Fix unbounded recursion when traceback contains an exception that has a reference to itself in its cause chain (hynek#739)

* Fix unbounded traceback recursion

* Update CHANGELOG.md

* Fixes & attributions

Co-authored-by: yuwu <47444399+leiyi2000@users.noreply.github.com>
Co-authored-by: William FH <13333726+hinthornw@users.noreply.github.com>

---------

Co-authored-by: Hynek Schlawack <hs@ox.cx>
Co-authored-by: yuwu <47444399+leiyi2000@users.noreply.github.com>
Co-authored-by: William FH <13333726+hinthornw@users.noreply.github.com>

Author: 4 people

CHANGELOG.md

@@ -37,6 +37,9 @@ You can find our backwards-compatibility policy [here](https://github.com/hynek/
 - `structlog.processors.MaybeTimeStamper` now respects the *key* argument when determining whether to overwrite the timestamp field.
   [#747](https://github.com/hynek/structlog/pull/747)
 
+- `structlog.tracebacks.extract()` no longer raises a *RecursionError* when the cause chain of an exception contains itself.
+  [#739](https://github.com/hynek/structlog/pull/739)
+
 
 ## [25.4.0](https://github.com/hynek/structlog/compare/25.3.0...25.4.0) - 2025-06-02
 

src/structlog/tracebacks.py

@@ -190,6 +190,7 @@ def extract(
     locals_hide_dunder: bool = True,
     locals_hide_sunder: bool = False,
     use_rich: bool = True,
+    _seen: set[int] | None = None,
 ) -> Trace:
     """
     Extract traceback information.
@@ -235,12 +236,23 @@ def extract(
 
     .. versionchanged:: 25.4.0
        Handle exception groups.
+
+    .. versionchanged:: 25.5.0
+       Handle loops in exception cause chain.
     """
 
     stacks: list[Stack] = []
     is_cause = False
 
+    if _seen is None:
+        _seen = set()
+
     while True:
+        exc_id = id(exc_value)
+        if exc_id in _seen:
+            break
+        _seen.add(exc_id)
+
         stack = Stack(
             exc_type=safe_str(exc_type.__name__),
             exc_value=safe_str(exc_value),
@@ -265,6 +277,7 @@ def extract(
                             locals_hide_dunder=locals_hide_dunder,
                             locals_hide_sunder=locals_hide_sunder,
                             use_rich=use_rich,
+                            _seen=_seen,
                         )
                     )
 

tests/test_tracebacks.py

@@ -1071,3 +1071,121 @@ def test_no_exception(
         logger.exception("onoes")
 
         assert [{"event": "onoes", "log_level": "error"}] == cap_logs.entries
+
+
+@pytest.mark.skipif(
+    sys.version_info < (3, 11), reason="Requires Python 3.11 or higher"
+)
+def test_reraise_error_from_exception_group():
+    """
+    There is no RecursionError when building the traceback for an exception
+    that has been re-raised from an ExceptionGroup.
+    """
+    inner_lineno = None
+    lineno = None
+
+    try:
+        try:
+            inner_lineno = get_next_lineno()
+            raise ExceptionGroup(  # noqa: F821
+                "Some error occurred",
+                [ValueError("value error")],
+            )
+        except ExceptionGroup as e:  # noqa: F821
+            lineno = get_next_lineno()
+            raise e.exceptions[0]  # noqa: B904
+    except Exception as e:
+        trace = tracebacks.extract(type(e), e, e.__traceback__)
+
+    assert lineno is not None
+    assert inner_lineno is not None
+    assert 2 == len(trace.stacks)
+    assert lineno == trace.stacks[0].frames[0].lineno
+    assert (
+        tracebacks.Stack(
+            exc_type="ValueError",
+            exc_value="value error",
+            syntax_error=None,
+            is_cause=False,
+            frames=[
+                tracebacks.Frame(
+                    filename=__file__,
+                    lineno=lineno,
+                    name="test_reraise_error_from_exception_group",
+                    locals=None,
+                )
+            ],
+            is_group=False,
+            exceptions=[],
+        )
+        == trace.stacks[0]
+    )
+    assert (
+        tracebacks.Stack(
+            exc_type="ExceptionGroup",
+            exc_value="Some error occurred (1 sub-exception)",
+            syntax_error=None,
+            is_cause=False,
+            frames=[
+                tracebacks.Frame(
+                    filename=__file__,
+                    lineno=inner_lineno,
+                    name="test_reraise_error_from_exception_group",
+                    locals=None,
+                ),
+            ],
+            is_group=True,
+            exceptions=[tracebacks.Trace(stacks=[])],
+        )
+        == trace.stacks[1]
+    )
+
+
+def test_exception_cycle():
+    """
+    There is no RecursionError when building the traceback for an exception
+    that has itself in its cause chain.
+    """
+    inner_lineno = None
+    lineno = None
+
+    try:
+        try:
+            exc = ValueError("onoes")
+            inner_lineno = get_next_lineno()
+            raise exc
+        except Exception as exc:
+            lineno = get_next_lineno()
+            raise exc from exc
+    except Exception as e:
+        trace = tracebacks.extract(type(e), e, e.__traceback__)
+
+    assert lineno is not None
+    assert inner_lineno is not None
+    assert 1 == len(trace.stacks)
+    assert lineno == trace.stacks[0].frames[0].lineno
+    assert (
+        tracebacks.Stack(
+            exc_type="ValueError",
+            exc_value="onoes",
+            syntax_error=None,
+            is_cause=False,
+            frames=[
+                tracebacks.Frame(
+                    filename=__file__,
+                    lineno=lineno,
+                    name="test_exception_cycle",
+                    locals=None,
+                ),
+                tracebacks.Frame(
+                    filename=__file__,
+                    lineno=inner_lineno,
+                    name="test_exception_cycle",
+                    locals=None,
+                ),
+            ],
+            is_group=False,
+            exceptions=[],
+        )
+        == trace.stacks[0]
+    )