Keep the frame stack balanced when tracing is switched

src/core/IronPython/Runtime/PythonContext.cs

@@ -3849,13 +3849,13 @@ public PythonTracebackListenersDispatcher(PythonContext parent) {
             void Debugging.ITraceCallback.OnTraceEvent(Debugging.TraceEventKind kind, string name, string sourceFileName, SourceSpan sourceSpan, Func<IDictionary<object, object>> scopeCallback, object payload, object customPayload) {
                 var listener = _parent._tracebackListeners.Value;
 
-                if (listener == null && (_parent.PythonOptions.Tracing || _parent._tracebackListenersCount > 0)) {
-                    // If tracing is enabled (globally or via SetTrace on any thread), we need to register a dummy
-                    // traceback listener for this thread, because of the FunctionStack handling done there.
+                if (listener == null) {
+                    // Code compiled for tracing pushes and pops its frames on the FunctionStack only through the listener, so this thread
+                    // needs a dummy listener even without a trace function. This must not depend on whether tracing is on right now.
                     _parent._tracebackListeners.Value = listener = new PythonTracebackListener(_parent, null, isDummyListener: true);
                 }
 
-                listener?.OnTraceEvent(kind, name, sourceFileName, sourceSpan, scopeCallback, payload, customPayload);
+                listener.OnTraceEvent(kind, name, sourceFileName, sourceSpan, scopeCallback, payload, customPayload);
             }
         }
 

src/core/IronPython/Runtime/PythonTracebackListener.cs

@@ -64,7 +64,7 @@ internal bool ExceptionThrown {
             }
         }
 
-        // dummy listener used to maintain the frame stack
+        // dummy listener used to maintain the frame stack on a thread without a trace function
         internal bool IsDummyListener { get; }
 
 #if PROFILE_SUPPORT
@@ -135,7 +135,7 @@ public void OnTraceEvent(Debugging.TraceEventKind kind, string name, string sour
                     pyFrame._lineNo = sourceSpan.Start.Line;
                 }
 
-                if (traceDispatchObject != null && !_exceptionThrown) {
+                if (traceDispatchObject != null && !_exceptionThrown && !IsDummyListener) {
                     DispatchTrace(thread, kind, payload, traceDispatch, traceDispatchObject, pyFrame);
                 }
             } finally {

tests/suite/modules/system_related/test_sys.py

@@ -253,6 +253,52 @@ def f():
         out = subprocess.check_output([sys.executable, '-X:Debug', '-c', code])
         self.assertEqual(out.strip(), b'True')
 
+    def test_settrace_no_frames(self):
+        # A traced function that calls sys.settrace(None) leaves no frames behind on the frame stack
+        def stop():
+            sys.settrace(None)
+        def f():
+            stop()
+
+        sys.settrace(lambda *args: None)
+        try:
+            f()
+        finally:
+            sys.settrace(None)
+        self.assertEqual(sys._getframe(0).f_code.co_name, 'test_settrace_no_frames')
+
+    def test_no_trace_events_after_settrace_none(self):
+        # After sys.settrace(None), no trace events are delivered, not even to frames that have a local trace function,
+        # also while another thread is still being traced
+        import threading
+        events = []
+        def tracer(frame, event, arg):
+            events.append((event, frame.f_code.co_name))
+            return tracer
+        def stop():
+            sys.settrace(None)
+        def f():
+            stop()
+            return 1
+
+        traced, finished = threading.Event(), threading.Event()
+        def other():
+            sys.settrace(lambda *args: None)
+            traced.set()
+            finished.wait()
+            sys.settrace(None)
+        thread = threading.Thread(target=other)
+        thread.start()
+        traced.wait()
+        sys.settrace(tracer)
+        try:
+            f()
+        finally:
+            sys.settrace(None)
+            finished.set()
+            thread.join()
+        self.assertEqual(events, [('call', 'f'), ('line', 'f'), ('call', 'stop'), ('line', 'stop')])
+
     @skipUnlessIronPython()
     @unittest.expectedFailure # dummy getrefcount removed in https://github.com/IronLanguages/ironpython3/pull/577
     def test_getrefcount(self):