Workaround for socket permission errors on Cygwin (#4584)

* Workaround for wrong errno on socket bind permission errors on Cygwin.

* [ci skip] Add period to comment

Co-authored-by: Steven Silvester <stslve@amazon.com>

Author: embray

notebook/notebookapp.py

@@ -1725,13 +1725,20 @@ def _bind_http_server_tcp(self):
             try:
                 self.http_server.listen(port, self.ip)
             except socket.error as e:
+                eacces = (errno.EACCES, getattr(errno, 'WSAEACCES', errno.EACCES))
+                if sys.platform == 'cygwin':
+                    # Cygwin has a bug that causes EPERM to be returned in this
+                    # case instead of EACCES:
+                    # https://cygwin.com/ml/cygwin/2019-04/msg00160.html
+                    eacces += (errno.EPERM,)
+
                 if e.errno == errno.EADDRINUSE:
                     if self.port_retries:
                         self.log.info(_('The port %i is already in use, trying another port.') % port)
                     else:
                         self.log.info(_('The port %i is already in use.') % port)
                     continue
-                elif e.errno in (errno.EACCES, getattr(errno, 'WSAEACCES', errno.EACCES)):
+                elif e.errno in eacces:
                     self.log.warning(_("Permission to listen on port %i denied.") % port)
                     continue
                 else: