http txns: Do not cache error responses

synapse/rest/client/transactions.py

@@ -87,9 +87,17 @@ def fetch_or_execute(self, txn_key, fn, *args, **kwargs):
 
         deferred = fn(*args, **kwargs)
 
-        # We don't add an errback to the raw deferred, so we ask ObservableDeferred
-        # to swallow the error. This is fine as the error will still be reported
-        # to the observers.
+        # if the request fails with a Twisted failure, remove it
+        # from the transaction map. This is done to ensure that we don't
+        # cache transient errors like rate-limiting errors, etc.
+        def remove_from_map(err):
+            self.transactions.pop(txn_key, None)
+            return err
+        deferred.addErrback(remove_from_map)
+
+        # We don't add any other errbacks to the raw deferred, so we ask
+        # ObservableDeferred to swallow the error. This is fine as the error will
+        # still be reported to the observers.
         observable = ObservableDeferred(deferred, consumeErrors=True)
         self.transactions[txn_key] = (observable, self.clock.time_msec())
         return observable.observe()