playbin: Handle error message with redirection indication

There are in the wild (mp4) streams that basically contain no tracks
but do have a redirect info[0], in which case, qtdemux won't be able
to expose any pad (there are no tracks) so can't post anything but
an error on the bus, as:
  - it can't send EOS downstream, it has no pad,
  - posting an EOS message will be useless as PAUSED state can't be
    reached and there is no sink in the pipeline meaning GstBin will
    simply ignore it

In that case, currently the application could try to handle that but it
is pretty complex as it will get the REDIRECT message on the bus at
which point it could set the URL but playbin will ignore it, as
it will only be for the next EOS, it thus need to set the pipeline to
NULL (READY won't do as it is already in READY at that point). And it
needs to figure out the following ERROR message on the bus needs to be
ignored, which is not really simple.

The approach here is to allow element to add details to the ERROR
message with a `redirect-location` field which elements like playbin handle
and use right away.

We could also use the element 'redirect' message in playbin, but the
issue with that approach is that the element will still emit the ERROR
message on the bus, leading to wrong behaviour. That can't be avoided
since in the case the app/parent pipeline is not handling the redirect
instruction, the ERROR message is necessary (and there is no way to
detect that the message has been "handled" from the element emitting the
redirect).

[0]: http://movietrailers.apple.com/movies/paramount/terminator-dark-fate/terminator-dark-fate-trailer-2_480p.mov

gst/playback/gstplaybin2.c

@@ -170,6 +170,10 @@
  * type. The new location may be a relative or an absolute URI. Examples
  * for such redirects can be found in many quicktime movie trailers.
  *
+ * NOTE: playbin will internally handle the redirect messages in the case
+ * that the redirecting stream doesn't contain any tracks and thus
+ * needs to report an error message on the bus.
+ *
  * ## Examples
  * |[
  * gst-launch-1.0 -v playbin uri=file:///path/to/somefile.mp4
@@ -3025,6 +3029,42 @@ gst_play_bin_handle_message (GstBin * bin, GstMessage * msg)
           no_more_pads_cb (NULL, group);
         }
       }
+    } else {
+      const GstStructure *details = NULL;
+
+      gst_message_parse_error_details (msg, &details);
+      if (details && gst_structure_has_field (details, "redirect-location")) {
+        gchar *uri = NULL;
+        const gchar *location =
+            gst_structure_get_string ((GstStructure *) details,
+            "redirect-location");
+
+        if (gst_uri_is_valid (location)) {
+          uri = g_strdup (location);
+        } else {
+          uri = gst_uri_join_strings (group->uri, location);
+        }
+
+        if (g_strcmp0 (uri, group->uri)) {
+          GST_PLAY_BIN_LOCK (playbin);
+          if (playbin->next_group && playbin->next_group->valid) {
+            GST_DEBUG_OBJECT (playbin,
+                "User already setup next uri %s, using it",
+                playbin->next_group->uri);
+          } else {
+            GST_DEBUG_OBJECT (playbin,
+                "Using newly configured redirect URI: %s", uri);
+            gst_play_bin_set_uri (playbin, uri);
+          }
+          GST_PLAY_BIN_UNLOCK (playbin);
+
+          setup_next_source (playbin, GST_STATE_PAUSED);
+          gst_message_unref (msg);
+          msg = NULL;
+        }
+
+        g_free (uri);
+      }
     }
   }