Clean up `ShardedWorkerHandlingConfig` (#9466)

* Split ShardedWorkerHandlingConfig

This is so that we have a type level understanding of when it is safe to
call `get_instance(..)` (as opposed to `should_handle(..)`).

* Remove special cases in ShardedWorkerHandlingConfig.

`ShardedWorkerHandlingConfig` tried to handle the various different ways
it was possible to configure federation senders and pushers. This led to
special cases that weren't hit during testing.

To fix this the handling of the different cases is moved from there and
`generic_worker` into the worker config class. This allows us to have
the logic in one place and allows the rest of the code to ignore the
different cases.

5 files changed, 116 insertions, 21 deletions

diff --git a/synapse/config/_base.py b/synapse/config/_base.py
index e89decda34..4026966711 100644
--- a/synapse/config/_base.py
+++ b/synapse/config/_base.py
@@ -844,22 +844,23 @@ class ShardedWorkerHandlingConfig:
 
     def should_handle(self, instance_name: str, key: str) -> bool:
         """Whether this instance is responsible for handling the given key."""
-        # If multiple instances are not defined we always return true
-        if not self.instances or len(self.instances) == 1:
-            return True
+        # If no instances are defined we assume some other worker is handling
+        # this.
+        if not self.instances:
+            return False
 
-        return self.get_instance(key) == instance_name
+        return self._get_instance(key) == instance_name
 
-    def get_instance(self, key: str) -> str:
+    def _get_instance(self, key: str) -> str:
         """Get the instance responsible for handling the given key.
 
-        Note: For things like federation sending the config for which instance
-        is sending is known only to the sender instance if there is only one.
-        Therefore `should_handle` should be used where possible.
+        Note: For federation sending and pushers the config for which instance
+        is sending is known only to the sender instance, so we don't expose this
+        method by default.
         """
 
         if not self.instances:
-            return "master"
+            raise Exception("Unknown worker")
 
         if len(self.instances) == 1:
             return self.instances[0]
@@ -876,4 +877,21 @@ class ShardedWorkerHandlingConfig:
         return self.instances[remainder]
 
 
+@attr.s
+class RoutableShardedWorkerHandlingConfig(ShardedWorkerHandlingConfig):
+    """A version of `ShardedWorkerHandlingConfig` that is used for config
+    options where all instances know which instances are responsible for the
+    sharded work.
+    """
+
+    def __attrs_post_init__(self):
+        # We require that `self.instances` is non-empty.
+        if not self.instances:
+            raise Exception("Got empty list of instances for shard config")
+
+    def get_instance(self, key: str) -> str:
+        """Get the instance responsible for handling the given key."""
+        return self._get_instance(key)
+
+
 __all__ = ["Config", "RootConfig", "ShardedWorkerHandlingConfig"]
diff --git a/synapse/config/_base.pyi b/synapse/config/_base.pyi
index 70025b5d60..db16c86f50 100644
--- a/synapse/config/_base.pyi
+++ b/synapse/config/_base.pyi
@@ -149,4 +149,6 @@ class ShardedWorkerHandlingConfig:
     instances: List[str]
     def __init__(self, instances: List[str]) -> None: ...
     def should_handle(self, instance_name: str, key: str) -> bool: ...
+
+class RoutableShardedWorkerHandlingConfig(ShardedWorkerHandlingConfig):
     def get_instance(self, key: str) -> str: ...
diff --git a/synapse/config/push.py b/synapse/config/push.py
index 3adbfb73e6..7831a2ef79 100644
--- a/synapse/config/push.py
+++ b/synapse/config/push.py
@@ -14,7 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from ._base import Config, ShardedWorkerHandlingConfig
+from ._base import Config
 
 
 class PushConfig(Config):
@@ -27,9 +27,6 @@ class PushConfig(Config):
             "group_unread_count_by_room", True
         )
 
-        pusher_instances = config.get("pusher_instances") or []
-        self.pusher_shard_config = ShardedWorkerHandlingConfig(pusher_instances)
-
         # There was a a 'redact_content' setting but mistakenly read from the
         # 'email'section'. Check for the flag in the 'push' section, and log,
         # but do not honour it to avoid nasty surprises when people upgrade.
diff --git a/synapse/config/server.py b/synapse/config/server.py
index 0bfd4398e2..2afca36e7d 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -397,7 +397,6 @@ class ServerConfig(Config):
         if self.public_baseurl is not None:
             if self.public_baseurl[-1] != "/":
                 self.public_baseurl += "/"
-        self.start_pushers = config.get("start_pushers", True)
 
         # (undocumented) option for torturing the worker-mode replication a bit,
         # for testing. The value defines the number of milliseconds to pause before
diff --git a/synapse/config/workers.py b/synapse/config/workers.py
index 7a0ca16da8..ac92375a85 100644
--- a/synapse/config/workers.py
+++ b/synapse/config/workers.py
@@ -17,9 +17,28 @@ from typing import List, Union
 
 import attr
 
-from ._base import Config, ConfigError, ShardedWorkerHandlingConfig
+from ._base import (
+    Config,
+    ConfigError,
+    RoutableShardedWorkerHandlingConfig,
+    ShardedWorkerHandlingConfig,
+)
 from .server import ListenerConfig, parse_listener_def
 
+_FEDERATION_SENDER_WITH_SEND_FEDERATION_ENABLED_ERROR = """
+The send_federation config option must be disabled in the main
+synapse process before they can be run in a separate worker.
+
+Please add ``send_federation: false`` to the main config
+"""
+
+_PUSHER_WITH_START_PUSHERS_ENABLED_ERROR = """
+The start_pushers config option must be disabled in the main
+synapse process before they can be run in a separate worker.
+
+Please add ``start_pushers: false`` to the main config
+"""
+
 
 def _instance_to_list_converter(obj: Union[str, List[str]]) -> List[str]:
     """Helper for allowing parsing a string or list of strings to a config
@@ -103,6 +122,7 @@ class WorkerConfig(Config):
         self.worker_replication_secret = config.get("worker_replication_secret", None)
 
         self.worker_name = config.get("worker_name", self.worker_app)
+        self.instance_name = self.worker_name or "master"
 
         self.worker_main_http_uri = config.get("worker_main_http_uri", None)
 
@@ -118,12 +138,41 @@ class WorkerConfig(Config):
                 )
             )
 
-        # Whether to send federation traffic out in this process. This only
-        # applies to some federation traffic, and so shouldn't be used to
-        # "disable" federation
-        self.send_federation = config.get("send_federation", True)
+        # Handle federation sender configuration.
+        #
+        # There are two ways of configuring which instances handle federation
+        # sending:
+        #   1. The old way where "send_federation" is set to false and running a
+        #      `synapse.app.federation_sender` worker app.
+        #   2. Specifying the workers sending federation in
+        #      `federation_sender_instances`.
+        #
+
+        send_federation = config.get("send_federation", True)
+
+        federation_sender_instances = config.get("federation_sender_instances")
+        if federation_sender_instances is None:
+            # Default to an empty list, which means "another, unknown, worker is
+            # responsible for it".
+            federation_sender_instances = []
 
-        federation_sender_instances = config.get("federation_sender_instances") or []
+            # If no federation sender instances are set we check if
+            # `send_federation` is set, which means use master
+            if send_federation:
+                federation_sender_instances = ["master"]
+
+            if self.worker_app == "synapse.app.federation_sender":
+                if send_federation:
+                    # If we're running federation senders, and not using
+                    # `federation_sender_instances`, then we should have
+                    # explicitly set `send_federation` to false.
+                    raise ConfigError(
+                        _FEDERATION_SENDER_WITH_SEND_FEDERATION_ENABLED_ERROR
+                    )
+
+                federation_sender_instances = [self.worker_name]
+
+        self.send_federation = self.instance_name in federation_sender_instances
         self.federation_shard_config = ShardedWorkerHandlingConfig(
             federation_sender_instances
         )
@@ -164,7 +213,37 @@ class WorkerConfig(Config):
                 "Must only specify one instance to handle `receipts` messages."
             )
 
-        self.events_shard_config = ShardedWorkerHandlingConfig(self.writers.events)
+        if len(self.writers.events) == 0:
+            raise ConfigError("Must specify at least one instance to handle `events`.")
+
+        self.events_shard_config = RoutableShardedWorkerHandlingConfig(
+            self.writers.events
+        )
+
+        # Handle sharded push
+        start_pushers = config.get("start_pushers", True)
+        pusher_instances = config.get("pusher_instances")
+        if pusher_instances is None:
+            # Default to an empty list, which means "another, unknown, worker is
+            # responsible for it".
+            pusher_instances = []
+
+            # If no pushers instances are set we check if `start_pushers` is
+            # set, which means use master
+            if start_pushers:
+                pusher_instances = ["master"]
+
+            if self.worker_app == "synapse.app.pusher":
+                if start_pushers:
+                    # If we're running pushers, and not using
+                    # `pusher_instances`, then we should have explicitly set
+                    # `start_pushers` to false.
+                    raise ConfigError(_PUSHER_WITH_START_PUSHERS_ENABLED_ERROR)
+
+                pusher_instances = [self.instance_name]
+
+        self.start_pushers = self.instance_name in pusher_instances
+        self.pusher_shard_config = ShardedWorkerHandlingConfig(pusher_instances)
 
         # Whether this worker should run background tasks or not.
         #