Debug set_session(0) calls preventing session_going_away()

libs/ardour/ardour/session_handle.h

@@ -22,6 +22,10 @@
 
 #include "ardour/libardour_visibility.h"
 
+#ifndef NDEBUG
+# define TRACE_SETSESSION_NULL
+#endif
+
 namespace ARDOUR {
 	class Session;
 
@@ -32,9 +36,10 @@ class LIBARDOUR_API SessionHandleRef : public PBD::ScopedConnectionList
 	virtual ~SessionHandleRef ();
 
 protected:
-	ARDOUR::Session&          _session;
 	virtual void session_going_away ();
 	virtual void insanity_check ();
+
+	ARDOUR::Session& _session;
 };
 
 class LIBARDOUR_API SessionHandlePtr
@@ -48,10 +53,15 @@ class LIBARDOUR_API SessionHandlePtr
 	virtual ARDOUR::Session* session() const { return _session; }
 
 protected:
+	virtual void session_going_away ();
+
 	ARDOUR::Session*          _session;
 	PBD::ScopedConnectionList _session_connections;
 
-	virtual void session_going_away ();
+#ifdef TRACE_SETSESSION_NULL
+private:
+	bool _gone_away_emitted;
+#endif
 };
 
 } /* namespace */

libs/ardour/audioengine.cc

@@ -463,7 +463,12 @@ AudioEngine::process_callback (pframes_t nframes)
 			/* fade out done */
 			PortManager::silence_outputs (nframes);
 			session_deleted = true;
+#ifdef TRACE_SETSESSION_NULL
+			_session_connections.drop_connections ();
+			_session = 0;
+#else
 			SessionHandlePtr::set_session (0);
+#endif
 			session_removal_countdown = -1; // reset to "not in progress"
 			session_remove_pending = false;
 			session_removed.signal(); // wakes up thread that initiated session removal

libs/ardour/session_handle.cc

@@ -19,6 +19,11 @@
 #include "pbd/demangle.h"
 #include "pbd/error.h"
 
+#ifdef TRACE_SETSESSION_NULL
+#include <cassert>
+#include "pbd/stacktrace.h"
+#endif
+
 #include "ardour/boost_debug.h"
 #include "ardour/session.h"
 #include "ardour/session_handle.h"
@@ -31,6 +36,9 @@ using namespace PBD;
 
 SessionHandlePtr::SessionHandlePtr (Session* s)
 	: _session (s)
+#ifdef TRACE_SETSESSION_NULL
+	, _gone_away_emitted (false)
+#endif
 {
 	if (_session) {
 		_session->DropReferences.connect_same_thread (_session_connections, std::bind (&SessionHandlePtr::session_going_away, this));
@@ -42,6 +50,32 @@ SessionHandlePtr::set_session (Session* s)
 {
 	_session_connections.drop_connections ();
 
+#ifdef TRACE_SETSESSION_NULL
+	/* DropReferences may already have been disconnected due to signal emission ordering.
+	 *
+	 * An instance of this class (e.g. Ardour_UI) will need to call ::set_session() on member instances.
+	 *
+	 * Yet, when session_going_away() first calls set_session (0) on an instance that has SessionHandlePtr members,
+	 * they will reach here, and disocnnect signal handlers. Their derived implementation of ::session_going_away()
+	 * will not be called.
+	 */
+	if (!_gone_away_emitted && _session && !s) {
+		/* if this assert goes off, some ::set_session() implementation calls
+		 * some_member->set_session (0);
+		 *
+		 * replace it with
+		 *
+		 * if (session) { 
+		 *   some_member->set_session (session);
+		 * }
+		 */
+		PBD::stacktrace (cerr, 10);
+		assert (0);
+		_gone_away_emitted = true;
+		session_going_away ();
+	}
+#endif
+
 	if (_session) {
 		_session = 0;
 	}
@@ -49,14 +83,24 @@ SessionHandlePtr::set_session (Session* s)
 	if (s) {
 		_session = s;
 		_session->DropReferences.connect_same_thread (_session_connections, std::bind (&SessionHandlePtr::session_going_away, this));
+#ifdef TRACE_SETSESSION_NULL
+		_gone_away_emitted = false;
+#endif
 	}
 }
 
 void
 SessionHandlePtr::session_going_away ()
 {
+#ifdef TRACE_SETSESSION_NULL
+	if (_session && !_gone_away_emitted) {
+		_gone_away_emitted = true;
 		set_session (0);
 	}
+#else
+	set_session (0);
+#endif
+}
 
 /*-------------------------*/