Make ReentrantFileLock thread-safe and, thereby, fix race condition in virtualenv.cli_run

docs/changelog/2516.bugfix.rst

@@ -0,0 +1,2 @@
+Make ``ReentrantFileLock`` thread-safe and,
+thereby, fix race condition in ``virtualenv.cli_run`` - by :user:`radoering`.

src/virtualenv/util/lock.py

@@ -23,13 +23,16 @@ def __init__(self, lock_file):
         self.thread_safe = RLock()
 
     def acquire(self, timeout=None, poll_interval=0.05):
-        with self.thread_safe:
-            if self.count == 0:
-                super().acquire(timeout, poll_interval)
-            self.count += 1
+        if not self.thread_safe.acquire(timeout=-1 if timeout is None else timeout):
+            raise Timeout(self.lock_file)
+        if self.count == 0:
+            super().acquire(timeout, poll_interval)
+        self.count += 1
 
     def release(self, force=False):
         with self.thread_safe:
+            if self.count > 0:
+                self.thread_safe.release()
             if self.count == 1:
                 super().release(force=force)
             self.count = max(self.count - 1, 0)

tests/unit/test_util.py

@@ -1,3 +1,9 @@
+import concurrent.futures
+import traceback
+
+import pytest
+
+from virtualenv.util.lock import ReentrantFileLock
 from virtualenv.util.subprocess import run_cmd
 
 
@@ -6,3 +12,25 @@ def test_run_fail(tmp_path):
     assert err
     assert not out
     assert code
+
+
+def test_reentrant_file_lock_is_thread_safe(tmp_path):
+    lock = ReentrantFileLock(tmp_path)
+    target_file = tmp_path / "target"
+    target_file.touch()
+
+    def recreate_target_file():
+        with lock.lock_for_key("target"):
+            target_file.unlink()
+            target_file.touch()
+
+    with concurrent.futures.ThreadPoolExecutor() as executor:
+        tasks = []
+        for _ in range(4):
+            tasks.append(executor.submit(recreate_target_file))
+        concurrent.futures.wait(tasks)
+        for task in tasks:
+            try:
+                task.result()
+            except Exception:
+                pytest.fail(traceback.format_exc())