Fix export encoder race condition

The CmdPipeWriter::Terminated signal is used to trigger the FileWritten
callback, which invokes ExportGraphBuilder::Encoder::copy_files

Encoder::filenames must not be destroyed before the callback
arrives. This is now guaranteed by the Encoder d'tor waiting
for for the encode process to terminate.

libs/ardour/ardour/export_graph_builder.h

@@ -120,7 +120,8 @@ class LIBARDOUR_API ExportGraphBuilder
 	void add_split_config (FileSpec const & config);
 
 	class Encoder {
-            public:
+	public:
+		~Encoder ();
 		template <typename T> std::shared_ptr<AudioGrapher::Sink<T> > init (FileSpec const & new_config);
 		void add_child (FileSpec const & new_config);
 		void remove_children ();

libs/ardour/export_graph_builder.cc

@@ -380,6 +380,13 @@ ExportGraphBuilder::Encoder::add_child (FileSpec const & new_config)
 	filenames.push_back (new_config.filename);
 }
 
+ExportGraphBuilder::Encoder::~Encoder ()
+{
+	if (pipe_writer) {
+		pipe_writer->flush ();
+	}
+}
+
 void
 ExportGraphBuilder::Encoder::destroy_writer (bool delete_out_file)
 {

libs/audiographer/audiographer/general/cmdpipe_writer.h

@@ -59,6 +59,12 @@ public:
 	samplecnt_t get_samples_written() const { return samples_written; }
 	void       reset_samples_written_count() { samples_written = 0; }
 
+	void flush (void)
+	{
+		_proc->close_stdin ();
+		_proc->wait ();
+	}
+
 	void close (void)
 	{
 		_proc->terminate ();
@@ -93,6 +99,7 @@ public:
 				if (_proc->start (ARDOUR::SystemExec::ShareWithParent)) {
 					throw ARDOUR::ExportFailed ("External encoder (ffmpeg) cannot be started.");
 				}
+				assert (_proc->is_running ());
 			} else {
 				_proc->close_stdin ();
 			}