Prevent Tox from hanging with --installpkg sdist due to orphaned build backend

docs/changelog/3530.bugfix.rst

@@ -0,0 +1,3 @@
+Prevent tox from hanging upon exit due to orphaned build threads and subprocesses when the ``--installpkg`` option is
+used with *sdist*.
+- by :user:`vytas7`

pyproject.toml

@@ -86,6 +86,7 @@ test = [
   "flaky>=3.8.1",
   "hatch-vcs>=0.5",
   "hatchling>=1.27",
+  "pdm-backend",
   "psutil>=7",
   "pytest>=8.4.1",
   "pytest-cov>=6.2.1",

src/tox/tox_env/python/virtual_env/package/pyproject.py

@@ -118,6 +118,12 @@ def root(self) -> Path:
 
     @root.setter
     def root(self, value: Path) -> None:
+        # Recreating the frontend with a new root would orphan the current frontend.backend_executor, if any, making tox
+        # hang upon exit waiting for its threads and subprocesses (#3512).
+        # Therefore, we make sure to close the existing back-end executor in the case of an existing PEP 517 frontend.
+        if self._frontend is not None:
+            self._frontend.backend_executor.close()
+
         self._root = value
         self._frontend_ = None  # force recreating the frontend with new root
 

tests/tox_env/python/virtual_env/package/conftest.py

@@ -2,7 +2,7 @@
 
 import sys
 from textwrap import dedent
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Callable
 
 import pytest
 
@@ -41,3 +41,41 @@ def pkg_with_extras_project(tmp_path_factory: pytest.TempPathFactory) -> Path:
     toml = '[build-system]\nrequires=["setuptools"]\nbuild-backend = "setuptools.build_meta"'
     (tmp_path / "pyproject.toml").write_text(toml)
     return tmp_path
+
+
+@pytest.fixture(scope="session")
+def pkg_with_pdm_backend(
+    tmp_path_factory: pytest.TempPathFactory,
+    pkg_builder: Callable[[Path, Path, list[str], bool], Path],
+) -> Path:
+    tmp_path = tmp_path_factory.mktemp("skeleton")
+
+    pyproject_toml = """
+    [build-system]
+    requires = ["pdm-backend"]
+    build-backend = "pdm.backend"
+
+    [project]
+    name = "skeleton"
+    description = "Just a skeleton for reproducing #3512."
+    version = "0.1.1337"
+    dependencies = [
+        "requests",
+    ]
+
+    [tool.pdm.build]
+    includes = [
+        "skeleton/",
+    ]
+    source-includes = [
+        "tox.ini",
+    ]
+    """
+    (tmp_path / "pyproject.toml").write_text(dedent(pyproject_toml))
+    (tmp_path / "skeleton").mkdir(exist_ok=True)
+    (tmp_path / "skeleton" / "__init__.py").touch()
+
+    dist = tmp_path / "dist"
+    pkg_builder(dist, tmp_path, ["sdist"], False)
+
+    return tmp_path

tests/tox_env/python/virtual_env/package/test_package_pyproject.py

@@ -464,3 +464,24 @@ def test_pyproject_config_settings_editable_legacy(
         "get_requires_for_build_wheel": {"C": "3"},
         "prepare_metadata_for_build_wheel": {"D": "4"},
     }
+
+
+@pytest.mark.usefixtures("enable_pip_pypi_access")
+def test_pyproject_installpkg_pep517_envs(tox_project: ToxProjectCreator, pkg_with_pdm_backend: Path) -> None:
+    # Regression test for #3512
+    tox_ini = """
+    [tox]
+    envlist = dummy1,dummy2
+
+    [testenv:dummy1]
+    commands =
+        python -c print(1)
+
+    [testenv:dummy2]
+    commands =
+        python -c print(42)
+    """
+    sdist = pkg_with_pdm_backend / "dist" / "skeleton-0.1.1337.tar.gz"
+    proj = tox_project({"tox.ini": tox_ini}, base=pkg_with_pdm_backend)
+    result = proj.run("--installpkg", str(sdist))
+    result.assert_success()