Wake up transaction queue when remote server comes back online (#6706)

This will be used to retry outbound transactions to a remote server if
we think it might have come back up.

synapse/federation/transport/server.py

@@ -44,6 +44,7 @@ from synapse.logging.opentracing import (
     tags,
     whitelisted_homeserver,
 )
+from synapse.server import HomeServer
 from synapse.types import ThirdPartyInstanceID, get_domain_from_id
 from synapse.util.ratelimitutils import FederationRateLimiter
 from synapse.util.versionstring import get_version_string
@@ -101,12 +102,17 @@ class NoAuthenticationError(AuthenticationError):
 
 
 class Authenticator(object):
-    def __init__(self, hs):
+    def __init__(self, hs: HomeServer):
         self._clock = hs.get_clock()
         self.keyring = hs.get_keyring()
         self.server_name = hs.hostname
         self.store = hs.get_datastore()
         self.federation_domain_whitelist = hs.config.federation_domain_whitelist
+        self.notifer = hs.get_notifier()
+
+        self.replication_client = None
+        if hs.config.worker.worker_app:
+            self.replication_client = hs.get_tcp_replication()
 
     # A method just so we can pass 'self' as the authenticator to the Servlets
     async def authenticate_request(self, request, content):
@@ -166,6 +172,17 @@ class Authenticator(object):
         try:
             logger.info("Marking origin %r as up", origin)
             await self.store.set_destination_retry_timings(origin, None, 0, 0)
+
+            # Inform the relevant places that the remote server is back up.
+            self.notifer.notify_remote_server_up(origin)
+            if self.replication_client:
+                # If we're on a worker we try and inform master about this. The
+                # replication client doesn't hook into the notifier to avoid
+                # infinite loops where we send a `REMOTE_SERVER_UP` command to
+                # master, which then echoes it back to us which in turn pokes
+                # the notifier.
+                self.replication_client.send_remote_server_up(origin)
+
         except Exception:
             logger.exception("Error resetting retry timings on %s", origin)