fixtures: fix tracebacks for higher-scoped failed fixtures getting longer and longer

Fix #12204.

changelog/12204.bugfix.rst

@@ -0,0 +1,4 @@
+Fix a regression in pytest 8.0 where tracebacks get longer and longer when multiple tests fail due to a shared higher-scope fixture which raised.
+
+The fix necessitated internal changes which may affect some plugins:
+- ``FixtureDef.cached_result[2]`` is now a tuple ``(exc, tb)`` instead of ``exc``.

src/_pytest/fixtures.py

@@ -8,6 +8,7 @@ import inspect
 import os
 from pathlib import Path
 import sys
+import types
 from typing import AbstractSet
 from typing import Any
 from typing import Callable
@@ -104,8 +105,8 @@ _FixtureCachedResult = Union[
         None,
         # Cache key.
         object,
-        # Exception if raised.
+        # The exception and the original traceback.
-        BaseException,
+        Tuple[BaseException, Optional[types.TracebackType]],
     ],
 ]
 
@@ -1049,8 +1050,8 @@ class FixtureDef(Generic[FixtureValue]):
             # numpy arrays (#6497).
             if my_cache_key is cache_key:
                 if self.cached_result[2] is not None:
-                    exc = self.cached_result[2]
+                    exc, exc_tb = self.cached_result[2]
-                    raise exc
+                    raise exc.with_traceback(exc_tb)
                 else:
                     result = self.cached_result[0]
                     return result
@@ -1126,7 +1127,7 @@ def pytest_fixture_setup(
             # Don't show the fixture as the skip location, as then the user
             # wouldn't know which test skipped.
             e._use_item_location = True
-        fixturedef.cached_result = (None, my_cache_key, e)
+        fixturedef.cached_result = (None, my_cache_key, (e, e.__traceback__))
         raise
     fixturedef.cached_result = (result, my_cache_key, None)
     return result

testing/python/fixtures.py

@@ -3397,6 +3397,28 @@ class TestErrors:
             ["*def gen(qwe123):*", "*fixture*qwe123*not found*", "*1 error*"]
         )
 
+    def test_cached_exception_doesnt_get_longer(self, pytester: Pytester) -> None:
+        """Regression test for #12204."""
+        pytester.makepyfile(
+            """
+            import pytest
+            @pytest.fixture(scope="session")
+            def bad(): 1 / 0
+
+            def test_1(bad): pass
+            def test_2(bad): pass
+            def test_3(bad): pass
+            """
+        )
+
+        result = pytester.runpytest_inprocess("--tb=native")
+        assert result.ret == ExitCode.TESTS_FAILED
+        failures = result.reprec.getfailures()  # type: ignore[attr-defined]
+        assert len(failures) == 3
+        lines1 = failures[1].longrepr.reprtraceback.reprentries[0].lines
+        lines2 = failures[2].longrepr.reprtraceback.reprentries[0].lines
+        assert len(lines1) == len(lines2)
+
 
 class TestShowFixtures:
     def test_funcarg_compat(self, pytester: Pytester) -> None: