Rate limit retries when fetching server keys.

synapse/crypto/keyring.py

@@ -87,8 +87,20 @@ class Keyring(object):
             return
 
         # Try to fetch the key from the remote server.
-        # TODO(markjh): Ratelimit requests to a given server.
 
+        retry_last_ts, retry_interval = (0, 0)
+        retry_timings = yield self.store.get_destination_retry_timings(
+            server_name
+        )
+        if retry_timings:
+            retry_last_ts, retry_interval = (
+                retry_timings.retry_last_ts, retry_timings.retry_interval
+            )
+            if retry_last_ts + retry_interval > int(self.clock.time_msec()):
+                logger.info("%s not ready for retry", server_name)
+                raise ValueError("No verification key found for given key ids")
+
+        try:
             (response, tls_certificate) = yield fetch_server_key(
                 server_name, self.hs.tls_context_factory
             )
@@ -153,3 +165,7 @@ class Keyring(object):
                     return
 
             raise ValueError("No verification key found for given key ids")
+
+        except:
+            self.set_retrying(server_name, retry_interval)
+            raise