rtpsession: Always keep at least one NACK on early RTCP

We recently added code to remove outdate NACK to avoid using bandwidth
for packet that have no chance of arriving on time. Though, this had a
side effect, which is that it was to get an early RTCP packet with no
feedback into it. This was pretty useless but also had a side effect,
which is that the RTX RTT value would never be updated. So we we stared
having late RTX request due to high RTT, we'd never manage to recover.

This fixes the regression by making sure we keep at least one NACK in
this situation. This is really light on the bandwidth and allow for
quick recover after the RTT have spiked higher then the jitterbuffer
capacity.

gst/rtpmanager/rtpsession.c

@@ -3666,6 +3666,14 @@ session_nack (const gchar * key, RTPSource * source, ReportData * data)
     if (nack_deadlines[i] >= data->current_time)
       break;
   }
+
+  if (data->is_early) {
+    /* don't remove them all if this is an early RTCP packet. It may happen
+     * that the NACKs are late due to high RTT, not sending NACKs at all would
+     * keep the RTX RTT stats high and maintain a dropping state. */
+    i = MIN (n_nacks - 1, i);
+  }
+
   if (i) {
     GST_WARNING ("Removing %u expired NACKS", i);
     rtp_source_clear_nacks (source, i);

tests/check/elements/rtpsession.c

@@ -1887,6 +1887,76 @@ GST_START_TEST (test_disable_probation)
 
 GST_END_TEST;
 
+GST_START_TEST (test_request_late_nack)
+{
+  SessionHarness *h = session_harness_new ();
+  GstBuffer *buf;
+  GstRTCPBuffer rtcp = GST_RTCP_BUFFER_INIT;
+  GstRTCPPacket rtcp_packet;
+  guint8 *fci_data;
+  guint32 fci_length;
+
+  g_object_set (h->internal_session, "internal-ssrc", 0xDEADBEEF, NULL);
+
+  /* Receive a RTP buffer from the wire */
+  fail_unless_equals_int (GST_FLOW_OK,
+      session_harness_recv_rtp (h, generate_test_buffer (0, 0x12345678)));
+
+  /* Wait for first regular RTCP to be sent so that we are clear to send early RTCP */
+  session_harness_produce_rtcp (h, 1);
+  gst_buffer_unref (session_harness_pull_rtcp (h));
+
+  /* request NACK immediately, but also advance the clock, so the request is
+   * now late, but it should be kept to avoid sendign an early rtcp without
+   * NACK. This would otherwise lead to a stall if the late packet was cause
+   * by high RTT, we need to send some RTX in order to update that statistic. */
+  session_harness_rtp_retransmission_request (h, 0x12345678, 1234, 0, 0, 0);
+  gst_test_clock_advance_time (h->testclock, 100 * GST_USECOND);
+
+  /* NACK should be produced immediately as early RTCP is allowed. Pull buffer
+     without advancing the clock to ensure this is the case */
+  buf = session_harness_pull_rtcp (h);
+
+  fail_unless (gst_rtcp_buffer_validate (buf));
+  gst_rtcp_buffer_map (buf, GST_MAP_READ, &rtcp);
+  fail_unless_equals_int (3, gst_rtcp_buffer_get_packet_count (&rtcp));
+  fail_unless (gst_rtcp_buffer_get_first_packet (&rtcp, &rtcp_packet));
+
+  /* first a Receiver Report */
+  fail_unless_equals_int (GST_RTCP_TYPE_RR,
+      gst_rtcp_packet_get_type (&rtcp_packet));
+  fail_unless (gst_rtcp_packet_move_to_next (&rtcp_packet));
+
+  /* then a SDES */
+  fail_unless_equals_int (GST_RTCP_TYPE_SDES,
+      gst_rtcp_packet_get_type (&rtcp_packet));
+  fail_unless (gst_rtcp_packet_move_to_next (&rtcp_packet));
+
+  /* and then our NACK */
+  fail_unless_equals_int (GST_RTCP_TYPE_RTPFB,
+      gst_rtcp_packet_get_type (&rtcp_packet));
+  fail_unless_equals_int (GST_RTCP_RTPFB_TYPE_NACK,
+      gst_rtcp_packet_fb_get_type (&rtcp_packet));
+
+  fail_unless_equals_int (0xDEADBEEF,
+      gst_rtcp_packet_fb_get_sender_ssrc (&rtcp_packet));
+  fail_unless_equals_int (0x12345678,
+      gst_rtcp_packet_fb_get_media_ssrc (&rtcp_packet));
+
+  fci_data = gst_rtcp_packet_fb_get_fci (&rtcp_packet);
+  fci_length =
+      gst_rtcp_packet_fb_get_fci_length (&rtcp_packet) * sizeof (guint32);
+  fail_unless_equals_int (4, fci_length);
+  fail_unless_equals_int (GST_READ_UINT32_BE (fci_data), 1234L << 16);
+
+  gst_rtcp_buffer_unmap (&rtcp);
+  gst_buffer_unref (buf);
+
+  session_harness_free (h);
+}
+
+GST_END_TEST;
+
 static Suite *
 rtpsession_suite (void)
 {
@@ -1917,6 +1987,7 @@ rtpsession_suite (void)
   tcase_add_test (tc_chain, test_disable_sr_timestamp);
   tcase_add_test (tc_chain, test_on_sending_nacks);
   tcase_add_test (tc_chain, test_disable_probation);
+  tcase_add_test (tc_chain, test_request_late_nack);
   return s;
 }