Add API to query plugin tail duration

So far only VST3 plugins have an actual implementation.

libs/ardour/ardour/plugin.h

@@ -175,6 +175,9 @@ public:
 	/** the max possible latency a plugin will have */
 	virtual samplecnt_t max_latency () const { return 0; }
 
+	samplecnt_t effective_tail() const;
+	PBD::Signal0<void> TailChanged;
+
 	virtual int  set_block_size (pframes_t nframes) = 0;
 	virtual bool requires_fixed_sized_buffers () const { return false; }
 	virtual bool inplace_broken () const { return false; }
@@ -423,6 +426,11 @@ protected:
 
 private:
 	virtual samplecnt_t plugin_latency () const = 0;
+	/** tail duration in samples. e.g. for reverb or delay plugins.
+	 *
+	 * The default when unknown is 2 sec */
+	virtual samplecnt_t plugin_tail () const;
+
 
 	/** Fill _presets with our presets */
 	virtual void find_presets () = 0;

libs/ardour/ardour/rc_configuration_vars.h

@@ -256,6 +256,9 @@ CONFIG_VARIABLE (uint32_t, plugin_scan_timeout, "plugin-scan-timeout", 150) /* d
 CONFIG_VARIABLE (uint32_t, limit_n_automatables, "limit-n-automatables", 512)
 CONFIG_VARIABLE (uint32_t, plugin_cache_version, "plugin-cache-version", 0)
 
+CONFIG_VARIABLE (float, tail_duration_sec, "tail-duration-sec", 2.0)
+CONFIG_VARIABLE (uint32_t, max_tail_samples, "max-tail-samples", 0xffffffff) // aka kInfiniteTail
+
 /* custom user plugin paths */
 CONFIG_VARIABLE (std::string, plugin_path_vst, "plugin-path-vst", "@default@")
 CONFIG_VARIABLE (std::string, plugin_path_lxvst, "plugin-path-lxvst", "@default@")

libs/ardour/ardour/vst3_plugin.h

@@ -180,6 +180,7 @@ public:
 
 	/* API for Ardour -- Setup/Processing */
 	uint32_t plugin_latency ();
+	uint32_t plugin_tail ();
 	bool     set_block_size (int32_t);
 	bool     activate ();
 	bool     deactivate ();
@@ -326,6 +327,7 @@ private:
 	bool                        _add_to_selection;
 
 	boost::optional<uint32_t> _plugin_latency;
+	boost::optional<uint32_t> _plugin_tail;
 
 	int _n_bus_in;
 	int _n_bus_out;
@@ -440,6 +442,7 @@ public:
 
 private:
 	samplecnt_t plugin_latency () const;
+	samplecnt_t plugin_tail () const;
 	void        init ();
 	void        find_presets ();
 	void        forward_resize_view (int w, int h);

libs/ardour/plugin.cc

@@ -317,6 +317,19 @@ Plugin::input_streams () const
 	return ChanCount::ZERO;
 }
 
+samplecnt_t
+Plugin::effective_tail () const
+{
+	/* consider adding a user-override per plugin; compare to HasLatency, Latent */
+	return max<samplecnt_t> (0, min<samplecnt_t> (plugin_tail (), Config->get_max_tail_samples ()));
+}
+
+samplecnt_t
+Plugin::plugin_tail () const
+{
+	return _session.sample_rate () * Config->get_tail_duration_sec ();
+}
+
 Plugin::IOPortDescription
 Plugin::describe_io_port (ARDOUR::DataType dt, bool input, uint32_t id) const
 {

libs/ardour/vst3_plugin.cc

@@ -819,6 +819,12 @@ VST3Plugin::set_block_size (pframes_t n_samples)
 	return 0;
 }
 
+samplecnt_t
+VST3Plugin::plugin_tail () const
+{
+	return _plug->plugin_tail ();
+}
+
 samplecnt_t
 VST3Plugin::plugin_latency () const
 {
@@ -1951,6 +1957,15 @@ VST3PI::plugin_latency ()
 	return _plugin_latency.value ();
 }
 
+uint32_t
+VST3PI::plugin_tail ()
+{
+	if (!_plugin_tail) { // XXX this is currently never reset
+		_plugin_tail = _processor->getTailSamples ();
+	}
+	return _plugin_tail.value ();
+}
+
 void
 VST3PI::set_owner (SessionObject* o)
 {