result: keep original traceback and reraise with it

The `Result` API allows the same exception to be raised multiple times.
In Python, `Exception.__traceback__` becomes longer everytime an
exception is raised. This means that on every raise, the exception's
traceback gets longer and longer.

To prevent this, save the original traceback and always raise using it.

Regressed in pluggy 1.1.0 (fbc4442).

changelog/504.bugfix.rst

@@ -0,0 +1 @@
+Fix a regression in pluggy 1.1.0 where using :func:`result.get_result() <pluggy.Result.get_result>` on the same failed :class:`~pluggy.Result` causes the exception's traceback to get longer and longer.

src/pluggy/_result.py

@@ -28,7 +28,7 @@ class Result(Generic[ResultType]):
     """An object used to inspect and set the result in a :ref:`hook wrapper
     <hookwrappers>`."""
 
-    __slots__ = ("_result", "_exception")
+    __slots__ = ("_result", "_exception", "_traceback")
 
     def __init__(
         self,
@@ -38,6 +38,8 @@ def __init__(
         """:meta private:"""
         self._result = result
         self._exception = exception
+        # Exception __traceback__ is mutable, this keeps the original.
+        self._traceback = exception.__traceback__ if exception is not None else None
 
     @property
     def excinfo(self) -> _ExcInfo | None:
@@ -46,7 +48,7 @@ def excinfo(self) -> _ExcInfo | None:
         if exc is None:
             return None
         else:
-            return (type(exc), exc, exc.__traceback__)
+            return (type(exc), exc, self._traceback)
 
     @property
     def exception(self) -> BaseException | None:
@@ -75,6 +77,7 @@ def force_result(self, result: ResultType) -> None:
         """
         self._result = result
         self._exception = None
+        self._traceback = None
 
     def force_exception(self, exception: BaseException) -> None:
         """Force the result to fail with ``exception``.
@@ -85,6 +88,7 @@ def force_exception(self, exception: BaseException) -> None:
         """
         self._result = None
         self._exception = exception
+        self._traceback = exception.__traceback__ if exception is not None else None
 
     def get_result(self) -> ResultType:
         """Get the result(s) for this hook call.
@@ -94,10 +98,11 @@ def get_result(self) -> ResultType:
         """
         __tracebackhide__ = True
         exc = self._exception
+        tb = self._traceback
         if exc is None:
             return cast(ResultType, self._result)
         else:
-            raise exc
+            raise exc.with_traceback(tb)
 
 
 # Historical name (pluggy<=1.2), kept for backward compatibility.

testing/test_result.py

@@ -0,0 +1,27 @@
+import traceback
+
+from pluggy import Result
+
+
+def test_exceptions_traceback_doesnt_get_longer_and_longer() -> None:
+    def bad() -> None:
+        1 / 0
+
+    result = Result.from_call(bad)
+
+    try:
+        result.get_result()
+    except Exception as exc:
+        tb1 = traceback.extract_tb(exc.__traceback__)
+
+    try:
+        result.get_result()
+    except Exception as exc:
+        tb2 = traceback.extract_tb(exc.__traceback__)
+
+    try:
+        result.get_result()
+    except Exception as exc:
+        tb3 = traceback.extract_tb(exc.__traceback__)
+
+    assert len(tb1) == len(tb2) == len(tb3)