3021: Add comment about IPv4/IPv6 trap in websocket client.

diff --git a/sdk/python/arvados/events.py b/sdk/python/arvados/events.py
index fc86cb46d4eecfbdd821f0e6f34da5df2061d437..94dd62b4ed1370cd239106c78abbbf8b249cb832 100644 (file)
--- a/sdk/python/arvados/events.py
+++ b/sdk/python/arvados/events.py
@@ -22,6 +22,11 @@ class EventClient(WebSocketClient):
             ssl_options['cert_reqs'] = ssl.CERT_NONE
         else:
             ssl_options['cert_reqs'] = ssl.CERT_REQUIRED
+
+        # Warning: If the host part of url resolves to both IPv6 and
+        # IPv4 addresses (common with "localhost"), only one of them
+        # will be attempted -- and it might not be the right one. See
+        # ws4py's WebSocketBaseClient.__init__.
         super(EventClient, self).__init__(url, ssl_options=ssl_options)
         self.filters = filters
         self.on_event = on_event