[#290] only anonymous user can log in without password

Logins without a password (ie. when password='', or when the secrets file
".irodsA" is missing from the irods environment) should only be allowed for the
anonymous user.

The need for this fix was discovered when the demonstration script at:

  #315 (comment)

ceased to trip the issue at hand (#315) when rebased to follow commit:

  91b1ada
  ( [#290] allow skipping of password file in anonymous user case )

Note that prior to the two #290 related commits (ie. this and the above-
mentioned one), instantiating an iRODSSession object from the user's
client environment without the secrets file present would raise a
FileNotFoundError.

As of this change, attempts by any other user than `anonymous' to log in
without auth credentials will instead raise an Exception of type:

  `NonAnonymousLoginWithoutPassword'

Author: d-w-moore

irods/session.py

@@ -19,6 +19,8 @@
 
 logger = logging.getLogger(__name__)
 
+class NonAnonymousLoginWithoutPassword(RuntimeError): pass
+
 class iRODSSession(object):
 
     @property
@@ -113,7 +115,13 @@ def _configure_account(self, **kwargs):
         except KeyError:
             pass
 
-        creds['password'] = self.get_irods_password(session_ = self, **creds)
+        missing_file_path = []
+        error_args = []
+        pw = creds['password'] = self.get_irods_password(session_ = self, file_path_if_not_found = missing_file_path, **creds)
+        if not pw and creds.get('irods_user_name') != 'anonymous':
+            if missing_file_path:
+                error_args += ["Authentication file not found at {!r}".format(missing_file_path[0])]
+            raise NonAnonymousLoginWithoutPassword(*error_args)
 
         return iRODSAccount(**creds)
 
@@ -201,7 +209,8 @@ def get_irods_env(env_file, session_ = None):
             return {}
 
     @staticmethod
-    def get_irods_password(session_ = None, **kwargs):
+    def get_irods_password(session_ = None, file_path_if_not_found = (), **kwargs):
+        path_memo  = []
         try:
             irods_auth_file = kwargs['irods_authentication_file']
         except KeyError:
@@ -219,9 +228,13 @@ def get_irods_password(session_ = None, **kwargs):
                 _retval = decode(f.read().rstrip('\n'), uid)
                 return _retval
         except IOError as exc:
-            if exc.errno != errno.ENOENT: raise # Auth file exists but can't be read
+            if exc.errno != errno.ENOENT:
+                raise  # Auth file exists but can't be read
+            path_memo = [ irods_auth_file ]
             return ''                           # No auth file (as with anonymous user)
         finally:
+            if isinstance(file_path_if_not_found, list) and path_memo:
+                file_path_if_not_found[:] = path_memo
             if session_ is not None and _retval:
                 session_._auth_file = irods_auth_file