From 0f0006cebcc95daca46fd4359ce540deb5f0ffc3 Mon Sep 17 00:00:00 2001
From: Robin Gareus <robin@gareus.org>
Date: Wed, 6 Nov 2024 03:41:17 +0100
Subject: [PATCH] Debug set_session(0) calls preventing session_going_away()

---
 libs/ardour/ardour/session_handle.h | 22 +++++++++++----
 libs/ardour/audioengine.cc          |  5 ++++
 libs/ardour/session_handle.cc       | 44 +++++++++++++++++++++++++++++
 3 files changed, 65 insertions(+), 6 deletions(-)

diff --git a/libs/ardour/ardour/session_handle.h b/libs/ardour/ardour/session_handle.h
index cf2e04dda5..0c660ca054 100644
--- a/libs/ardour/ardour/session_handle.h
+++ b/libs/ardour/ardour/session_handle.h
@@ -22,24 +22,29 @@
 
 #include "ardour/libardour_visibility.h"
 
+#ifndef NDEBUG
+# define TRACE_SETSESSION_NULL
+#endif
+
 namespace ARDOUR {
 	class Session;
 
 class LIBARDOUR_API SessionHandleRef : public PBD::ScopedConnectionList
 {
-  public:
+public:
 	SessionHandleRef (ARDOUR::Session& s);
 	virtual ~SessionHandleRef ();
 
-  protected:
-	ARDOUR::Session&          _session;
+protected:
 	virtual void session_going_away ();
 	virtual void insanity_check ();
+
+	ARDOUR::Session& _session;
 };
 
 class LIBARDOUR_API SessionHandlePtr
 {
-  public:
+public:
 	SessionHandlePtr (ARDOUR::Session* s);
 	SessionHandlePtr () : _session (0) {}
 	virtual ~SessionHandlePtr () {}
@@ -47,11 +52,16 @@ class LIBARDOUR_API SessionHandlePtr
 	virtual void set_session (ARDOUR::Session *);
 	virtual ARDOUR::Session* session() const { return _session; }
 
-  protected:
+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 */
diff --git a/libs/ardour/audioengine.cc b/libs/ardour/audioengine.cc
index 99bb95f12b..481c6dab16 100644
--- a/libs/ardour/audioengine.cc
+++ b/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
diff --git a/libs/ardour/session_handle.cc b/libs/ardour/session_handle.cc
index a8dc792f3f..8a0f246a26 100644
--- a/libs/ardour/session_handle.cc
+++ b/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;
 	}
@@ -49,13 +83,23 @@ 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
 }
 
 /*-------------------------*/