; // default if no last used PTs are known
+
+			for (int i = 0; i < G_N_ELEMENTS(ps->ssrc_in[0]->tracker.last_pts); i++) {
+				int pt_idx = ps->ssrc_in[0]->tracker.last_pt_idx - i;
+				pt_idx += G_N_ELEMENTS(ps->ssrc_in[0]->tracker.last_pts);
+				pt_idx %= G_N_ELEMENTS(ps->ssrc_in[0]->tracker.last_pts);
+				int last_pt = ps->ssrc_in[0]->tracker.last_pts[pt_idx];
+				if (last_pt == 255)
+					break;
 
-			for (int i = 0; i < ps->ssrc_in[0]->tracker.most_len; i++) {
-				unsigned char most_pt = ps->ssrc_in[0]->tracker.most[i];
 				shutdown = false;
-				// we are good if the most used PT is
+				// we are good if the last used PT is
 				// either us
-				if (ch->handler->source_pt.payload_type == most_pt)
+				if (ch->handler->source_pt.payload_type == last_pt)
 					break;
 				// or our input PT (which is the audio PT if we are supplemental)
-				if (ch->handler->input_handler && ch->handler->input_handler->source_pt.payload_type == most_pt)
+				if (ch->handler->input_handler
+						&& ch->handler->input_handler->source_pt.payload_type == last_pt)
 					break;
 
 				// looks like codec change, but...
 				shutdown = true;
 
-				// another possibility is that the most used PT is actually a supplemental type. check this,
-				// and if true move on to the next most used PT.
+				// another possibility is that the most used PT is actually a supplemental type.
+				// check this, and if true move on to the next most used PT.
 				rtp_payload_type *pt = t_hash_table_lookup(ps->media->codecs.codecs,
-						GUINT_TO_POINTER(most_pt));
+						GUINT_TO_POINTER(last_pt));
 				if (pt && pt->codec_def && pt->codec_def->supplemental)
 					continue;
 

daemon/ssrc.c

@@ -672,8 +672,10 @@ void payload_tracker_init(struct payload_tracker *t) {
 	memset(&t->count, 0, sizeof(t->count));
 	memset(&t->idx, -1, sizeof(t->idx));
 	memset(&t->most, -1, sizeof(t->most));
+	memset(&t->last_pts, 255, sizeof(t->last_pts));
 	t->last_idx = 0;
 	t->most_len = 0;
+	t->last_pt_idx = -1;
 }
 //#define PT_DBG(x...) ilog(LOG_DEBUG, x)
 #define PT_DBG(x...) ((void)0)
@@ -683,6 +685,10 @@ void payload_tracker_add(struct payload_tracker *t, int pt) {
 
 	mutex_lock(&t->lock);
 
+	int pt_idx = (t->last_pt_idx + 1) % G_N_ELEMENTS(t->last_pts);
+	t->last_pts[pt_idx] = pt;
+	t->last_pt_idx = pt_idx;
+
 	PT_DBG("new pt: %i", pt);
 	PT_DBG("last idx: %u", t->last_idx);
 	int old_pt = t->last[t->last_idx];

include/ssrc.h

@@ -35,6 +35,9 @@ struct payload_tracker {
 	unsigned char idx[128]; // each pt's index into most[]
 	unsigned char most[128]; // sorted list of pts
 	unsigned int most_len; // idx for new entries
+
+	unsigned char last_pts[16];
+	int last_pt_idx;
 };
 struct ssrc_ctx {
 	struct ssrc_entry_call *parent;