Don't preimport threading early (#1897)

4 files changed, 43 insertions, 5 deletions

diff --git a/docs/changelog/1897.bugfix.rst b/docs/changelog/1897.bugfix.rst
new file mode 100644
index 0000000..59ff426
--- /dev/null
+++ b/docs/changelog/1897.bugfix.rst
@@ -0,0 +1 @@
+No longer preimport threading to fix support for `gpython <https://pypi.org/project/pygolang/#gpython>`_ and `gevent <https://www.gevent.org/>`_ - by :user:`navytux`.
diff --git a/src/virtualenv/create/via_global_ref/_virtualenv.py b/src/virtualenv/create/via_global_ref/_virtualenv.py
index b399da4..31f9b81 100644
--- a/src/virtualenv/create/via_global_ref/_virtualenv.py
+++ b/src/virtualenv/create/via_global_ref/_virtualenv.py
@@ -39,18 +39,33 @@ if sys.version_info > (3, 4):
     # https://docs.python.org/3/library/importlib.html#setting-up-an-importer
     from importlib.abc import MetaPathFinder
     from importlib.util import find_spec
-    from threading import Lock
     from functools import partial
 
     class _Finder(MetaPathFinder):
         """A meta path finder that allows patching the imported distutils modules"""
 
         fullname = None
-        lock = Lock()
+
+        # lock[0] is threading.Lock(), but initialized lazily to avoid importing threading very early at startup,
+        # because there are gevent-based applications that need to be first to import threading by themselves.
+        # See https://github.com/pypa/virtualenv/issues/1895 for details.
+        lock = []
 
         def find_spec(self, fullname, path, target=None):
             if fullname in _DISTUTILS_PATCH and self.fullname is None:
-                with self.lock:
+                # initialize lock[0] lazily
+                if len(self.lock) == 0:
+                    import threading
+
+                    lock = threading.Lock()
+                    # there is possibility that two threads T1 and T2 are simultaneously running into find_spec,
+                    # observing .lock as empty, and further going into hereby initialization. However due to the GIL,
+                    # list.append() operation is atomic and this way only one of the threads will "win" to put the lock
+                    # - that every thread will use - into .lock[0].
+                    # https://docs.python.org/3/faq/library.html#what-kinds-of-global-value-mutation-are-thread-safe
+                    self.lock.append(lock)
+
+                with self.lock[0]:
                     self.fullname = fullname
                     try:
                         spec = find_spec(fullname, path)
diff --git a/tests/conftest.py b/tests/conftest.py
index e69e4d7..b2ddf03 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -188,11 +188,11 @@ COVERAGE_RUN = os.environ.get(str(COV_ENV_VAR))
 
 
 @pytest.fixture(autouse=True)
-def coverage_env(monkeypatch, link):
+def coverage_env(monkeypatch, link, request):
     """
     Enable coverage report collection on the created virtual environments by injecting the coverage project
     """
-    if COVERAGE_RUN:
+    if COVERAGE_RUN and "no_coverage" not in request.fixturenames:
         # we inject right after creation, we cannot collect coverage on site.py - used for helper scripts, such as debug
         from virtualenv import run
 
@@ -230,6 +230,12 @@ def coverage_env(monkeypatch, link):
         yield finish
 
 
+# no_coverage tells coverage_env to disable coverage injection for no_coverage user.
+@pytest.fixture
+def no_coverage():
+    pass
+
+
 class EnableCoverage(object):
     _COV_FILE = Path(coverage.__file__)
     _ROOT_COV_FILES_AND_FOLDERS = [i for i in _COV_FILE.parents[1].iterdir() if i.name.startswith("coverage")]
diff --git a/tests/unit/create/test_creator.py b/tests/unit/create/test_creator.py
index 5a24df6..10090c7 100644
--- a/tests/unit/create/test_creator.py
+++ b/tests/unit/create/test_creator.py
@@ -557,3 +557,19 @@ def test_zip_importer_can_import_setuptools(tmp_path):
     env = os.environ.copy()
     env[str("PYTHONPATH")] = str(zip_path)
     subprocess.check_call([str(result.creator.exe), "-c", "from setuptools.dist import Distribution"], env=env)
+
+
+# verify that python in created virtualenv does not preimport threading.
+# https://github.com/pypa/virtualenv/issues/1895
+#
+# coverage is disabled, because when coverage is active, it imports threading in default mode.
+@pytest.mark.xfail(
+    IS_PYPY and PY3 and sys.platform.startswith("darwin"), reason="https://foss.heptapod.net/pypy/pypy/-/issues/3269",
+)
+def test_no_preimport_threading(tmp_path, no_coverage):
+    session = cli_run([ensure_text(str(tmp_path))])
+    out = subprocess.check_output(
+        [str(session.creator.exe), "-c", r"import sys; print('\n'.join(sorted(sys.modules)))"], universal_newlines=True,
+    )
+    imported = set(out.splitlines())
+    assert "threading" not in imported