Add ability to run replication protocol over redis. (#7040)

This is configured via the `redis` config options.

synapse/replication/tcp/redis.py

@@ -0,0 +1,181 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import logging
+from typing import TYPE_CHECKING
+
+import txredisapi
+
+from synapse.logging.context import PreserveLoggingContext
+from synapse.metrics.background_process_metrics import run_as_background_process
+from synapse.replication.tcp.commands import (
+    Command,
+    ReplicateCommand,
+    parse_command_from_line,
+)
+from synapse.replication.tcp.protocol import AbstractConnection
+
+if TYPE_CHECKING:
+    from synapse.replication.tcp.handler import ReplicationCommandHandler
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class RedisSubscriber(txredisapi.SubscriberProtocol, AbstractConnection):
+    """Connection to redis subscribed to replication stream.
+
+    Parses incoming messages from redis into replication commands, and passes
+    them to `ReplicationCommandHandler`
+
+    Due to the vagaries of `txredisapi` we don't want to have a custom
+    constructor, so instead we expect the defined attributes below to be set
+    immediately after initialisation.
+
+    Attributes:
+        handler: The command handler to handle incoming commands.
+        stream_name: The *redis* stream name to subscribe to (not anything to
+            do with Synapse replication streams).
+        outbound_redis_connection: The connection to redis to use to send
+            commands.
+    """
+
+    handler = None  # type: ReplicationCommandHandler
+    stream_name = None  # type: str
+    outbound_redis_connection = None  # type: txredisapi.RedisProtocol
+
+    def connectionMade(self):
+        logger.info("Connected to redis instance")
+        self.subscribe(self.stream_name)
+        self.send_command(ReplicateCommand())
+
+        self.handler.new_connection(self)
+
+    def messageReceived(self, pattern: str, channel: str, message: str):
+        """Received a message from redis.
+        """
+
+        if message.strip() == "":
+            # Ignore blank lines
+            return
+
+        try:
+            cmd = parse_command_from_line(message)
+        except Exception:
+            logger.exception(
+                "[%s] failed to parse line: %r", message,
+            )
+            return
+
+        # Now lets try and call on_<CMD_NAME> function
+        run_as_background_process(
+            "replication-" + cmd.get_logcontext_id(), self.handle_command, cmd
+        )
+
+    async def handle_command(self, cmd: Command):
+        """Handle a command we have received over the replication stream.
+
+        By default delegates to on_<COMMAND>, which should return an awaitable.
+
+        Args:
+            cmd: received command
+        """
+        handled = False
+
+        # First call any command handlers on this instance. These are for redis
+        # specific handling.
+        cmd_func = getattr(self, "on_%s" % (cmd.NAME,), None)
+        if cmd_func:
+            await cmd_func(cmd)
+            handled = True
+
+        # Then call out to the handler.
+        cmd_func = getattr(self.handler, "on_%s" % (cmd.NAME,), None)
+        if cmd_func:
+            await cmd_func(cmd)
+            handled = True
+
+        if not handled:
+            logger.warning("Unhandled command: %r", cmd)
+
+    def connectionLost(self, reason):
+        logger.info("Lost connection to redis instance")
+        self.handler.lost_connection(self)
+
+    def send_command(self, cmd: Command):
+        """Send a command if connection has been established.
+
+        Args:
+            cmd (Command)
+        """
+        string = "%s %s" % (cmd.NAME, cmd.to_line())
+        if "\n" in string:
+            raise Exception("Unexpected newline in command: %r", string)
+
+        encoded_string = string.encode("utf-8")
+
+        async def _send():
+            with PreserveLoggingContext():
+                # Note that we use the other connection as we can't send
+                # commands using the subscription connection.
+                await self.outbound_redis_connection.publish(
+                    self.stream_name, encoded_string
+                )
+
+        run_as_background_process("send-cmd", _send)
+
+
+class RedisDirectTcpReplicationClientFactory(txredisapi.SubscriberFactory):
+    """This is a reconnecting factory that connects to redis and immediately
+    subscribes to a stream.
+
+    Args:
+        hs
+        outbound_redis_connection: A connection to redis that will be used to
+            send outbound commands (this is seperate to the redis connection
+            used to subscribe).
+    """
+
+    maxDelay = 5
+    continueTrying = True
+    protocol = RedisSubscriber
+
+    def __init__(
+        self, hs: "HomeServer", outbound_redis_connection: txredisapi.RedisProtocol
+    ):
+
+        super().__init__()
+
+        # This sets the password on the RedisFactory base class (as
+        # SubscriberFactory constructor doesn't pass it through).
+        self.password = hs.config.redis.redis_password
+
+        self.handler = hs.get_tcp_replication()
+        self.stream_name = hs.hostname
+
+        self.outbound_redis_connection = outbound_redis_connection
+
+    def buildProtocol(self, addr):
+        p = super().buildProtocol(addr)  # type: RedisSubscriber
+
+        # We do this here rather than add to the constructor of `RedisSubcriber`
+        # as to do so would involve overriding `buildProtocol` entirely, however
+        # the base method does some other things than just instantiating the
+        # protocol.
+        p.handler = self.handler
+        p.outbound_redis_connection = self.outbound_redis_connection
+        p.stream_name = self.stream_name
+
+        return p