Fed server: use a linearizer for ongoing transactions

We don't want to process the same transaction multiple times concurrently, so
use a linearizer.

synapse/federation/federation_server.py

@@ -53,6 +53,7 @@ def __init__(self, hs):
         self.auth = hs.get_auth()
 
         self._server_linearizer = Linearizer("fed_server")
+        self._transaction_linearizer = Linearizer("fed_txn_handler")
 
         # We cache responses to state queries, as they take a while and often
         # come in waves.
@@ -111,12 +112,39 @@ def on_backfill_request(self, origin, room_id, versions, limit):
     def on_incoming_transaction(self, transaction_data):
         # keep this as early as possible to make the calculated origin ts as
         # accurate as possible.
-        request_time = int(self._clock.time_msec())
+        request_time = self._clock.time_msec()
 
         transaction = Transaction(**transaction_data)
 
+        if not transaction.transaction_id:
+            raise Exception("Transaction missing transaction_id")
+        if not transaction.origin:
+            raise Exception("Transaction missing origin")
+
         logger.debug("[%s] Got transaction", transaction.transaction_id)
 
+        # use a linearizer to ensure that we don't process the same transaction
+        # multiple times in parallel.
+        with (yield self._transaction_linearizer.queue(
+                (transaction.origin, transaction.transaction_id),
+        )):
+            result = yield self._handle_incoming_transaction(
+                transaction, request_time,
+            )
+
+        defer.returnValue(result)
+
+    @defer.inlineCallbacks
+    def _handle_incoming_transaction(self, transaction, request_time):
+        """ Process an incoming transaction and return the HTTP response
+
+        Args:
+            transaction (Transaction): incoming transaction
+            request_time (int): timestamp that the HTTP request arrived at
+
+        Returns:
+            Deferred[(int, object)]: http response code and body
+        """
         response = yield self.transaction_actions.have_responded(transaction)
 
         if response: