Too many drop-references handlers (#9363)

This fixes a random crash with stop-and-forget capture.

When aborting capture, the disk-writer can emit
 midi_write_source->drop_references ()
in the butler thread, which leads to a direct call to
Session::remove_source.

This can happen before or after Region::source_deleted
is called which is initiated from the same signal.

Furthermore it was possible that Region::source deleted
was called concurrently from the UI thread via SourceRemoved
for whole file regions, which lead to memory corruption.

libs/ardour/ardour/region.h

@@ -545,6 +545,8 @@ private:
 	void register_properties ();
 
 	void use_sources (SourceList const &);
+
+	std::atomic<int> _source_deleted;
 };
 
 } /* namespace ARDOUR */

libs/ardour/audiosource.cc

@@ -116,11 +116,14 @@ AudioSource::AudioSource (Session& s, const XMLNode& node)
 
 AudioSource::~AudioSource ()
 {
-	/* shouldn't happen but make sure we don't leak file descriptors anyway */
-
+#ifndef NDEBUG
+	/* shouldn't happen but make sure we don't leak file descriptors anyway,
+	 * (it can happen with stop-and-forget capture)
+	 */
 	if (peak_leftover_cnt) {
 		cerr << "AudioSource destroyed with leftover peak data pending" << endl;
 	}
+#endif
 
 	if (-1 != _peakfile_fd) {
 		close (_peakfile_fd);

libs/ardour/region.cc

@@ -246,7 +246,7 @@ Region::register_properties ()
 Region::Region (Session& s, timepos_t const & start, timecnt_t const & length, const string& name, DataType type)
 	: SessionObject(s, name)
 	, _type (type)
-        , REGION_DEFAULT_STATE (start,length)
+	, REGION_DEFAULT_STATE (start,length)
 	, _last_length (length)
 	, _first_edit (EditChangesNothing)
 	, _layer (0)
@@ -1552,6 +1552,9 @@ Region::size_equivalent (std::shared_ptr<const Region> other) const
 void
 Region::source_deleted (std::weak_ptr<Source>)
 {
+	if (_source_deleted.fetch_add (1)) {
+		return;
+	}
 	drop_sources ();
 
 	if (!_session.deletion_in_progress()) {
@@ -1957,6 +1960,7 @@ Region::drop_sources ()
 void
 Region::use_sources (SourceList const & s)
 {
+	_source_deleted.store (0);
 	set<std::shared_ptr<Source> > unique_srcs;
 
 	for (SourceList::const_iterator i = s.begin (); i != s.end(); ++i) {

libs/ardour/session.cc

@@ -4666,8 +4666,6 @@ Session::remove_source (std::weak_ptr<Source> src, bool drop_references)
 		return;
 	}
 
-	assert (!source->used ());
-
 	if (!in_cleanup () && !loading ()) {
 		/* save state so we don't end up with a session file
 		 * referring to non-existent sources.

libs/ardour/source.cc

@@ -96,6 +96,7 @@ Source::Source (Session& s, const XMLNode& node)
 Source::~Source ()
 {
 	DEBUG_TRACE (DEBUG::Destruction, string_compose ("Source %1 destructor %2\n", _name, this));
+	assert (!used ());
 }
 
 void
@@ -448,20 +449,13 @@ Source::dec_use_count ()
 {
 #ifndef NDEBUG
 	int oldval = _use_count.fetch_sub (1);
-        if (oldval <= 0) {
-                cerr << "Bad use dec for " << name() << endl;
-                abort ();
-        }
-        assert (oldval > 0);
-#else
-        _use_count.fetch_sub (1);
-#endif
-
-	try {
-		std::shared_ptr<Source> sptr = shared_from_this();
-	} catch (...) {
-		/* no shared_ptr available, relax; */
+	if (oldval <= 0) {
+		cerr << "Bad use dec for " << name() << endl;
 	}
+	assert (oldval > 0);
+#else
+	_use_count.fetch_sub (1);
+#endif
 }
 
 bool