reenable greendns in nova.

Back in the days of centos 6 and python 2.6 eventlet
greendns monkeypatching broke ipv6. As a result nova
has run without greendns monkey patching ever since.
This removes that old workaround allowing modern
eventlet to use greendns for non blocking dns lookups.

Closes-Bug: #1964149
Change-Id: Ia511879d2f5f50a3f63d180258abccf046a7264e

nova/monkey_patch.py

@@ -22,22 +22,11 @@
 
 
 def _monkey_patch():
-    # See https://bugs.launchpad.net/nova/+bug/1164822
-    # TODO(mdbooth): This feature was deprecated and removed in eventlet at
-    # some point but brought back in version 0.21.0, presumably because some
-    # users still required it to work round issues. However, there have been a
-    # number of greendns fixes in eventlet since then. Specifically, it looks
-    # as though the originally reported IPv6 issue may have been fixed in
-    # version 0.24.0. We should remove this when we can confirm that the
-    # original issue is fixed.
-    # NOTE(artom) eventlet processes environment variables at import-time. We
-    # therefore set this here, before importing eventlet, in order to correctly
-    # disable greendns.
-    os.environ['EVENTLET_NO_GREENDNS'] = 'yes'
-
     # NOTE(mdbooth): Anything imported here will not be monkey patched. It is
     # important to take care not to import anything here which requires monkey
     # patching.
+    # NOTE(artom) eventlet processes environment variables at import-time.
+    # as such any eventlet configuration should happen here if needed.
     import eventlet
     import sys
 

releasenotes/notes/greendns-34df7f9fba952bcd.yaml

@@ -0,0 +1,14 @@
+---
+fixes:
+  - |
+    During the havana cycle it was discovered that eventlet
+    monkey patching of greendns broke ipv6.
+    https://bugs.launchpad.net/nova/+bug/1164822
+    Since then nova has been disabling eventlet monkey patching
+    of greendns. Eventlet adressed the ipv6 limitation in v0.17
+    with the introduction of python 3 support in 2015. Nova
+    however continued to disable it, which can result i slow dns
+    queries blocking the entire nova api or other binary
+    because socket.getaddrinfo becomes a blocking call into glibc
+    see: https://bugs.launchpad.net/nova/+bug/1964149 for
+    more details.