Export LV2 plugin latency over a control port

plugin/hydrumkit.ttl

@@ -5,6 +5,7 @@
 @prefix rdfs:  <http://www.w3.org/2000/01/rdf-schema#> .
 @prefix state: <http://lv2plug.in/ns/ext/state#> .
 @prefix ui:    <http://lv2plug.in/ns/extensions/ui#> .
+@prefix units: <http://lv2plug.in/ns/extensions/units#> .
 @prefix urid:  <http://lv2plug.in/ns/ext/urid#> .
 @prefix work:  <http://lv2plug.in/ns/ext/worker#> .
 @prefix param: <http://lv2plug.in/ns/ext/parameters#> .
@@ -70,6 +71,16 @@
                         pg:group <http://lv2.incal.net/plugins/hydrumkit/out> ;
                         pg:role pg:rightChannel
                 ]
+        ] , [
+                a lv2:OutputPort ,
+                        lv2:ControlPort ;
+                lv2:index 4 ;
+                lv2:symbol "latency" ;
+                lv2:name "latency" ;
+                lv2:minimum 0 ;
+                lv2:maximum 192000 ;
+                lv2:portProperty lv2:reportsLatency, lv2:integer ;
+                units:unit units:frame ;
         ];
 	state:state [
 		<http://lv2.incal.net/plugins/hydrumkit#drumkit> <none>

plugin/plugin.c

@@ -28,6 +28,7 @@
 #define SAMPLER_NOTIFY 1
 #define SAMPLER_OUT_L 2
 #define SAMPLER_OUT_R 3
+#define SAMPLER_LATENCY 4
 
 #define NUM_VOICES 64
 
@@ -40,6 +41,7 @@ struct sampler_plugin {
   LV2_Atom_Sequence *notify_port;
   float *output_port_l;
   float *output_port_r;
+  float *output_latency;
 
   LV2_Atom_Forge_Frame notify_frame;
   LV2_Atom_Forge forge;
@@ -138,6 +140,9 @@ static void connect_port(LV2_Handle instance, uint32_t port, void *data) {
   case SAMPLER_OUT_R:
     plugin->output_port_r = (float *)data;
     break;
+  case SAMPLER_LATENCY:
+    plugin->output_latency = (float *)data;
+    break;
   default:
     break;
   }
@@ -266,6 +271,10 @@ static void run(LV2_Handle instance, uint32_t sample_count) {
   }
 
   render(plugin, plugin->frame_offset, sample_count);
+
+  if (plugin->output_latency) {
+    *(plugin->output_latency) = (float)sampler_get_latency(plugin->sampler);
+  }
 }
 
 static LV2_State_Status save(LV2_Handle instance,