Fix evaluation of variables from chained exception frames (#2018)

* Fix evaluation of variables from chained exception frames

* Move frame registration into a shared helper function

Author: pdepetro

src/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_suspended_frames.py

@@ -394,16 +394,27 @@ def track(self, thread_id, frames_list, frame_custom_thread_id=None):
 
             frame_ids_from_thread = self._thread_id_to_frame_ids.setdefault(coroutine_or_main_thread_id, [])
 
-            self._thread_id_to_frames_list[coroutine_or_main_thread_id] = frames_list
-            for frame in frames_list:
+            def _register_frame(frame):
                 frame_id = id(frame)
                 self._frame_id_to_frame[frame_id] = frame
                 _FrameVariable(self.py_db, frame, self._register_variable)  # Instancing is enough to register.
                 self._suspended_frames_manager._variable_reference_to_frames_tracker[frame_id] = self
                 frame_ids_from_thread.append(frame_id)
-
                 self._frame_id_to_main_thread_id[frame_id] = thread_id
 
+            self._thread_id_to_frames_list[coroutine_or_main_thread_id] = frames_list
+            for frame in frames_list:
+                _register_frame(frame)
+
+            # Also track frames from chained exceptions (e.g. __cause__ / __context__)
+            # so that variable evaluation works for chained exception frames displayed
+            # in the call stack.
+            chained = getattr(frames_list, 'chained_frames_list', None)
+            while chained is not None and len(chained) > 0:
+                for frame in chained:
+                    _register_frame(frame)
+                chained = getattr(chained, 'chained_frames_list', None)
+
             frame = None
 
     def untrack_all(self):

src/debugpy/_vendored/pydevd/tests_python/test_debugger_json.py

@@ -926,6 +926,80 @@ def additional_output_checks(writer, stdout, stderr):
         writer.finished_ok = True
 
 
+def test_case_chained_exception_variables(case_setup_dap, pyfile):
+    """
+    When stopped on a chained exception, variable evaluation must work for
+    frames belonging to the chained (cause) exception, not just the primary one.
+    """
+
+    @pyfile
+    def target():
+        def inner():
+            cause_var = "from_cause"  # noqa
+            raise RuntimeError("the cause")
+
+        def outer():
+            outer_var = "from_outer"  # noqa
+            try:
+                inner()
+            except Exception as e:
+                raise ValueError("the effect") from e  # raise line
+
+        outer()
+
+    def check_test_suceeded_msg(self, stdout, stderr):
+        return "the cause" in "".join(stderr)
+
+    def additional_output_checks(writer, stdout, stderr):
+        assert 'raise RuntimeError("the cause")' in stderr
+        assert 'raise ValueError("the effect") from e' in stderr
+
+    with case_setup_dap.test_file(
+        target,
+        EXPECTED_RETURNCODE=1,
+        check_test_suceeded_msg=check_test_suceeded_msg,
+        additional_output_checks=additional_output_checks,
+    ) as writer:
+        json_facade = JsonFacade(writer)
+
+        json_facade.write_launch(justMyCode=False)
+        json_facade.write_set_exception_breakpoints(["uncaught"])
+        json_facade.write_make_initial_run()
+
+        json_hit = json_facade.wait_for_thread_stopped(
+            reason="exception", line=writer.get_line_index_with_content("raise line")
+        )
+
+        stack_frames = json_hit.stack_trace_response.body.stackFrames
+
+        # Find the chained exception frames.
+        chained_frames = [f for f in stack_frames if f["name"].startswith("[Chained Exc:")]
+        assert len(chained_frames) > 0, "Expected chained exception frames in stack trace"
+
+        # Verify variables can be retrieved for chained frames (this is the
+        # operation that previously failed with "Unable to find thread to
+        # evaluate variable reference.").
+        for chained_frame in chained_frames:
+            variables_response = json_facade.get_variables_response(chained_frame["id"])
+            assert variables_response.success
+
+        # Find the inner() chained frame and verify its local variable.
+        inner_frames = [f for f in chained_frames if "inner" in f["name"]]
+        assert len(inner_frames) == 1
+        variables_response = json_facade.get_variables_response(inner_frames[0]["id"])
+        var_names = [v["name"] for v in variables_response.body.variables]
+        assert "cause_var" in var_names, "Expected 'cause_var' in chained frame variables, got: %s" % var_names
+
+        # Also verify that primary frame variables still work.
+        primary_frame_id = json_hit.frame_id
+        variables_response = json_facade.get_variables_response(primary_frame_id)
+        assert variables_response.success
+
+        json_facade.write_continue()
+
+        writer.finished_ok = True
+
+
 def test_case_throw_exc_reason_shown(case_setup_dap):
 
     def check_test_suceeded_msg(self, stdout, stderr):

src/debugpy/_vendored/pydevd/tests_python/test_suspended_frames_manager.py

@@ -156,3 +156,73 @@ def test_get_child_variables():
                 raise AssertionError("Expected to find variable named: %s" % (TOO_LARGE_ATTR,))
             if not found_len:
                 raise AssertionError("Expected to find variable named: len()")
+
+
+def test_chained_exception_frames_tracked():
+    """
+    When an exception has chained causes (__cause__ / __context__), the chained
+    frames are shown in the call stack.  Variable evaluation must also work for
+    those frames, which requires them to be registered in the
+    SuspendedFramesManager.  Uses a 3-level chain to verify all levels are walked.
+    """
+    from _pydevd_bundle.pydevd_suspended_frames import SuspendedFramesManager
+    from _pydevd_bundle.pydevd_constants import EXCEPTION_TYPE_USER_UNHANDLED
+
+    def level0():
+        local0 = "from_level_0"  # noqa
+        raise RuntimeError("level_0")
+
+    def level1():
+        local1 = "from_level_1"  # noqa
+        try:
+            level0()
+        except Exception as e:
+            raise TypeError("level_1") from e
+
+    def level2():
+        local2 = "from_level_2"  # noqa
+        try:
+            level1()
+        except Exception as e:
+            raise ValueError("level_2") from e
+
+    try:
+        level2()
+    except Exception:
+        exc_type, exc_desc, trace_obj = sys.exc_info()
+        frame = sys._getframe()
+        frames_list = pydevd_frame_utils.create_frames_list_from_traceback(
+            trace_obj, frame, exc_type, exc_desc,
+            exception_type=EXCEPTION_TYPE_USER_UNHANDLED,
+        )
+
+    # Collect all chained levels.
+    chained_levels = []
+    cur = frames_list
+    while getattr(cur, "chained_frames_list", None) is not None:
+        chained_levels.append(cur.chained_frames_list)
+        cur = cur.chained_frames_list
+    assert len(chained_levels) == 2
+
+    suspended_frames_manager = SuspendedFramesManager()
+    with suspended_frames_manager.track_frames(_DummyPyDB()) as tracker:
+        thread_id = "thread1"
+        tracker.track(thread_id, frames_list)
+
+        # Primary and all chained frames must be tracked.
+        for f in frames_list:
+            assert suspended_frames_manager.get_thread_id_for_variable_reference(id(f)) == thread_id
+        for level in chained_levels:
+            for f in level:
+                assert suspended_frames_manager.get_thread_id_for_variable_reference(id(f)) == thread_id
+
+        # Variable retrieval must work for the deepest chained frames.
+        for f in chained_levels[-1]:
+            assert suspended_frames_manager.get_variable(id(f)).get_children_variables() is not None
+
+    # After untracking, all references must be gone.
+    for f in frames_list:
+        assert suspended_frames_manager.get_thread_id_for_variable_reference(id(f)) is None
+    for level in chained_levels:
+        for f in level:
+            assert suspended_frames_manager.get_thread_id_for_variable_reference(id(f)) is None