srtsrc: Fix timestamping

SRT provides the original timestamp of a packet (with drift/skew corrected for
local clock), which is what should be used for timestamping the outgoing
buffers. This ensures that we output the packets with the same timestamp (and by
extension rate) as the original feed.

Also detect if packets were dropped (by checking the sequence number) and
properly set DISCONT flag on the outgoing buffer.

Finally answer the latency queries

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1658>

ext/srt/gstsrtobject.c

@@ -1247,7 +1247,8 @@ gst_srt_object_wait_caller (GstSRTObject * srtobject,
 
 gssize
 gst_srt_object_read (GstSRTObject * srtobject,
-    guint8 * data, gsize size, GCancellable * cancellable, GError ** error)
+    guint8 * data, gsize size, GCancellable * cancellable, GError ** error,
+    SRT_MSGCTRL * mctrl)
 {
   gssize len = 0;
   gint poll_timeout;
@@ -1335,7 +1336,8 @@ gst_srt_object_read (GstSRTObject * srtobject,
     }
 
 
-    len = srt_recvmsg (rsock, (char *) (data), size);
+    srt_msgctrl_init (mctrl);
+    len = srt_recvmsg2 (rsock, (char *) (data), size, mctrl);
 
     if (len == SRT_ERROR) {
       gint srt_errno = srt_getlasterror (NULL);

ext/srt/gstsrtobject.h

@@ -98,7 +98,8 @@ gboolean        gst_srt_object_set_uri (GstSRTObject * srtobject, const gchar *u
 gssize          gst_srt_object_read     (GstSRTObject * srtobject,
                                          guint8 *data, gsize size,
                                          GCancellable *cancellable,
-                                         GError **err);
+                                         GError **err,
+					 SRT_MSGCTRL *mctrl);
 
 gssize          gst_srt_object_write    (GstSRTObject * srtobject,
                                          GstBufferList * headers,

ext/srt/gstsrtsrc.c

@@ -97,6 +97,9 @@ gst_srt_src_start (GstBaseSrc * bsrc)
     g_clear_error (&error);
   }
 
+  /* Reset expected pktseq */
+  self->next_pktseq = 0;
+
   return ret;
 }
 
@@ -118,6 +121,11 @@ gst_srt_src_fill (GstPushSrc * src, GstBuffer * outbuf)
   GstMapInfo info;
   GError *err = NULL;
   gssize recv_len;
+  GstClock *clock;
+  GstClockTime base_time;
+  GstClockTime capture_time;
+  GstClockTime delay;
+  SRT_MSGCTRL mctrl;
 
   if (g_cancellable_is_cancelled (self->cancellable)) {
     ret = GST_FLOW_FLUSHING;
@@ -130,11 +138,30 @@ gst_srt_src_fill (GstPushSrc * src, GstBuffer * outbuf)
     goto out;
   }
 
+  /* Get clock and values */
+  clock = gst_element_get_clock (GST_ELEMENT (src));
+  base_time = gst_element_get_base_time (GST_ELEMENT (src));
+
   recv_len = gst_srt_object_read (self->srtobject, info.data,
-      gst_buffer_get_size (outbuf), self->cancellable, &err);
+      gst_buffer_get_size (outbuf), self->cancellable, &err, &mctrl);
+
+  /* Capture clock values ASAP */
+  capture_time = gst_clock_get_time (clock);
+#if SRT_VERSION_VALUE >= 0x10402
+  /* Use SRT clock value if available (SRT > 1.4.2) */
+  delay = (srt_time_now () - mctrl.srctime) * GST_USECOND;
+#else
+  /* Else use the unix epoch monotonic clock */
+  delay = (g_get_real_time () - mctrl.srctime) * GST_USECOND;
+#endif
+  gst_object_unref (clock);
 
   gst_buffer_unmap (outbuf, &info);
 
+  GST_LOG_OBJECT (src,
+      "recv_len:%" G_GSIZE_FORMAT " pktseq:%d msgno:%d srctime:%"
+      G_GUINT64_FORMAT, recv_len, mctrl.pktseq, mctrl.msgno, mctrl.srctime);
+
   if (g_cancellable_is_cancelled (self->cancellable)) {
     ret = GST_FLOW_FLUSHING;
     goto out;
@@ -150,6 +177,29 @@ gst_srt_src_fill (GstPushSrc * src, GstBuffer * outbuf)
     goto out;
   }
 
+  /* Detect discontinuities */
+  if (mctrl.pktseq != self->next_pktseq) {
+    GST_WARNING_OBJECT (src, "discont detected %d (expected: %d)",
+        mctrl.pktseq, self->next_pktseq);
+    GST_BUFFER_FLAG_SET (outbuf, GST_BUFFER_FLAG_DISCONT);
+  }
+  /* pktseq is a 31bit field */
+  self->next_pktseq = (mctrl.pktseq + 1) % G_MAXINT32;
+
+  /* Subtract the base_time (since the pipeline started) ... */
+  if (capture_time > base_time)
+    capture_time -= base_time;
+  else
+    capture_time = 0;
+  /* And adjust by the delay */
+  if (capture_time > delay)
+    capture_time -= delay;
+  else
+    capture_time = 0;
+  GST_BUFFER_TIMESTAMP (outbuf) = capture_time;
+
+  GST_DEBUG_OBJECT (src, "delay:%" GST_TIME_FORMAT, GST_TIME_ARGS (delay));
+
   gst_buffer_resize (outbuf, 0, recv_len);
 
   GST_LOG_OBJECT (src,
@@ -173,7 +223,8 @@ gst_srt_src_init (GstSRTSrc * self)
 
   gst_base_src_set_format (GST_BASE_SRC (self), GST_FORMAT_TIME);
   gst_base_src_set_live (GST_BASE_SRC (self), TRUE);
-  gst_base_src_set_do_timestamp (GST_BASE_SRC (self), TRUE);
+  /* We do the timing ourselves */
+  gst_base_src_set_do_timestamp (GST_BASE_SRC (self), FALSE);
 
   gst_srt_object_set_uri (self->srtobject, GST_SRT_DEFAULT_URI, NULL);
 
@@ -234,6 +285,24 @@ gst_srt_src_get_property (GObject * object,
   }
 }
 
+static gboolean
+gst_srt_src_query (GstBaseSrc * basesrc, GstQuery * query)
+{
+  GstSRTSrc *self = GST_SRT_SRC (basesrc);
+
+  if (GST_QUERY_TYPE (query) == GST_QUERY_LATENCY) {
+    gint latency;
+    if (!gst_structure_get_int (self->srtobject->parameters, "latency",
+            &latency))
+      latency = GST_SRT_DEFAULT_LATENCY;
+    gst_query_set_latency (query, TRUE, latency * GST_MSECOND,
+        latency * GST_MSECOND);
+    return TRUE;
+  } else {
+    return GST_BASE_SRC_CLASS (parent_class)->query (basesrc, query);
+  }
+}
+
 static void
 gst_srt_src_class_init (GstSRTSrcClass * klass)
 {
@@ -285,6 +354,7 @@ gst_srt_src_class_init (GstSRTSrcClass * klass)
   gstbasesrc_class->stop = GST_DEBUG_FUNCPTR (gst_srt_src_stop);
   gstbasesrc_class->unlock = GST_DEBUG_FUNCPTR (gst_srt_src_unlock);
   gstbasesrc_class->unlock_stop = GST_DEBUG_FUNCPTR (gst_srt_src_unlock_stop);
+  gstbasesrc_class->query = GST_DEBUG_FUNCPTR (gst_srt_src_query);
 
   gstpushsrc_class->fill = GST_DEBUG_FUNCPTR (gst_srt_src_fill);
 }

ext/srt/gstsrtsrc.h

@@ -49,6 +49,8 @@ struct _GstSRTSrc {
 
   GstSRTObject *srtobject;
   GCancellable *cancellable;
+
+  guint32       next_pktseq;
 };
 
 struct _GstSRTSrcClass {