patch to metricpublisher to lock connection before performing redis operations

ZEN-14322.patch

diff --git a/Products/ZenHub/metricpublisher/publisher.py b/Products/ZenHub/metricpublisher/publisher.py
index 96872c2..7c4ae85 100644
--- a/Products/ZenHub/metricpublisher/publisher.py
+++ b/Products/ZenHub/metricpublisher/publisher.py
@@ -20,6 +20,7 @@ from twisted.web.iweb import IBodyProducer
 from twisted.web.http_headers import Headers
 from zope.interface import implements
 from txredis import RedisClientFactory
+from threading import Lock
 
 import json as _stdlib_json
 from .compat import json
@@ -135,6 +136,7 @@ class RedisListPublisher(BasePublisher):
         self._connection = reactor.connectTCP(self._host,
                                               self._port,
                                               self._redis)
+        self._connection_lock = Lock()
         reactor.addSystemEventTrigger('before', 'shutdown', self._shutdown)
 
     def build_metric(self, metric, value, timestamp, tags):
@@ -180,6 +182,11 @@ class RedisListPublisher(BasePublisher):
 
             @defer.inlineCallbacks
             def _flush():
+                # try to get lock on connection
+                if not self._connection_lock.acquire(False):
+                    # could not get lock, another call to _flush() must be running
+                    yield defer.succeed(0)
+                    raise StopIteration
                 client = self._redis.client
                 try:
                     yield client.multi()
@@ -195,6 +202,8 @@ class RedisListPublisher(BasePublisher):
                     except Exception:
                         pass
                     self._publish_failed(e, metrics=metrics)
+                finally:
+                    self._connection_lock.release()
 
             return _flush()