Fix race condition error in http.get_rest_hosts when `fallback_real…

…time_host` is set

ably/http/http.py

@@ -146,7 +146,8 @@ def get_rest_hosts(self):
         if host is None:
             return hosts
 
-        if time.time() > self.__host_expires:
+        # unstore saved fallback host after fallbackRetryTimeout (RSC15f)
+        if self.__host_expires is not None and time.time() > self.__host_expires:
             self.__host = None
             self.__host_expires = None
             return hosts

test/unit/http_test.py

@@ -0,0 +1,13 @@
+from ably import AblyRest
+
+
+def test_http_get_rest_hosts_works_when_fallback_realtime_host_is_set():
+    ably = AblyRest(token="foo")
+    ably.options.fallback_realtime_host = ably.options.get_rest_hosts()[0]
+    assert ably.http.get_rest_hosts()
+
+
+def test_http_get_rest_hosts_works_when_fallback_realtime_host_is_not_set():
+    ably = AblyRest(token="foo")
+    ably.options.fallback_realtime_host = None
+    assert ably.http.get_rest_hosts()