pytester: avoid unraisableexception gc collects in inline runs to speed up test suite

Because `pytester.runpytest()` executes the full session cycle (including
`pytest_unconfigure`), it was calling `gc.collect()` in a loop multiple
times—even for small, fast tests. This significantly increased the total
test suite runtime.

To optimize performance, disable the gc runs in inline pytester runs
entirely, matching the behavior before pytest-dev#12958.

Locally the test suite runtime improved dramatically, dropping from 425s
to 160s.

Fixes pytest-dev#13482.

Co-authored-by: Bruno Oliveira <[email protected]>

Author: bluetech

src/_pytest/pytester.py

@@ -65,6 +65,7 @@
 from _pytest.reports import CollectReport
 from _pytest.reports import TestReport
 from _pytest.tmpdir import TempPathFactory
+from _pytest.unraisableexception import gc_collect_iterations_key
 from _pytest.warning_types import PytestFDWarning
 
 
@@ -1115,12 +1116,16 @@ def inline_run(
 
             rec = []
 
-            class Collect:
+            class PytesterHelperPlugin:
                 @staticmethod
                 def pytest_configure(config: Config) -> None:
                     rec.append(self.make_hook_recorder(config.pluginmanager))
 
-            plugins.append(Collect())
+                    # The unraisable plugin GC collect slows down inline
+                    # pytester runs too much.
+                    config.stash[gc_collect_iterations_key] = 0
+
+            plugins.append(PytesterHelperPlugin())
             ret = main([str(x) for x in args], plugins=plugins)
             if len(rec) == 1:
                 reprec = rec.pop()

src/_pytest/unraisableexception.py

@@ -24,10 +24,12 @@
     from exceptiongroup import ExceptionGroup
 
 
-def gc_collect_harder() -> None:
-    # A single collection doesn't necessarily collect everything.
-    # Constant determined experimentally by the Trio project.
-    for _ in range(5):
+# This is a stash item and not a simple constant to allow pytester to override it.
+gc_collect_iterations_key = StashKey[int]()
+
+
+def gc_collect_harder(iterations: int) -> None:
+    for _ in range(iterations):
         gc.collect()
 
 
@@ -84,9 +86,12 @@ def collect_unraisable(config: Config) -> None:
 def cleanup(
     *, config: Config, prev_hook: Callable[[sys.UnraisableHookArgs], object]
 ) -> None:
+    # A single collection doesn't necessarily collect everything.
+    # Constant determined experimentally by the Trio project.
+    gc_collect_iterations = config.stash.get(gc_collect_iterations_key, 5)
     try:
         try:
-            gc_collect_harder()
+            gc_collect_harder(gc_collect_iterations)
             collect_unraisable(config)
         finally:
             sys.unraisablehook = prev_hook

testing/test_unraisableexception.py

@@ -1,7 +1,5 @@
 from __future__ import annotations
 
-from collections.abc import Generator
-import contextlib
 import gc
 import sys
 from unittest import mock
@@ -229,19 +227,13 @@ def _set_gc_state(enabled: bool) -> bool:
     return was_enabled
 
 
-@contextlib.contextmanager
-def _disable_gc() -> Generator[None]:
-    was_enabled = _set_gc_state(enabled=False)
-    try:
-        yield
-    finally:
-        _set_gc_state(enabled=was_enabled)
-
-
 def test_refcycle_unraisable(pytester: Pytester) -> None:
     # see: https://github.com/pytest-dev/pytest/issues/10404
     pytester.makepyfile(
         test_it="""
+        # Should catch the unraisable exception even if gc is disabled.
+        import gc; gc.disable()
+
         import pytest
 
         class BrokenDel:
@@ -256,23 +248,22 @@ def test_it():
         """
     )
 
-    with _disable_gc():
-        result = pytester.runpytest()
+    result = pytester.runpytest_subprocess(
+        "-Wdefault::pytest.PytestUnraisableExceptionWarning"
+    )
 
-    # TODO: should be a test failure or error
-    assert result.ret == pytest.ExitCode.INTERNAL_ERROR
+    assert result.ret == 0
 
     result.assert_outcomes(passed=1)
     result.stderr.fnmatch_lines("ValueError: del is broken")
 
 
-@pytest.mark.filterwarnings("default::pytest.PytestUnraisableExceptionWarning")
 def test_refcycle_unraisable_warning_filter(pytester: Pytester) -> None:
-    # note that the host pytest warning filter is disabled and the pytester
-    # warning filter applies during config teardown of unraisablehook.
-    # see: https://github.com/pytest-dev/pytest/issues/10404
     pytester.makepyfile(
         test_it="""
+        # Should catch the unraisable exception even if gc is disabled.
+        import gc; gc.disable()
+
         import pytest
 
         class BrokenDel:
@@ -287,17 +278,18 @@ def test_it():
         """
     )
 
-    with _disable_gc():
-        result = pytester.runpytest("-Werror")
+    result = pytester.runpytest_subprocess(
+        "-Werror::pytest.PytestUnraisableExceptionWarning"
+    )
 
-    # TODO: should be a test failure or error
-    assert result.ret == pytest.ExitCode.INTERNAL_ERROR
+    # TODO: Should be a test failure or error. Currently the exception
+    # propagates all the way to the top resulting in exit code 1.
+    assert result.ret == 1
 
     result.assert_outcomes(passed=1)
     result.stderr.fnmatch_lines("ValueError: del is broken")
 
 
-@pytest.mark.filterwarnings("default::pytest.PytestUnraisableExceptionWarning")
 def test_create_task_raises_unraisable_warning_filter(pytester: Pytester) -> None:
     # note that the host pytest warning filter is disabled and the pytester
     # warning filter applies during config teardown of unraisablehook.
@@ -306,6 +298,9 @@ def test_create_task_raises_unraisable_warning_filter(pytester: Pytester) -> Non
     # the issue
     pytester.makepyfile(
         test_it="""
+        # Should catch the unraisable exception even if gc is disabled.
+        import gc; gc.disable()
+
         import asyncio
         import pytest
 
@@ -318,11 +313,11 @@ def test_scheduler_must_be_created_within_running_loop() -> None:
         """
     )
 
-    with _disable_gc():
-        result = pytester.runpytest("-Werror")
+    result = pytester.runpytest_subprocess("-Werror")
 
-    # TODO: should be a test failure or error
-    assert result.ret == pytest.ExitCode.INTERNAL_ERROR
+    # TODO: Should be a test failure or error. Currently the exception
+    # propagates all the way to the top resulting in exit code 1.
+    assert result.ret == 1
 
     result.assert_outcomes(passed=1)
     result.stderr.fnmatch_lines("RuntimeWarning: coroutine 'my_task' was never awaited")