lamemp3enc: implement sinkpad get_caps() function to proxy rate and channels restrictions from downstream

The element downstream of mp3enc might only accept certain sample rates or channels,
make sure we relay any restrictions that do exist to upstream when it does a
get_caps() on the sink pad. That way upstream elements like audioresample or
audioconvert can pick a sample rate / channel configuration that will be accepted,
instead of just negotiating to the highest, which might then be rejected.

https://bugzilla.gnome.org/show_bug.cgi?id=641151

ext/lame/gstlamemp3enc.c

@@ -389,6 +389,61 @@ setup_failed:
   }
 }
 
+static GstCaps *
+gst_lamemp3enc_sink_getcaps (GstPad * pad)
+{
+  const GstCaps *templ_caps;
+  GstLameMP3Enc *lame;
+  GstCaps *allowed = NULL;
+  GstCaps *caps, *filter_caps;
+  gint i, j;
+
+  lame = GST_LAMEMP3ENC (gst_pad_get_parent (pad));
+
+  /* we want to be able to communicate to upstream elements like audioconvert
+   * and audioresample any rate/channel restrictions downstream (e.g. muxer
+   * only accepting certain sample rates) */
+  templ_caps = gst_pad_get_pad_template_caps (pad);
+  allowed = gst_pad_get_allowed_caps (lame->srcpad);
+  if (!allowed || gst_caps_is_empty (allowed) || gst_caps_is_any (allowed)) {
+    caps = gst_caps_copy (templ_caps);
+    goto done;
+  }
+
+  filter_caps = gst_caps_new_empty ();
+
+  for (i = 0; i < gst_caps_get_size (templ_caps); i++) {
+    GQuark q_name;
+
+    q_name = gst_structure_get_name_id (gst_caps_get_structure (templ_caps, i));
+
+    /* pick rate + channel fields from allowed caps */
+    for (j = 0; j < gst_caps_get_size (allowed); j++) {
+      const GstStructure *allowed_s = gst_caps_get_structure (allowed, j);
+      const GValue *val;
+      GstStructure *s;
+
+      s = gst_structure_id_empty_new (q_name);
+      if ((val = gst_structure_get_value (allowed_s, "rate")))
+        gst_structure_set_value (s, "rate", val);
+      if ((val = gst_structure_get_value (allowed_s, "channels")))
+        gst_structure_set_value (s, "channels", val);
+
+      gst_caps_merge_structure (filter_caps, s);
+    }
+  }
+
+  caps = gst_caps_intersect (filter_caps, templ_caps);
+  gst_caps_unref (filter_caps);
+
+done:
+
+  gst_caps_replace (&allowed, NULL);
+  gst_object_unref (lame);
+
+  return caps;
+}
+
 static gint64
 gst_lamemp3enc_get_latency (GstLameMP3Enc * lame)
 {
@@ -453,6 +508,8 @@ gst_lamemp3enc_init (GstLameMP3Enc * lame)
       GST_DEBUG_FUNCPTR (gst_lamemp3enc_chain));
   gst_pad_set_setcaps_function (lame->sinkpad,
       GST_DEBUG_FUNCPTR (gst_lamemp3enc_sink_setcaps));
+  gst_pad_set_getcaps_function (lame->sinkpad,
+      GST_DEBUG_FUNCPTR (gst_lamemp3enc_sink_getcaps));
   gst_element_add_pad (GST_ELEMENT (lame), lame->sinkpad);
 
   lame->srcpad =