Fix client reader sharding tests (#7853)

* Fix client reader sharding tests

* Newsfile

* Fix typing

* Update changelog.d/7853.misc

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

* Move mocking of http_client to tests

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

tests/server.py

@@ -237,6 +237,7 @@ class ThreadedMemoryReactorClock(MemoryReactorClock):
     def __init__(self):
         self.threadpool = ThreadPool(self)
 
+        self._tcp_callbacks = {}
         self._udp = []
         lookups = self.lookups = {}
 
@@ -268,6 +269,29 @@ class ThreadedMemoryReactorClock(MemoryReactorClock):
     def getThreadPool(self):
         return self.threadpool
 
+    def add_tcp_client_callback(self, host, port, callback):
+        """Add a callback that will be invoked when we receive a connection
+        attempt to the given IP/port using `connectTCP`.
+
+        Note that the callback gets run before we return the connection to the
+        client, which means callbacks cannot block while waiting for writes.
+        """
+        self._tcp_callbacks[(host, port)] = callback
+
+    def connectTCP(self, host, port, factory, timeout=30, bindAddress=None):
+        """Fake L{IReactorTCP.connectTCP}.
+        """
+
+        conn = super().connectTCP(
+            host, port, factory, timeout=timeout, bindAddress=None
+        )
+
+        callback = self._tcp_callbacks.get((host, port))
+        if callback:
+            callback()
+
+        return conn
+
 
 class ThreadPool:
     """
@@ -486,7 +510,7 @@ class FakeTransport(object):
         try:
             self.other.dataReceived(to_write)
         except Exception as e:
-            logger.warning("Exception writing to protocol: %s", e)
+            logger.exception("Exception writing to protocol: %s", e)
             return
 
         self.buffer = self.buffer[len(to_write) :]