GUI Dialog to inform the user about missing files

So far this is only used for MIDI, missing audio files are
already handled by asking a user to locate them.
The same approach doesn't work for MIDI, since there cannot be
externally referenced (embedded) .mid files (since MIDI is destructive).

gtk2_ardour/ardour_ui_session.cc

@@ -59,6 +59,7 @@
 #include "ardour_message.h"
 #include "ardour_ui.h"
 #include "engine_dialog.h"
+#include "missing_filesource_dialog.h"
 #include "missing_plugin_dialog.h"
 #include "opts.h"
 #include "public_editor.h"
@@ -454,6 +455,13 @@ ARDOUR_UI::load_session_stage_two (const std::string& path, const std::string& s
 		goto out;
 	}
 
+	{
+		list<string> const u = new_session->missing_filesources (DataType::MIDI);
+		if (!u.empty()) {
+			MissingFileSourceDialog d (_session, u, DataType::MIDI);
+			d.run ();
+		}
+	}
 	{
 		list<string> const u = new_session->unknown_processors ();
 		if (!u.empty()) {

gtk2_ardour/missing_filesource_dialog.cc

@@ -0,0 +1,64 @@
+/*
+ * Copyright (C) 2010-2011 Carl Hetherington <carl@carlh.net>
+ * Copyright (C) 2011-2012 David Robillard <d@drobilla.net>
+ * Copyright (C) 2011-2016 Paul Davis <paul@linuxaudiosystems.com>
+ * Copyright (C) 2013-2020 Robin Gareus <robin@gareus.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include <gtkmm/label.h>
+#include "pbd/compose.h"
+#include "missing_filesource_dialog.h"
+#include "pbd/i18n.h"
+
+using namespace Gtk;
+using namespace std;
+using namespace ARDOUR;
+using namespace PBD;
+
+MissingFileSourceDialog::MissingFileSourceDialog (Session * s, list<string> const & plugins, DataType dt)
+	: ArdourDialog (_("Missing Source Files"), true, false)
+{
+	/* This dialog is always shown programatically. Center the window.*/
+	set_position (Gtk::WIN_POS_CENTER);
+
+	set_session (s);
+
+	add_button (_("OK"), RESPONSE_OK);
+	set_default_response (RESPONSE_OK);
+
+	Label* m = manage (new Label);
+
+	stringstream t;
+	t << string_compose (_("This session misses following %1 files.\nThey have been replaced with silence:\n\n"), dt.to_string());
+
+	for (list<string>::const_iterator i = plugins.begin(); i != plugins.end(); ++i) {
+		t << *i << "\n";
+	}
+
+	t << _("\nThe Regions and edits have been retained.\n"
+	       "If this is unexpected, manually loacate the files and restore them in the session folder.\n");
+
+	if (dt == DataType::MIDI) {
+		t << _("Editing the MIDI files by adding new content will re-create the file and disable this warning,\n"
+		       "but also prevent future recovery of the original in the exising region(s).\n");
+	}
+
+	m->set_markup (t.str ());
+	get_vbox()->pack_start (*m, false, false);
+
+	show_all ();
+}

gtk2_ardour/missing_filesource_dialog.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (C) 2013-2015 Paul Davis <paul@linuxaudiosystems.com>
+ * Copyright (C) 2013-2020 Robin Gareus <robin@gareus.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef __gtk_ardour_missing_filesource_dialog_h__
+#define __gtk_ardour_missing_filesource_dialog_h__
+
+#include <list>
+#include <string>
+
+#include "ardour/data_type.h"
+
+#include "ardour_dialog.h"
+
+namespace ARDOUR {
+	class Session;
+}
+
+class MissingFileSourceDialog : public ArdourDialog
+{
+public:
+	MissingFileSourceDialog (ARDOUR::Session *, std::list<std::string> const &, ARDOUR::DataType);
+};
+
+#endif

gtk2_ardour/wscript

@@ -161,6 +161,7 @@ gtk2_ardour_sources = [
         'midi_velocity_dialog.cc',
         'mini_timeline.cc',
         'missing_file_dialog.cc',
+        'missing_filesource_dialog.cc',
         'missing_plugin_dialog.cc',
         'mixer_group_tabs.cc',
         'mixer_strip.cc',