vaapivideocontext: only vaapisink process app context

gst.vaapi.app.Display context is made for applications that will
provide the VA display and the native display to used by the
pipeline, when are using vaapisink as overlay. There are no use
case for encoders, decoders, neither for the postprocessor.

In the case of the vaapisink, it shall query for gst.vaapi.Display
upstream first, and then, if there is no reply,
gst.vaapi.app.Display context will be posted in the bus for the
application. If the application replies, a GstVaapiDisplay object
is instantiated given the context info, otherwise a
GstVaapiDisplay is created with the normal algorithm to guess the
graphics platform. Either way, the instantiated GstVaapiDisplay
is propagated among the pipeline and the have-message bus message.

Also only vaapisink will process the gst.vaapi.app.Display, if
and only if, it doesn't have a display already set. This is
caused because if vaapisink is in a bin (playsink, for example)
the need-context is posted twice, leading to an error state.

https://bugzilla.gnome.org/show_bug.cgi?id=790999

gst/vaapi/gstvaapipluginbase.c

@@ -91,9 +91,12 @@ gst_vaapi_plugin_base_set_context (GstVaapiPluginBase * plugin,
 {
   GstVaapiDisplay *display = NULL;
 
-  if (gst_vaapi_video_context_get_display (context, &display))
+  /* gst.vaapi.app.Display is only attended _if_ the element is
+   * vaapisink and it doesn't have a display set yet */
+  if (gst_vaapi_video_context_get_display (context,
+          GST_IS_VIDEO_SINK (plugin) && !plugin->display, &display)) {
     plugin_set_display (plugin, display);
-
+  }
 #if USE_GST_GL_HELPERS
   {
     GstGLDisplay *gl_display = NULL;

gst/vaapi/gstvaapivideocontext.c

@@ -72,11 +72,12 @@ gst_vaapi_video_context_new_with_display (GstVaapiDisplay * display,
 }
 
 gboolean
-gst_vaapi_video_context_get_display (GstContext * context,
+gst_vaapi_video_context_get_display (GstContext * context, gboolean app_context,
     GstVaapiDisplay ** display_ptr)
 {
   const GstStructure *structure;
   const gchar *type;
+  GstVaapiDisplay *display = NULL;
 
   g_return_val_if_fail (GST_IS_CONTEXT (context), FALSE);
 
@@ -86,7 +87,9 @@ gst_vaapi_video_context_get_display (GstContext * context,
     structure = gst_context_get_structure (context);
     return gst_structure_get (structure, GST_VAAPI_DISPLAY_CONTEXT_TYPE_NAME,
         GST_TYPE_VAAPI_DISPLAY, display_ptr, NULL);
-  } else if (!g_strcmp0 (type, GST_VAAPI_DISPLAY_APP_CONTEXT_TYPE_NAME)) {
+  }
+
+  if (app_context && !g_strcmp0 (type, GST_VAAPI_DISPLAY_APP_CONTEXT_TYPE_NAME)) {
     VADisplay va_display = NULL;
     structure = gst_context_get_structure (context);
 
@@ -96,12 +99,22 @@ gst_vaapi_video_context_get_display (GstContext * context,
       Display *x11_display = NULL;
       if (gst_structure_get (structure, "x11-display", G_TYPE_POINTER,
               &x11_display, NULL)) {
-        *display_ptr =
+        display =
             gst_vaapi_display_x11_new_with_va_display (va_display, x11_display);
-        return TRUE;
       }
 #endif
-      GST_WARNING ("Not support if only VADisplay provided");
+
+      _init_context_debug ();
+
+      if (!display) {
+        GST_CAT_WARNING (GST_CAT_CONTEXT,
+            "Cannot create GstVaapiDisplay if only VADisplay is provided");
+        return FALSE;
+      }
+      GST_CAT_INFO (GST_CAT_CONTEXT,
+          "new display with context %" GST_PTR_FORMAT, display);
+      *display_ptr = display;
+      return TRUE;
     }
   }
 
@@ -211,6 +224,33 @@ found:
   gst_query_unref (query);
 }
 
+static gboolean
+_gst_vaapi_sink_find_context (GstElement * element)
+{
+  GstQuery *query;
+  GstMessage *msg;
+  gboolean found;
+
+  /* 1. Query upstream for an already created GstVaapiDisplay */
+  query = gst_query_new_context (GST_VAAPI_DISPLAY_CONTEXT_TYPE_NAME);
+  found = _gst_context_get_from_query (element, query, GST_PAD_SINK);
+  gst_query_unref (query);
+  if (found)
+    return TRUE;
+
+  /* 2. Post a GST_MESSAGE_NEED_CONTEXT message on the bus with a
+   * gst.vaapi.app.Display context from the application */
+  msg = gst_message_new_need_context (GST_OBJECT_CAST (element),
+      GST_VAAPI_DISPLAY_APP_CONTEXT_TYPE_NAME);
+  if (!gst_element_post_message (element, msg)) {
+    _init_context_debug ();
+    GST_CAT_INFO_OBJECT (GST_CAT_CONTEXT, element, "No bus attached");
+    gst_message_unref (msg);
+  }
+
+  return FALSE;
+}
+
 gboolean
 gst_vaapi_video_context_prepare (GstElement * element,
     GstVaapiDisplay ** display_ptr)
@@ -227,22 +267,21 @@ gst_vaapi_video_context_prepare (GstElement * element,
     return TRUE;
   }
 
-  _gst_context_query (element, GST_VAAPI_DISPLAY_CONTEXT_TYPE_NAME);
+  if (GST_IS_VIDEO_SINK (element)) {
+    if (!_gst_vaapi_sink_find_context (element) && *display_ptr) {
+      /* Propagate if display was created from application */
+      gst_vaapi_video_context_propagate (element, *display_ptr);
+    }
+  } else {
+    _gst_context_query (element, GST_VAAPI_DISPLAY_CONTEXT_TYPE_NAME);
+  }
 
   if (*display_ptr) {
     GST_LOG_OBJECT (element, "found a display %" GST_PTR_FORMAT, *display_ptr);
     return TRUE;
   }
 
-  _gst_context_query (element, GST_VAAPI_DISPLAY_APP_CONTEXT_TYPE_NAME);
-
-  if (*display_ptr) {
-    GST_LOG_OBJECT (element,
-        "got a display with va display from app %" GST_PTR_FORMAT,
-        *display_ptr);
-  }
-
-  return *display_ptr != NULL;
+  return FALSE;
 }
 
 /* 5) Create a context by itself and post a GST_MESSAGE_HAVE_CONTEXT message

gst/vaapi/gstvaapivideocontext.h

@@ -44,7 +44,7 @@ gst_vaapi_video_context_new_with_display (GstVaapiDisplay * display,
 
 G_GNUC_INTERNAL
 gboolean
-gst_vaapi_video_context_get_display (GstContext * context,
+gst_vaapi_video_context_get_display (GstContext * context, gboolean app_context,
     GstVaapiDisplay ** display_ptr);
 
 G_GNUC_INTERNAL