Add logging for redis connection setup

changelog.d/9590.misc

@@ -0,0 +1 @@
+Add logging for redis connection setup.

synapse/replication/tcp/redis.py

@@ -20,6 +20,10 @@
 import attr
 import txredisapi
 
+from twisted.internet.address import IPv4Address, IPv6Address
+from twisted.internet.interfaces import IAddress, IConnector
+from twisted.python.failure import Failure
+
 from synapse.logging.context import PreserveLoggingContext, make_deferred_yieldable
 from synapse.metrics.background_process_metrics import (
     BackgroundProcessLoggingContext,
@@ -253,6 +257,37 @@ async def _send_ping(self):
             except Exception:
                 logger.warning("Failed to send ping to a redis connection")
 
+    # ReconnectingClientFactory has some logging (if you enable `self.noisy`), but
+    # it's rubbish. We add our own here.
+
+    def startedConnecting(self, connector: IConnector):
+        logger.info(
+            "Connecting to redis server %s", format_address(connector.getDestination())
+        )
+        super().startedConnecting(connector)
+
+    def clientConnectionFailed(self, connector: IConnector, reason: Failure):
+        logger.info(
+            "Connection to redis server %s failed: %s",
+            format_address(connector.getDestination()),
+            reason.value,
+        )
+        super().clientConnectionFailed(connector, reason)
+
+    def clientConnectionLost(self, connector: IConnector, reason: Failure):
+        logger.info(
+            "Connection to redis server %s lost: %s",
+            format_address(connector.getDestination()),
+            reason.value,
+        )
+        super().clientConnectionLost(connector, reason)
+
+
+def format_address(address: IAddress) -> str:
+    if isinstance(address, (IPv4Address, IPv6Address)):
+        return "%s:%i" % (address.host, address.port)
+    return str(address)
+
 
 class RedisDirectTcpReplicationClientFactory(SynapseRedisFactory):
     """This is a reconnecting factory that connects to redis and immediately