Make a safer way to check if an interface has an attribute

src/_zkapauthorizer/foolscap.py

@@ -141,6 +141,20 @@ def add_arguments(schema, kwargs):
     return modified_schema
 
 
+def remoteinterface_hasattr(ri, name):
+    """
+    :param InterfaceClass ri: A ``RemoteInterface`` to inspect.
+    :param str name: The name of an attribute.
+
+    :return bool: ``True`` if and only if ``ri`` has the attribute named by
+        ``name``, ``False`` otherwise.
+    """
+    try:
+        ri[name]
+    except KeyError:
+        return False
+    return True
+
 
 class RIPrivacyPassAuthorizedStorageServer(RemoteInterface):
     """
@@ -162,7 +176,7 @@ class RIPrivacyPassAuthorizedStorageServer(RemoteInterface):
 
     add_lease = add_passes(RIStorageServer["add_lease"])
 
-    if "renew_lease" in RIStorageServer:
+    if remoteinterface_hasattr(RIStorageServer, "renew_lease"):
         # Tahoe-LAFS 1.16.0 drops renew_lease from the interface.  Do likewise
         # here, if we discover we have a version of Tahoe that has done so.
         # If Tahoe has dropped this method then nothing in Tahoe is going to