Fix fair internal lock used from eventlet.spawn_n

The fasteners lib in version 0.15.0 removed the
threading.current_thread workaround for eventlet[1] because eventlet
seemed to fixed the current_thread issues tracked in [2]. However the
fix for [2] only fixed half of the problem. The threading.current_thread
call works if it is called from thread created by eventlet.spawn.
However if the thread is created with eventlet.spawn_n then
threading.current_thread is still broken and returns the ID of the
python native thread.

The fasteners' ReaderWriterLock depends heavily on
threading.current_thread to decide which thread holds a lock and to
allow re-entry of that thread. This leads to the situation that
multiple threads created from spawn_n could take the same
ReaderWriterLock at the same time.

The fair internal lock in oslo.concurrency uses ReaderWriterLock and
as a result such lock is broken for threads created with spawn_n.

Note that this issue was raised with eventlet in [3] when the nova team
detected it via a direct usage of ReaderWriterLock in the nova test
code. As [3] did not lead to a solution in eventlet nova implemented a
nova local fix for the test code in [4].

However now we detected that oslo.concurrency is affected by this issue
as well.

This patch restores the workaround that was removed by [1].

Note that a fasteners issue [5] also opened to restore the
workaround[1].

[1] 467ed75ee1
[2] https://github.com/eventlet/eventlet/issues/172
[3] https://github.com/eventlet/eventlet/issues/731
[4] https://review.opendev.org/c/openstack/nova/+/813114
[5] https://github.com/harlowja/fasteners/issues/96

Closes-Bug: #1988311
Change-Id: Ia873bcc6b07121c9bd0b94c593567d537b4c1112

oslo_concurrency/lockutils.py

@@ -32,6 +32,14 @@ from oslo_utils import timeutils
 
 from oslo_concurrency._i18n import _
 
+try:
+    # import eventlet optionally
+    import eventlet
+    from eventlet import patcher as eventlet_patcher
+except ImportError:
+    eventlet = None
+    eventlet_patcher = None
+
 
 LOG = logging.getLogger(__name__)
 
@@ -79,12 +87,23 @@ def get_lock_path(conf):
     return conf.oslo_concurrency.lock_path
 
 
-InterProcessLock = fasteners.InterProcessLock
-ReaderWriterLock = fasteners.ReaderWriterLock
-"""A reader/writer lock.
+class ReaderWriterLock(fasteners.ReaderWriterLock):
+    """A reader/writer lock.
 
-.. versionadded:: 0.4
-"""
+    .. versionadded:: 0.4
+    """
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        # Until https://github.com/eventlet/eventlet/issues/731 is resolved
+        # we need to use eventlet.getcurrent instead of
+        # threading.current_thread if we are running in a monkey patched
+        # environment
+        if eventlet is not None and eventlet_patcher is not None:
+            if eventlet_patcher.is_monkey_patched('thread'):
+                self._current_thread = eventlet.getcurrent
+
+
+InterProcessLock = fasteners.InterProcessLock
 
 
 class FairLocks(object):

oslo_concurrency/tests/unit/test_lockutils_eventlet.py

@@ -99,18 +99,6 @@ class TestInternalLock(test_base.BaseTestCase):
         )
 
     def test_fair_lock_with_spawn_n(self):
-        # This is bug 1988311 where spawn_n does not work with fair locks
-        ex = self.assertRaises(
-            AssertionError,
-            self._test_internal_lock_with_two_threads,
-            fair=True,
-            spawn=eventlet.spawn_n,
+        self._test_internal_lock_with_two_threads(
+            fair=True, spawn=eventlet.spawn_n
         )
-        self.assertEqual(
-            "'finished' is not None: "
-            "Two threads was able to take the same lock",
-            str(ex),
-        )
-        # self._test_internal_lock_with_two_threads(
-        #     fair=True, spawn=eventlet.spawn_n
-        # )