Ensure that we never stop reconnecting to redis (#9391)

changelog.d/9391.bugfix

@@ -0,0 +1 @@
+Fix bug where Synapse would occaisonally stop reconnecting after the connection was lost.

synapse/replication/tcp/redis.py

@@ -15,8 +15,9 @@
 
 import logging
 from inspect import isawaitable
-from typing import TYPE_CHECKING, Optional, Type, cast
+from typing import TYPE_CHECKING, Generic, Optional, Type, TypeVar, cast
 
+import attr
 import txredisapi
 
 from synapse.logging.context import PreserveLoggingContext, make_deferred_yieldable
@@ -42,6 +43,24 @@ if TYPE_CHECKING:
 
 logger = logging.getLogger(__name__)
 
+T = TypeVar("T")
+V = TypeVar("V")
+
+
+@attr.s
+class ConstantProperty(Generic[T, V]):
+    """A descriptor that returns the given constant, ignoring attempts to set
+    it.
+    """
+
+    constant = attr.ib()  # type: V
+
+    def __get__(self, obj: Optional[T], objtype: Type[T] = None) -> V:
+        return self.constant
+
+    def __set__(self, obj: Optional[T], value: V):
+        pass
+
 
 class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
     """Connection to redis subscribed to replication stream.
@@ -195,6 +214,10 @@ class SynapseRedisFactory(txredisapi.RedisFactory):
     we detect dead connections.
     """
 
+    # We want to *always* retry connecting, txredisapi will stop if there is a
+    # failure during certain operations, e.g. during AUTH.
+    continueTrying = cast(bool, ConstantProperty(True))
+
     def __init__(
         self,
         hs: "HomeServer",
@@ -243,7 +266,6 @@ class RedisDirectTcpReplicationClientFactory(SynapseRedisFactory):
     """
 
     maxDelay = 5
-    continueTrying = True
     protocol = RedisSubscriber
 
     def __init__(