fix: Add cycle detection to exceptions_from_error

sentry_sdk/utils.py

@@ -819,6 +819,8 @@ def exceptions_from_error(
     parent_id: int = 0,
     source: "Optional[str]" = None,
     full_stack: "Optional[list[dict[str, Any]]]" = None,
+    seen_exceptions: "Optional[list]" = None,
+    seen_exception_ids: "Optional[Set[int]]" = None,
 ) -> "Tuple[int, List[Dict[str, Any]]]":
     """
     Creates the list of exceptions.
@@ -828,6 +830,17 @@ def exceptions_from_error(
     https://develop.sentry.dev/sdk/event-payloads/exception/
     """
 
+    if seen_exception_ids is None:
+        seen_exceptions = []
+        seen_exception_ids = set()
+
+    if exc_value is not None and id(exc_value) in seen_exception_ids:
+        return (exception_id, [])
+
+    if exc_value is not None:
+        seen_exceptions.append(exc_value)
+        seen_exception_ids.add(id(exc_value))
+
     parent = single_exception_from_error_tuple(
         exc_type=exc_type,
         exc_value=exc_value,
@@ -866,6 +879,8 @@ def exceptions_from_error(
                 exception_id=exception_id,
                 source="__cause__",
                 full_stack=full_stack,
+                seen_exceptions=seen_exceptions,
+                seen_exception_ids=seen_exception_ids,
             )
             exceptions.extend(child_exceptions)
 
@@ -888,6 +903,8 @@ def exceptions_from_error(
                 exception_id=exception_id,
                 source="__context__",
                 full_stack=full_stack,
+                seen_exceptions=seen_exceptions,
+                seen_exception_ids=seen_exception_ids,
             )
             exceptions.extend(child_exceptions)
 
@@ -905,6 +922,8 @@ def exceptions_from_error(
                 parent_id=parent_id,
                 source="exceptions[%s]" % idx,
                 full_stack=full_stack,
+                seen_exceptions=seen_exceptions,
+                seen_exception_ids=seen_exception_ids,
             )
             exceptions.extend(child_exceptions)
 

tests/test_exceptiongroup.py

@@ -306,3 +306,182 @@ def test_simple_exception():
 
     exception_values = event["exception"]["values"]
     assert exception_values == expected_exception_values
+
+
+@minimum_python_311
+def test_exceptiongroup_starlette_collapse():
+    """
+    Simulates the Starlette collapse_excgroups() pattern where a single-exception
+    ExceptionGroup is caught and the inner exception is unwrapped and re-raised.
+
+    See: https://github.com/Kludex/starlette/blob/0e88e92b592bfa11fd92e331869a8d49ba34b541/starlette/_utils.py#L79-L87
+
+    When using FastAPI with multiple BaseHTTPMiddleware instances, anyio wraps
+    exceptions in ExceptionGroups. Starlette's collapse_excgroups() then unwraps
+    single-exception groups and re-raises the inner exception.
+
+    When re-raising the unwrapped exception, Python implicitly sets __context__
+    on it pointing back to the ExceptionGroup (because the re-raise happens
+    inside the except block that caught the ExceptionGroup), creating a cycle:
+
+        ExceptionGroup -> .exceptions[0] -> ValueError -> __context__ -> ExceptionGroup
+
+    Without cycle detection in exceptions_from_error(), this causes infinite
+    recursion and a silent RecursionError that drops the event.
+    """
+    exception_group = None
+
+    try:
+        try:
+            raise RuntimeError("something")
+        except RuntimeError:
+            raise ExceptionGroup(
+                "nested",
+                [
+                    ValueError(654),
+                ],
+            )
+    except ExceptionGroup as exc:
+        exception_group = exc
+
+        # Simulate Starlette's collapse_excgroups() as seen here:
+        # https://github.com/Kludex/starlette/blob/0e88e92b592bfa11fd92e331869a8d49ba34b541/starlette/_utils.py#L79-L87
+        #
+        # When an ExceptionGroup contains a single exception, collapse_excgroups
+        # unwraps it and re-raises the inner exception. This causes Python to
+        # implicitly set unwrapped.__context__ = ExceptionGroup (because the
+        # re-raise happens inside the except block handling the ExceptionGroup),
+        # creating a cycle:
+        #   exception_group -> .exceptions[0] -> ValueError -> __context__ -> exception_group
+        unwrapped = exc.exceptions[0]
+        try:
+            raise unwrapped
+        except Exception:
+            pass
+
+    (event, _) = event_from_exception(
+        exception_group,
+        client_options={
+            "include_local_variables": True,
+            "include_source_context": True,
+            "max_value_length": 1024,
+        },
+        mechanism={"type": "test_suite", "handled": False},
+    )
+
+    values = event["exception"]["values"]
+
+    # For this test the stacktrace and the module is not important
+    for x in values:
+        if "stacktrace" in x:
+            del x["stacktrace"]
+        if "module" in x:
+            del x["module"]
+
+    expected_values = [
+        {
+            "mechanism": {
+                "exception_id": 2,
+                "handled": False,
+                "parent_id": 0,
+                "source": "exceptions[0]",
+                "type": "chained",
+            },
+            "type": "ValueError",
+            "value": "654",
+        },
+        {
+            "mechanism": {
+                "exception_id": 1,
+                "handled": False,
+                "parent_id": 0,
+                "source": "__context__",
+                "type": "chained",
+            },
+            "type": "RuntimeError",
+            "value": "something",
+        },
+        {
+            "mechanism": {
+                "exception_id": 0,
+                "handled": False,
+                "is_exception_group": True,
+                "type": "test_suite",
+            },
+            "type": "ExceptionGroup",
+            "value": "nested",
+        },
+    ]
+
+    assert values == expected_values
+
+
+@minimum_python_311
+def test_cyclic_exception_group_cause():
+    """
+    Test case related to `test_exceptiongroup_starlette_collapse` above. We want to make sure that
+    the same cyclic loop cannot happen via the __cause__ as well as the __context__
+    """
+    # Construct the exact cyclic structure that anyio/Starlette creates when
+    # an exception propagates through multiple BaseHTTPMiddleware layers.
+    original = ValueError("original error")
+    group = ExceptionGroup("unhandled errors in a TaskGroup", [original])
+    original.__cause__ = group
+    original.__suppress_context__ = True
+
+    # When the ExceptionGroup is the top-level exception, exceptions_from_error
+    # is called directly (not walk_exception_chain which has cycle detection).
+    (event, _) = event_from_exception(
+        group,
+        client_options={
+            "include_local_variables": True,
+            "include_source_context": True,
+            "max_value_length": 1024,
+        },
+        mechanism={"type": "test_suite", "handled": False},
+    )
+
+    exception_values = event["exception"]["values"]
+
+    # Must produce a finite list of exceptions without hitting RecursionError.
+    assert len(exception_values) >= 1
+    exc_types = [v["type"] for v in exception_values]
+    assert "ExceptionGroup" in exc_types
+    assert "ValueError" in exc_types
+
+
+@minimum_python_311
+def test_deeply_nested_cyclic_exception_group():
+    """
+    Related to the `test_exceptiongroup_starlette_collapse` test above.
+
+    Testing a more complex cycle: ExceptionGroup -> ValueError -> __cause__ ->
+    ExceptionGroup (nested) -> TypeError -> __cause__ -> original ExceptionGroup
+    """
+    inner_error = TypeError("inner")
+    outer_error = ValueError("outer")
+    inner_group = ExceptionGroup("inner group", [inner_error])
+    outer_group = ExceptionGroup("outer group", [outer_error])
+
+    # Create a cycle spanning two ExceptionGroups
+    outer_error.__cause__ = inner_group
+    outer_error.__suppress_context__ = True
+    inner_error.__cause__ = outer_group
+    inner_error.__suppress_context__ = True
+
+    (event, _) = event_from_exception(
+        outer_group,
+        client_options={
+            "include_local_variables": True,
+            "include_source_context": True,
+            "max_value_length": 1024,
+        },
+        mechanism={"type": "test_suite", "handled": False},
+    )
+
+    exception_values = event["exception"]["values"]
+    assert len(exception_values) >= 1
+    exc_types = [v["type"] for v in exception_values]
+    assert "ExceptionGroup" in exc_types
+    assert "ValueError" in exc_types
+    assert "TypeError" in exc_types