Free-threading: run with pytest-run-paralell

.github/workflows/tests.yaml

@@ -35,6 +35,8 @@ jobs:
           python-version: ${{ matrix.python }}
           allow-prereleases: true
       - run: uv run --locked tox run -e ${{ matrix.tox || format('py{0}', matrix.python) }}
+      - if: endsWith(matrix.python, 't')
+        run: uv run --locked tox run -e parallel
   typing:
     runs-on: ubuntu-latest
     steps:

pyproject.toml

@@ -48,6 +48,7 @@ pre-commit = [
 ]
 tests = [
     "pytest",
+    "pytest-run-parallel; python_full_version >= '3.13'",
 ]
 typing = [
     "mypy",
@@ -67,7 +68,10 @@ testpaths = ["tests"]
 filterwarnings = [
     "error",
 ]
-
+markers = [
+    # Needed when pytest-run-parallel is not installed
+    "thread_unsafe: mark test as not safe to run in multiple threads",
+]
 [tool.coverage.run]
 branch = true
 source = ["markupsafe", "tests"]
@@ -124,8 +128,8 @@ tag-only = [
 
 [tool.tox]
 env_list = [
-    "py3.14", "py3.14t", "py3.13", "py3.13t",
-    "py3.12", "py3.11", "py3.10", "py3.9",
+    "py3.14", "py3.14t", "parallel",
+    "py3.13", "py3.13t", "py3.12", "py3.11", "py3.10", "py3.9",
     "pypy3.11",
     "style",
     "typing",
@@ -145,6 +149,15 @@ commands = [[
     {replace = "posargs", default = [], extend = true},
 ]]
 
+[tool.tox.env.parallel]
+description = "check for free threading issues"
+base_python = ["3.14t"]
+commands = [[
+    "pytest", "-v", "--tb=short", "--basetemp=env_tmp_dir",
+    "--parallel-threads=8",
+    {replace = "posargs", default = [], extend = true},
+]]
+
 [tool.tox.env.style]
 description = "run all pre-commit hooks on all files"
 dependency_groups = ["pre-commit"]

tests/test_ext_init.py

@@ -10,6 +10,7 @@
     _speedups = None  # type: ignore[assignment]
 
 
+@pytest.mark.thread_unsafe(reason="Tampers with sys.modules")
 @pytest.mark.skipif(_speedups is None, reason="speedups unavailable")
 def test_ext_init() -> None:
     """Test that the extension module uses multi-phase init by checking that

tests/test_leak.py

@@ -2,9 +2,12 @@
 
 import gc
 
+import pytest
+
 from markupsafe import escape
 
 
+@pytest.mark.thread_unsafe(reason="Tests gc.get_objects()")
 def test_markup_leaks() -> None:
     counts = set()
     # Try to start with a "clean" count. Works for PyPy but not 3.13 JIT.