Ensure that we can bind and connect to localhost

otherwise fallback to 127.0.0.1 for defaults

jupyterhub/utils.py

@@ -6,10 +6,12 @@
 from binascii import b2a_hex
 import errno
 import hashlib
+from hmac import compare_digest
 import os
 import socket
+from threading import Thread
 import uuid
-from hmac import compare_digest
+import warnings
 
 from tornado import web, gen, ioloop
 from tornado.httpclient import AsyncHTTPClient, HTTPError
@@ -192,3 +194,36 @@ def url_path_join(*pieces):
         result = '/'
 
     return result
+
+def localhost():
+    """Return localhost or 127.0.0.1"""
+    if hasattr(localhost, '_localhost'):
+        return localhost._localhost
+    binder = connector = None
+    try:
+        binder = socket.socket()
+        binder.bind(('localhost', 0))
+        binder.listen(1)
+        port = binder.getsockname()[1]
+        def accept():
+            try:
+                conn, addr = binder.accept()
+            except ConnectionAbortedError:
+                pass
+            else:
+                conn.close()
+        t = Thread(target=accept)
+        t.start()
+        connector = socket.create_connection(('localhost', port), timeout=10)
+        t.join(timeout=10)
+    except (socket.error, socket.gaierror) as e:
+        warnings.warn("localhost doesn't appear to work, using 127.0.0.1\n%s" % e, RuntimeWarning)
+        localhost._localhost = '127.0.0.1'
+    else:
+        localhost._localhost = 'localhost'
+    finally:
+        if binder:
+            binder.close()
+        if connector:
+            connector.close()
+    return localhost._localhost