codecs: gstvp8decoder: add support for render delay

Some decoding APIs support delayed output for performance reasons.
One example would be to request decoding for multiple frames and
then query for the oldest frame in the output queue.

This also increases throughput for transcoding and improves seek
performance when supported by the underlying backend.

Introduce support in the vp8 base class, so that backends that
support render delays can actually implement it.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2150>

gst-libs/gst/codecs/gstvp8decoder.c

@@ -29,6 +29,7 @@
 #include <config.h>
 #endif
 
+#include <gst/base/base.h>
 #include "gstvp8decoder.h"
 
 GST_DEBUG_CATEGORY (gst_vp8_decoder_debug);
@@ -42,8 +43,19 @@ struct _GstVp8DecoderPrivate
   gboolean had_sequence;
   GstVp8Parser parser;
   gboolean wait_keyframe;
+  guint preferred_output_delay;
+  /* for delayed output */
+  GstQueueArray *output_queue;
+  gboolean is_live;
 };
 
+typedef struct
+{
+  GstVideoCodecFrame *frame;
+  GstVp8Picture *picture;
+  GstVp8Decoder *self;
+} GstVp8DecoderOutputFrame;
+
 #define parent_class gst_vp8_decoder_parent_class
 G_DEFINE_ABSTRACT_TYPE_WITH_CODE (GstVp8Decoder, gst_vp8_decoder,
     GST_TYPE_VIDEO_DECODER,
@@ -60,6 +72,11 @@ static gboolean gst_vp8_decoder_flush (GstVideoDecoder * decoder);
 static GstFlowReturn gst_vp8_decoder_drain (GstVideoDecoder * decoder);
 static GstFlowReturn gst_vp8_decoder_handle_frame (GstVideoDecoder * decoder,
     GstVideoCodecFrame * frame);
+static void gst_vp8_decoder_clear_output_frame (GstVp8DecoderOutputFrame *
+    output_frame);
+static void gst_vp8_decoder_drain_output_queue (GstVp8Decoder * self,
+    guint num);
+
 
 static void
 gst_vp8_decoder_class_init (GstVp8DecoderClass * klass)
@@ -93,6 +110,11 @@ gst_vp8_decoder_start (GstVideoDecoder * decoder)
   gst_vp8_parser_init (&priv->parser);
   priv->wait_keyframe = TRUE;
 
+  priv->output_queue =
+      gst_queue_array_new_for_struct (sizeof (GstVp8DecoderOutputFrame), 1);
+  gst_queue_array_set_clear_func (priv->output_queue,
+      (GDestroyNotify) gst_vp8_decoder_clear_output_frame);
+
   return TRUE;
 }
 
@@ -106,12 +128,14 @@ gst_vp8_decoder_reset (GstVp8Decoder * self)
   gst_vp8_picture_clear (&self->alt_ref_picture);
 
   priv->wait_keyframe = TRUE;
+  gst_queue_array_clear (priv->output_queue);
 }
 
 static gboolean
 gst_vp8_decoder_stop (GstVideoDecoder * decoder)
 {
   GstVp8Decoder *self = GST_VP8_DECODER (decoder);
+  GstVp8DecoderPrivate *priv = self->priv;
 
   if (self->input_state) {
     gst_video_codec_state_unref (self->input_state);
@@ -119,6 +143,7 @@ gst_vp8_decoder_stop (GstVideoDecoder * decoder)
   }
 
   gst_vp8_decoder_reset (self);
+  gst_queue_array_free (priv->output_queue);
 
   return TRUE;
 }
@@ -157,6 +182,7 @@ gst_vp8_decoder_set_format (GstVideoDecoder * decoder,
 {
   GstVp8Decoder *self = GST_VP8_DECODER (decoder);
   GstVp8DecoderPrivate *priv = self->priv;
+  GstQuery *query;
 
   GST_DEBUG_OBJECT (decoder, "Set format");
 
@@ -168,6 +194,11 @@ gst_vp8_decoder_set_format (GstVideoDecoder * decoder,
   priv->width = GST_VIDEO_INFO_WIDTH (&state->info);
   priv->height = GST_VIDEO_INFO_HEIGHT (&state->info);
 
+  query = gst_query_new_latency ();
+  if (gst_pad_peer_query (GST_VIDEO_DECODER_SINK_PAD (self), query))
+    gst_query_parse_latency (query, &priv->is_live, NULL, NULL);
+  gst_query_unref (query);
+
   return TRUE;
 }
 
@@ -266,11 +297,27 @@ gst_vp8_decoder_drain (GstVideoDecoder * decoder)
 
   GST_DEBUG_OBJECT (self, "drain");
 
+  gst_vp8_decoder_drain_output_queue (GST_VP8_DECODER (decoder), 0);
   gst_vp8_decoder_reset (self);
 
   return GST_FLOW_OK;
 }
 
+static void
+gst_vp8_decoder_clear_output_frame (GstVp8DecoderOutputFrame * output_frame)
+{
+  if (!output_frame)
+    return;
+
+  if (output_frame->frame) {
+    gst_video_decoder_release_frame (GST_VIDEO_DECODER (output_frame->self),
+        output_frame->frame);
+    output_frame->frame = NULL;
+  }
+
+  gst_vp8_picture_clear (&output_frame->picture);
+}
+
 static GstFlowReturn
 gst_vp8_decoder_handle_frame (GstVideoDecoder * decoder,
     GstVideoCodecFrame * frame)
@@ -284,12 +331,17 @@ gst_vp8_decoder_handle_frame (GstVideoDecoder * decoder,
   GstVp8ParserResult pres;
   GstVp8Picture *picture = NULL;
   GstFlowReturn ret = GST_FLOW_OK;
+  GstVp8DecoderOutputFrame output_frame;
 
   GST_LOG_OBJECT (self,
       "handle frame, PTS: %" GST_TIME_FORMAT ", DTS: %"
       GST_TIME_FORMAT, GST_TIME_ARGS (GST_BUFFER_PTS (in_buf)),
       GST_TIME_ARGS (GST_BUFFER_DTS (in_buf)));
 
+  if (klass->get_preferred_output_delay)
+    priv->preferred_output_delay =
+        klass->get_preferred_output_delay (self, priv->is_live);
+
   if (!gst_buffer_map (in_buf, &map, GST_MAP_READ)) {
     GST_ERROR_OBJECT (self, "Cannot map buffer");
 
@@ -372,10 +424,13 @@ gst_vp8_decoder_handle_frame (GstVideoDecoder * decoder,
 
     ret = gst_video_decoder_finish_frame (GST_VIDEO_DECODER (self), frame);
   } else {
-    g_assert (klass->output_picture);
-    ret = klass->output_picture (self, frame, picture);
+    output_frame.frame = frame;
+    output_frame.picture = picture;
+    output_frame.self = self;
+    gst_queue_array_push_tail_struct (priv->output_queue, &output_frame);
   }
 
+  gst_vp8_decoder_drain_output_queue (self, priv->preferred_output_delay);
   return ret;
 
 unmap_and_error:
@@ -396,3 +451,17 @@ error:
     return ret;
   }
 }
+
+static void
+gst_vp8_decoder_drain_output_queue (GstVp8Decoder * self, guint num)
+{
+  GstVp8DecoderPrivate *priv = self->priv;
+  GstVp8DecoderClass *klass = GST_VP8_DECODER_GET_CLASS (self);
+  g_assert (klass->output_picture);
+
+  while (gst_queue_array_get_length (priv->output_queue) > num) {
+    GstVp8DecoderOutputFrame *output_frame = (GstVp8DecoderOutputFrame *)
+        gst_queue_array_pop_head_struct (priv->output_queue);
+    klass->output_picture (self, output_frame->frame, output_frame->picture);
+  }
+}

gst-libs/gst/codecs/gstvp8decoder.h

@@ -121,6 +121,21 @@ struct _GstVp8DecoderClass
                                         GstVideoCodecFrame * frame,
                                         GstVp8Picture * picture);
 
+  /**
+   * GstVp8DecoderClass::get_preferred_output_delay:
+   * @decoder: a #GstVp8Decoder
+   * @is_live: whether upstream is live or not
+   *
+   * Optional. Called by baseclass to query whether delaying output is
+   * preferred by subclass or not.
+   *
+   * Returns: the number of perferred delayed output frame
+   *
+   * Since: 1.20
+   */
+  guint           (*get_preferred_output_delay)   (GstVp8Decoder * decoder,
+                                                   gboolean is_live);
+
   /*< private >*/
   gpointer padding[GST_PADDING_LARGE];
 };