Replace missing .mid files with silence

This fixes a crash: missing playlist due to missing .mid,
and retains regions for missing MIDI files.

As opposed to missing Audio, we cannot use a SilentFileSource,
because MIDI files are destructive.

This also adds an API to query missing files that have been replaced
with silence to report them to the user.

4 files changed, 39 insertions, 16 deletions

diff --git a/libs/ardour/ardour/session.h b/libs/ardour/ardour/session.h
index daaecd8a38..8fe209a552 100644
--- a/libs/ardour/ardour/session.h
+++ b/libs/ardour/ardour/session.h
@@ -1161,6 +1161,8 @@ public:
 
 	std::list<std::string> unknown_processors () const;
 
+	std::list<std::string> missing_filesources (DataType) const;
+
 	/** Emitted when a feedback cycle has been detected within Ardour's signal
 	    processing path.  Until it is fixed (by the user) some (unspecified)
 	    routes will not be run.
diff --git a/libs/ardour/ardour/source.h b/libs/ardour/ardour/source.h
index 56e72acd42..9604062b8e 100644
--- a/libs/ardour/ardour/source.h
+++ b/libs/ardour/ardour/source.h
@@ -56,6 +56,7 @@ public:
 		Destructive = 0x80,
 		Empty = 0x100, /* used for MIDI only */
 		RF64_RIFF = 0x200,
+		Missing = 0x400, /* used for MIDI only */
 	};
 
 	typedef Glib::Threads::Mutex::Lock Lock;
diff --git a/libs/ardour/session.cc b/libs/ardour/session.cc
index de68cd9830..cfc7f9b278 100644
--- a/libs/ardour/session.cc
+++ b/libs/ardour/session.cc
@@ -103,6 +103,7 @@
 #include "ardour/route_graph.h"
 #include "ardour/route_group.h"
 #include "ardour/rt_tasklist.h"
+#include "ardour/silentfilesource.h"
 #include "ardour/send.h"
 #include "ardour/selection.h"
 #include "ardour/session.h"
@@ -6338,6 +6339,22 @@ Session::unknown_processors () const
 	return p;
 }
 
+list<string>
+Session::missing_filesources (DataType dt) const
+{
+	list<string> p;
+	for (SourceMap::const_iterator i = sources.begin(); i != sources.end(); ++i) {
+		if (dt == DataType::AUDIO && 0 != boost::dynamic_pointer_cast<SilentFileSource> (i->second)) {
+			p.push_back (i->second->name());
+		}
+		else if (dt == DataType::MIDI && 0 != boost::dynamic_pointer_cast<SMFSource> (i->second) && (i->second->flags() & Source::Missing) != 0) {
+			p.push_back (i->second->name());
+		}
+	}
+	p.sort ();
+	return p;
+}
+
 void
 Session::initialize_latencies ()
 {
diff --git a/libs/ardour/smf_source.cc b/libs/ardour/smf_source.cc
index e0d65176c3..164b5971ab 100644
--- a/libs/ardour/smf_source.cc
+++ b/libs/ardour/smf_source.cc
@@ -156,23 +156,24 @@ SMFSource::SMFSource (Session& s, const XMLNode& node, bool must_exist)
 		}
 
 	} catch (MissingSource& err) {
-
-		if (_flags & Source::Empty) {
-			/* we don't care that the file was not found, because
-			   it was empty. But FileSource::init() will have
-			   failed to set our _path correctly, so we have to do
-			   this ourselves. Use the first entry in the search
-			   path for MIDI files, which is assumed to be the
-			   correct "main" location.
-			*/
-			std::vector<string> sdirs = s.source_search_path (DataType::MIDI);
-			_path = Glib::build_filename (sdirs.front(), _path);
-			/* This might be important, too */
-			_file_is_new = true;
-		} else {
-			/* pass it on */
-			throw;
+		if (0 == (_flags & Source::Empty)) {
+			/* Don't throw, create the source.
+			 * Since MIDI is writable, we cannot use a SilentFileSource.
+			 */
+			_flags = Source::Flag (_flags | Source::Empty | Source::Missing);
 		}
+
+		/* we don't care that the file was not found, because
+			 it was empty. But FileSource::init() will have
+			 failed to set our _path correctly, so we have to do
+			 this ourselves. Use the first entry in the search
+			 path for MIDI files, which is assumed to be the
+			 correct "main" location.
+		*/
+		std::vector<string> sdirs = s.source_search_path (DataType::MIDI);
+		_path = Glib::build_filename (sdirs.front(), _path);
+		/* This might be important, too */
+		_file_is_new = true;
 	}
 
 	if (!(_flags & Source::Empty)) {
@@ -451,6 +452,7 @@ SMFSource::append_event_beats (const Glib::Threads::Mutex::Lock&   lock,
 	Evoral::SMF::append_event_delta(delta_time_ticks, ev.size(), ev.buffer(), event_id);
 	_last_ev_time_beats = time;
 	_flags = Source::Flag (_flags & ~Empty);
+	_flags = Source::Flag (_flags & ~Missing);
 }
 
 /** Append an event with a timestamp in samples (samplepos_t) */
@@ -501,6 +503,7 @@ SMFSource::append_event_samples (const Glib::Threads::Mutex::Lock& lock,
 	Evoral::SMF::append_event_delta(delta_time_ticks, ev.size(), ev.buffer(), event_id);
 	_last_ev_time_samples = ev.time();
 	_flags = Source::Flag (_flags & ~Empty);
+	_flags = Source::Flag (_flags & ~Missing);
 }
 
 XMLNode&