Fix server_name missing in RedisSubscriber (complexity)

See #18670 (comment)

Error:

```
  File "/home/runner/work/synapse/synapse/synapse/replication/tcp/redis.py", line 127, in __init__
    name="replication_command_handler", server_name=self.server_name
builtins.AttributeError: 'RedisSubscriber' object has no attribute 'server_name'
```

`self.server_name` is not set in the constructor since we set this after it's created, see
https://github.com/element-hq/synapse/blob/06108100c848ab3f6b9e541b7769bbc63adc2bf6/synapse/replication/tcp/redis.py#L383-L391

Author: MadLittleMods

synapse/replication/tcp/redis.py

@@ -36,7 +36,11 @@
 from twisted.internet.interfaces import IAddress, IConnector
 from twisted.python.failure import Failure
 
-from synapse.logging.context import PreserveLoggingContext, make_deferred_yieldable
+from synapse.logging.context import (
+    PreserveLoggingContext,
+    current_context,
+    make_deferred_yieldable,
+)
 from synapse.metrics import SERVER_NAME_LABEL
 from synapse.metrics.background_process_metrics import (
     BackgroundProcessLoggingContext,
@@ -117,15 +121,33 @@ class RedisSubscriber(SubscriberProtocol):
     def __init__(self, *args: Any, **kwargs: Any):
         super().__init__(*args, **kwargs)
 
+        # Capture the current context so we can use it later when `server_name` is set.
+        self._sentinel_context = current_context()
         # a logcontext which we use for processing incoming commands. We declare it as a
         # background process so that the CPU stats get reported to prometheus.
-        with PreserveLoggingContext():
-            # thanks to `PreserveLoggingContext()`, the new logcontext is guaranteed to
-            # capture the sentinel context as its containing context and won't prevent
-            # GC of / unintentionally reactivate what would be the current context.
-            self._logging_context = BackgroundProcessLoggingContext(
-                name="replication_command_handler", server_name=self.server_name
-            )
+        self._logging_context: Optional[BackgroundProcessLoggingContext] = None
+
+    def _get_logging_context(self) -> BackgroundProcessLoggingContext:
+        """
+        We lazily create the logging context so that `self.server_name` is set and
+        available. See `RedisDirectTcpReplicationClientFactory.buildProtocol` for more
+        details on why we set `self.server_name` after the fact instead of in the
+        constructor.
+        """
+        assert self.server_name is not None, (
+            "self.server_name must be set before using _get_logging_context()"
+        )
+        if self._logging_context is None:
+            # a logcontext which we use for processing incoming commands. We declare it as a
+            # background process so that the CPU stats get reported to prometheus.
+            with PreserveLoggingContext(self._sentinel_context):
+                # thanks to `PreserveLoggingContext()`, the new logcontext is guaranteed to
+                # capture the sentinel context as its containing context and won't prevent
+                # GC of / unintentionally reactivate what would be the current context.
+                self._logging_context = BackgroundProcessLoggingContext(
+                    name="replication_command_handler", server_name=self.server_name
+                )
+        return self._logging_context
 
     def connectionMade(self) -> None:
         logger.info("Connected to redis")
@@ -159,7 +181,7 @@ async def _send_subscribe(self) -> None:
 
     def messageReceived(self, pattern: str, channel: str, message: str) -> None:
         """Received a message from redis."""
-        with PreserveLoggingContext(self._logging_context):
+        with PreserveLoggingContext(self._get_logging_context()):
             self._parse_and_dispatch_message(message)
 
     def _parse_and_dispatch_message(self, message: str) -> None:
@@ -218,7 +240,7 @@ def connectionLost(self, reason: Failure) -> None:  # type: ignore[override]
 
         # mark the logging context as finished by triggering `__exit__()`
         with PreserveLoggingContext():
-            with self._logging_context:
+            with self._get_logging_context():
                 pass
             # the sentinel context is now active, which may not be correct.
             # PreserveLoggingContext() will restore the correct logging context.