audiodecoder: copy rate and channels from input before fixating output caps

For default caps generation when handling gap events that are sent
before any buffer, try to use caps that are closer to what upstream
provided to avoid fixating rate or channels to 1 as default.

So there are the steps:
1) Try to set rate, channels and channel-mask from upstream if provided
2) Fixate the rate and channels to the default rate and channels from
   audio lib
3) Fixate the caps just to be sure everything is fixed
4) If no channel-mask was provided and channels > 2, use a default
   channel-mask (taken from audioconvert code)

https://bugzilla.gnome.org/show_bug.cgi?id=722144

gst-libs/gst/audio/gstaudiodecoder.c

@@ -279,6 +279,69 @@ struct _GstAudioDecoderPrivate
   GList *pending_events;
 };
 
+//* Default channel layouts taken from audioconvert */
+static const GstAudioChannelPosition default_positions[8][8] = {
+  /* 1 channel */
+  {
+        GST_AUDIO_CHANNEL_POSITION_MONO,
+      },
+  /* 2 channels */
+  {
+        GST_AUDIO_CHANNEL_POSITION_FRONT_LEFT,
+        GST_AUDIO_CHANNEL_POSITION_FRONT_RIGHT,
+      },
+  /* 3 channels (2.1) */
+  {
+        GST_AUDIO_CHANNEL_POSITION_FRONT_LEFT,
+        GST_AUDIO_CHANNEL_POSITION_FRONT_RIGHT,
+        GST_AUDIO_CHANNEL_POSITION_LFE1,
+      },
+  /* 4 channels (4.0) */
+  {
+        GST_AUDIO_CHANNEL_POSITION_FRONT_LEFT,
+        GST_AUDIO_CHANNEL_POSITION_FRONT_RIGHT,
+        GST_AUDIO_CHANNEL_POSITION_REAR_LEFT,
+        GST_AUDIO_CHANNEL_POSITION_REAR_RIGHT,
+      },
+  /* 5 channels */
+  {
+        GST_AUDIO_CHANNEL_POSITION_FRONT_LEFT,
+        GST_AUDIO_CHANNEL_POSITION_FRONT_RIGHT,
+        GST_AUDIO_CHANNEL_POSITION_REAR_LEFT,
+        GST_AUDIO_CHANNEL_POSITION_REAR_RIGHT,
+        GST_AUDIO_CHANNEL_POSITION_FRONT_CENTER,
+      },
+  /* 6 channels (5.1) */
+  {
+        GST_AUDIO_CHANNEL_POSITION_FRONT_LEFT,
+        GST_AUDIO_CHANNEL_POSITION_FRONT_RIGHT,
+        GST_AUDIO_CHANNEL_POSITION_REAR_LEFT,
+        GST_AUDIO_CHANNEL_POSITION_REAR_RIGHT,
+        GST_AUDIO_CHANNEL_POSITION_FRONT_CENTER,
+        GST_AUDIO_CHANNEL_POSITION_LFE1,
+      },
+  /* 7 channels (6.1) */
+  {
+        GST_AUDIO_CHANNEL_POSITION_FRONT_LEFT,
+        GST_AUDIO_CHANNEL_POSITION_FRONT_RIGHT,
+        GST_AUDIO_CHANNEL_POSITION_REAR_LEFT,
+        GST_AUDIO_CHANNEL_POSITION_REAR_RIGHT,
+        GST_AUDIO_CHANNEL_POSITION_FRONT_CENTER,
+        GST_AUDIO_CHANNEL_POSITION_LFE1,
+        GST_AUDIO_CHANNEL_POSITION_REAR_CENTER,
+      },
+  /* 8 channels (7.1) */
+  {
+        GST_AUDIO_CHANNEL_POSITION_FRONT_LEFT,
+        GST_AUDIO_CHANNEL_POSITION_FRONT_RIGHT,
+        GST_AUDIO_CHANNEL_POSITION_REAR_LEFT,
+        GST_AUDIO_CHANNEL_POSITION_REAR_RIGHT,
+        GST_AUDIO_CHANNEL_POSITION_FRONT_CENTER,
+        GST_AUDIO_CHANNEL_POSITION_LFE1,
+        GST_AUDIO_CHANNEL_POSITION_SIDE_LEFT,
+        GST_AUDIO_CHANNEL_POSITION_SIDE_RIGHT,
+      }
+};
 
 static void gst_audio_decoder_finalize (GObject * object);
 static void gst_audio_decoder_set_property (GObject * object,
@@ -1761,6 +1824,12 @@ static gboolean
 gst_audio_decoder_negotiate_default_caps (GstAudioDecoder * dec)
 {
   GstCaps *caps;
+  gint i;
+  gint channels = 0;
+  gint rate;
+  guint64 channel_mask = 0;
+  gint caps_size;
+  GstStructure *structure;
 
   caps = gst_pad_get_current_caps (dec->srcpad);
   if (caps && !gst_audio_info_from_caps (&dec->priv->ctx.info, caps))
@@ -1770,7 +1839,62 @@ gst_audio_decoder_negotiate_default_caps (GstAudioDecoder * dec)
   if (!caps || gst_caps_is_empty (caps) || gst_caps_is_any (caps))
     return FALSE;
 
+  /* before fixating, try to use whatever upstream provided */
+  caps_size = gst_caps_get_size (caps);
+  if (dec->priv->ctx.input_caps) {
+    GstCaps *sinkcaps = dec->priv->ctx.input_caps;
+    GstStructure *structure = gst_caps_get_structure (sinkcaps, 0);
+
+    if (gst_structure_get_int (structure, "rate", &rate)) {
+      for (i = 0; i < caps_size; i++) {
+        gst_structure_set (gst_caps_get_structure (caps, i), "rate",
+            G_TYPE_INT, rate, NULL);
+      }
+    }
+
+    if (gst_structure_get_int (structure, "channels", &channels)) {
+      for (i = 0; i < caps_size; i++) {
+        gst_structure_set (gst_caps_get_structure (caps, i), "channels",
+            G_TYPE_INT, channels, NULL);
+      }
+    }
+
+    if (gst_structure_get (structure, "channel-mask", GST_TYPE_BITMASK,
+            &channel_mask, NULL)) {
+      for (i = 0; i < caps_size; i++) {
+        gst_structure_set (gst_caps_get_structure (caps, i), "channel-mask",
+            GST_TYPE_BITMASK, channel_mask, NULL);
+      }
+    }
+  }
+
+  for (i = 0; i < caps_size; i++) {
+    structure = gst_caps_get_structure (caps, i);
+    gst_structure_fixate_field_nearest_int (structure,
+        "channels", GST_AUDIO_DEF_CHANNELS);
+    gst_structure_fixate_field_nearest_int (structure,
+        "rate", GST_AUDIO_DEF_RATE);
+  }
   caps = gst_caps_fixate (caps);
+  structure = gst_caps_get_structure (caps, 0);
+
+  /* Need to add a channel-mask if channels > 2 */
+  gst_structure_get_int (structure, "channels", &channels);
+  if (channels > 2 && !gst_structure_has_field (structure, "channel-mask")) {
+    if (channels <= 8) {
+      channel_mask = 0;
+      for (i = 0; i < channels; i++)
+        channel_mask |=
+            G_GUINT64_CONSTANT (1) << default_positions[channels - 1][i];
+
+      gst_structure_set (structure, "channel-mask",
+          GST_TYPE_BITMASK, channel_mask, NULL);
+    } else {
+      GST_WARNING_OBJECT (dec, "No default channel-mask for %d channels",
+          channels);
+    }
+  }
+
   if (!caps || !gst_audio_info_from_caps (&dec->priv->ctx.info, caps))
     return FALSE;