Create new Redis connection when REDIS_URI changes

When the REDIS_URI configuration setting is changed, the Redis
client should be reconfigured with a new connection pool, just
like we do with the database.

It appears redis-py manages the connection pool by itself and
doesn't expose ways to explicitely destroy it or close all
connections (this is done automatically when all connections
loose scope).

This fix ensures that the unit tests don't accidentally work on
the Redis database configured in MUTALYZER_SETTINGS, which was
quite an unfortunate bug.

mutalyzer/redisclient.py

@@ -32,18 +32,36 @@ class LazyClient(util.LazyObject):
     """
     def _setup(self):
         """
-        Instantiate the Redis interface. This called the first time Redis is
-        used.
+        Instantiate the Redis interface. This is called the first time Redis
+        is used.
         """
-        if settings.REDIS_URI is None:
+        self.configure(settings.REDIS_URI)
+
+    def configure(self, uri):
+        """
+        Configure the client with a new connectionpool for the given URI.
+        """
+        if uri is None:
             import mockredis
             self._wrapped = mockredis.MockRedis(strict=True)
         else:
-            self._wrapped = redis.StrictRedis.from_url(settings.REDIS_URI,
+            self._wrapped = redis.StrictRedis.from_url(uri,
                                                        decode_responses=True,
                                                        encoding='utf-8')
 
 
+def configure_client(uri):
+    """
+    (Re)configure the client with the given URI.
+    """
+    global client
+    client.configure(uri)
+
+
+# Reconfigure the client if configuration is updated.
+settings.on_update(configure_client, 'REDIS_URI')
+
+
 #: Global :class:`LazyClient` instance. Use this for all communication with
 #: Redis.
 client = LazyClient()