Add ability to run multiple pusher instances (#7855)

This reuses the same scheme as federation sender sharding

synapse/config/federation.py

@@ -13,42 +13,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from hashlib import sha256
-from typing import List, Optional
+from typing import Optional
 
-import attr
 from netaddr import IPSet
 
-from ._base import Config, ConfigError
-
-
-@attr.s
-class ShardedFederationSendingConfig:
-    """Algorithm for choosing which federation sender instance is responsible
-    for which destionation host.
-    """
-
-    instances = attr.ib(type=List[str])
-
-    def should_send_to(self, instance_name: str, destination: str) -> bool:
-        """Whether this instance is responsible for sending transcations for
-        the given host.
-        """
-
-        # If multiple federation senders are not defined we always return true.
-        if not self.instances or len(self.instances) == 1:
-            return True
-
-        # We shard by taking the hash, modulo it by the number of federation
-        # senders and then checking whether this instance matches the instance
-        # at that index.
-        #
-        # (Technically this introduces some bias and is not entirely uniform, but
-        # since the hash is so large the bias is ridiculously small).
-        dest_hash = sha256(destination.encode("utf8")).digest()
-        dest_int = int.from_bytes(dest_hash, byteorder="little")
-        remainder = dest_int % (len(self.instances))
-        return self.instances[remainder] == instance_name
+from ._base import Config, ConfigError, ShardedWorkerHandlingConfig
 
 
 class FederationConfig(Config):
@@ -61,7 +30,7 @@ class FederationConfig(Config):
         self.send_federation = config.get("send_federation", True)
 
         federation_sender_instances = config.get("federation_sender_instances") or []
-        self.federation_shard_config = ShardedFederationSendingConfig(
+        self.federation_shard_config = ShardedWorkerHandlingConfig(
             federation_sender_instances
         )