rtspsrc: improve recovery from failed seek

In case server-side fails to perform seek, i.e. PLAY at non-zero requested
position, recovery so far would arrange for streaming to continue, albeit
having lost position tracking in the process.  So, query position prior
to seek and use upon failed seek.

gst/rtsp/gstrtspsrc.c

@@ -1716,6 +1716,33 @@ gst_rtspsrc_connection_receive (GstRTSPSrc * src, GstRTSPConnection * conn,
   return ret;
 }
 
+static void
+gst_rtspsrc_get_position (GstRTSPSrc * src)
+{
+  GstQuery *query;
+  GList *walk;
+
+  query = gst_query_new_position (GST_FORMAT_TIME);
+  /*  should be known somewhere down the stream (e.g. jitterbuffer) */
+  for (walk = src->streams; walk; walk = g_list_next (walk)) {
+    GstRTSPStream *stream = (GstRTSPStream *) walk->data;
+    GstFormat fmt;
+    gint64 pos;
+
+    if (stream->srcpad) {
+      if (gst_pad_query (stream->srcpad, query)) {
+        gst_query_parse_position (query, &fmt, &pos);
+        GST_DEBUG_OBJECT (src, "retaining position %" GST_TIME_FORMAT,
+            GST_TIME_ARGS (pos));
+        src->last_pos = pos;
+        return;
+      }
+    }
+  }
+
+  src->last_pos = 0;
+}
+
 static gboolean
 gst_rtspsrc_do_seek (GstRTSPSrc * src, GstSegment * segment)
 {
@@ -1808,8 +1835,11 @@ gst_rtspsrc_perform_seek (GstRTSPSrc * src, GstEvent * event)
   playing = (src->state == GST_RTSP_STATE_PLAYING);
 
   /* if we were playing, pause first */
-  if (playing)
+  if (playing) {
+    /* obtain current position in case seek fails */
+    gst_rtspsrc_get_position (src);
     gst_rtspsrc_pause (src, FALSE);
+  }
 
   gst_rtspsrc_do_seek (src, &seeksegment);
 
@@ -5885,7 +5915,7 @@ gst_rtspsrc_play (GstRTSPSrc * src, GstSegment * segment)
       /* NOTE the above also disables npt based eos detection */
       /* and below forces position to 0,
        * which is visible feedback we lost the plot */
-      segment->start = segment->last_stop = 0;
+      segment->start = segment->last_stop = src->last_pos;
     }
 
     gst_rtsp_message_unset (&request);

gst/rtsp/gstrtspsrc.h

@@ -233,7 +233,9 @@ struct _GstRTSPSrc {
 
   /* supported methods */
   gint               methods;
+
   gboolean           seekable;
+  GstClockTime       last_pos;
 
   /* session management */
   GstElement      *manager;