Add API to unset last capture source(s)

This is in preparation to clear the list when
switching snapshots to prevent destroying references.

https://discourse.ardour.org/t/discard-last-take-impacts-previous-snapshots-is-this-wanted-behaviour/111420/6?u=x42

libs/ardour/ardour/disk_writer.h

@@ -84,6 +84,7 @@ public:
 	bool configure_io (ChanCount in, ChanCount out);
 
 	std::list<std::shared_ptr<Source> >& last_capture_sources () { return _last_capture_sources; }
+	void reset_last_capture_sources ();
 
 	bool record_enabled () const { return _record_enabled.load(); }
 	bool record_safe () const { return _record_safe.load(); }
@@ -192,6 +193,7 @@ private:
 	std::atomic<int> _record_safe;
 	std::atomic<int> _samples_pending_write;
 	std::atomic<int> _num_captured_loops;
+	std::atomic<int> _reset_last_capture_sources;
 
 	std::shared_ptr<SMFSource> _midi_write_source;
 

libs/ardour/ardour/session.h

@@ -872,6 +872,7 @@ public:
 
 	int remove_last_capture ();
 	void get_last_capture_sources (std::list<std::shared_ptr<Source> >&);
+	void reset_last_capture_sources ();
 
 	/** handlers should return -1 for "stop cleanup",
 	    0 for "yes, delete this playlist",

libs/ardour/ardour/track.h

@@ -144,6 +144,7 @@ public:
 	void request_input_monitoring (bool);
 	void ensure_input_monitoring (bool);
 	std::list<std::shared_ptr<Source> > & last_capture_sources ();
+	void reset_last_capture_sources ();
 	std::string steal_write_source_name ();
 	void reset_write_sources (bool mark_write_complete);
 	float playback_buffer_load () const;

libs/ardour/disk_writer.cc

@@ -69,6 +69,7 @@ DiskWriter::DiskWriter (Session& s, Track& t, string const & str, DiskIOProcesso
 	_record_safe.store (0);
 	_samples_pending_write.store (0);
 	_num_captured_loops.store (0);
+	_reset_last_capture_sources.store (0);
 }
 
 DiskWriter::~DiskWriter ()
@@ -281,6 +282,12 @@ DiskWriter::calculate_record_range (Temporal::OverlapType ot, samplepos_t transp
 	                                                      _first_recordable_sample, _last_recordable_sample, rec_nframes, rec_offset));
 }
 
+void
+DiskWriter::reset_last_capture_sources ()
+{
+	_reset_last_capture_sources.store (1);
+}
+
 void
 DiskWriter::engage_record_enable ()
 {
@@ -561,7 +568,9 @@ DiskWriter::run (BufferSet& bufs, samplepos_t start_sample, samplepos_t end_samp
 
 	}
 
-	if (can_record && !_last_capture_sources.empty ()) {
+	int canderef (1);
+	const bool reset_capture_src = _reset_last_capture_sources.compare_exchange_strong (canderef, 0);
+	if ((reset_capture_src || can_record) && !_last_capture_sources.empty ()) {
 		_last_capture_sources.clear ();
 	}
 

libs/ardour/session.cc

@@ -4856,6 +4856,20 @@ Session::get_last_capture_sources (std::list<std::shared_ptr<Source> >& srcs)
 	}
 }
 
+void
+Session::reset_last_capture_sources ()
+{
+	std::shared_ptr<RouteList const> rl = routes.reader ();
+	for (auto const& i : *rl) {
+		std::shared_ptr<Track> tr = std::dynamic_pointer_cast<Track> (i);
+		if (!tr) {
+			continue;
+		}
+		tr->reset_last_capture_sources ();
+
+	}
+}
+
 /* Source Management */
 
 void

libs/ardour/track.cc

@@ -585,6 +585,12 @@ Track::last_capture_sources ()
 	return _disk_writer->last_capture_sources ();
 }
 
+void
+Track::reset_last_capture_sources ()
+{
+	_disk_writer->reset_last_capture_sources ();
+}
+
 std::string
 Track::steal_write_source_name()
 {