Fix AttributeError on select.poll()

Change-Id: Iefcae89cdedd13535c3135ca7227362f1cf9abc8
Closes-Bug: #1889396
Co-Authored-By: Mitchell Jameson <mitchell@arista.com>

networking_arista/ml2/mechanism_arista.py

@@ -14,7 +14,6 @@
 # limitations under the License.
 
 import json
-from multiprocessing import Queue
 
 from neutron_lib.api.definitions import portbindings
 from neutron_lib import constants as n_const
@@ -32,6 +31,45 @@ from networking_arista.ml2 import arista_sync
 from networking_arista.ml2 import arista_trunk
 from networking_arista.ml2.rpc.arista_eapi import AristaRPCWrapperEapi
 
+# When used as a Neutron plugin, neutron-lib imports this code. However earlier
+# neutron-lib has already imported 'multiprocessing'. This means the python
+# module cache (sys.modules) contains a version of 'multiprocessing' where
+# select.poll() exists.
+#
+# Further down we import arista_sync, which spawns a greenthread. This
+# greenthread then uses a green version of 'multiprocessing' where
+# select.poll() has been removed.
+#
+# Doing here multiprocessing.Queue.put() and in the greenthread
+# multiprocessing.Queue.get(timeout=...) leads to:
+# AttributeError: module 'select' has no attribute 'poll'
+#
+# We can't do eventlet.monkey_patch() early enough (before the first
+# 'mutiprocessing' import) as we would have to do it in neutron-lib and it's
+# forbidden, see https://review.opendev.org/#/c/333017/
+#
+# The solution is to let the python module cache here forget the already
+# imported 'multiprocessing' and re-import a green one. Here again
+# eventlet.monkey_patch() doesn't seem to help as it doesn't seem to touch
+# 'multiprocessing'. Thus we use eventlet.import_patched() instead:
+import eventlet
+import sys
+modules_to_forget = []
+for imported_module_name in sys.modules:
+    if imported_module_name.startswith('multiprocessing'):
+        modules_to_forget.append(imported_module_name)
+for module_to_forget in modules_to_forget:
+    del sys.modules[module_to_forget]
+
+for module_to_forget in modules_to_forget:
+    try:
+        eventlet.import_patched(module_to_forget)
+    except ImportError:
+        pass
+
+# import a green 'multiprocessing':
+multiprocessing = eventlet.import_patched('multiprocessing')
+from networking_arista.ml2 import arista_sync  # noqa: E402
 
 LOG = logging.getLogger(__name__)
 cfg.CONF.import_group('ml2_arista', 'networking_arista.common.config')
@@ -70,7 +108,7 @@ class AristaDriver(driver_api.MechanismDriver):
         self.eapi = AristaRPCWrapperEapi()
         self.mlag_pairs = dict()
 
-        self.provision_queue = Queue()
+        self.provision_queue = multiprocessing.Queue()
         self.trunk_driver = None
         self.vif_details = {portbindings.VIF_DETAILS_CONNECTIVITY:
                             portbindings.CONNECTIVITY_L2}