Read the environment variable EVENTLET_MONKEYPATCH

Since eventlet 0.39.0 [1] it is possible to you set an environment
variable ``EVENTLET_MONKEYPATCH=1`` to get patching as early as
possible in Python startup, which may avoid some problems.

[1]b754135b04

Related-Bug: #2091538
Topic: eventlet-deprecation
Change-Id: I928e32ea736bdb3357fdbb1de6646e69db3159d7

neutron/common/eventlet_utils.py

@@ -13,6 +13,8 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+import os
+
 import eventlet
 
 
@@ -22,6 +24,13 @@ IS_MONKEY_PATCHED = False
 def monkey_patch():
     global IS_MONKEY_PATCHED
     if not IS_MONKEY_PATCHED:
+        # This environment variable will be used in eventlet 0.39.0
+        # https://github.com/eventlet/eventlet/commit/
+        # b754135b045306022a537b5797f2cb2cf47ba49b
+        if os.getenv('EVENTLET_MONKEYPATCH') == '1':
+            IS_MONKEY_PATCHED = True
+            return
+
         eventlet.monkey_patch()
 
         # pylint: disable=import-outside-toplevel

releasenotes/notes/env_variable_eventlet_monkeypatch-fccede7812d60c38.yaml

@@ -0,0 +1,12 @@
+---
+features:
+  - |
+    Since eventlet 0.39.0, a new environment variable
+    ``EVENTLET_MONKEYPATCH=1`` can be defined to get patching as early as
+    possible in Python startup.
+upgrade:
+  - |
+    Neutron can read the environment variable ``EVENTLET_MONKEYPATCH=1`` in
+    order to not monkey patch the system libraries. If this environment
+    variable is set and the eventlet version is lower, the process won't be
+    patched.