Support 'store' argument in Credentials.add

Previously, there was not way to access `add_cred_from` from
the high-level API.  Now, `Credentials.add` accepts the
`store` method.

Closes #14

Author: DirectXMan12

gssapi/creds.py

@@ -276,7 +276,8 @@ def inquire_by_mech(self, mech, name=True, init_lifetime=True,
                                               res.usage)
 
     def add(self, desired_name, desired_mech, usage='both',
-            init_lifetime=None, accept_lifetime=None, impersonator=None):
+            init_lifetime=None, accept_lifetime=None, impersonator=None,
+            store=None):
         """Acquire more credentials to add to the current set
 
         This method works like :meth:`acquire`, except that it adds the
@@ -289,6 +290,17 @@ def add(self, desired_name, desired_mech, usage='both',
         impersonate the given name using the impersonator credentials.
         This requires the Services4User extension.
 
+        If the `store` argument is used, the credentials will be acquired
+        from the given credential store (if supported).  Otherwise, the
+        credentials are acquired from the default store.
+
+        The credential store information is a dictionary containing
+        mechanisms-specific keys and values pointing to a credential store
+        or stores.
+
+        Note that the `store` argument is not compatible with the
+        `impersonator` argument.
+
         Args:
             desired_name (Name): the name associated with the
                 credentials
@@ -302,13 +314,30 @@ def add(self, desired_name, desired_mech, usage='both',
                 credentials, or None for indefinite
             impersonator (Credentials): the credentials to use to impersonate
                 the given name, or None to not acquire normally
+            store (dict): the credential store information pointing to the
+                credential store from which to acquire the credentials,
+                or None for the default store
 
         Returns:
             Credentials: the credentials set containing the current credentials
                 and the newly acquired ones.
         """
 
-        if impersonator is not None:
+        if store is not None and impersonator is not None:
+            raise ValueError('You cannot use both the `impersonator` and '
+                             '`store` arguments at the same time')
+
+        if store is not None:
+            if rcred_cred_store is None:
+                raise NotImplementedError("Your GSSAPI implementation does "
+                                          "not have support for manipulating "
+                                          "credential stores")
+
+            res = rcred_cred_store.add_cred_from(store, self, desired_name,
+                                                 desired_mech, usage,
+                                                 init_lifetime,
+                                                 accept_lifetime)
+        elif impersonator is not None:
             if rcred_s4u is None:
                 raise NotImplementedError("Your GSSAPI implementation does "
                                           "not have support for S4U")

gssapi/tests/test_high_level.py

@@ -115,6 +115,10 @@ def true_false_perms(*all_elems_tuple):
 class CredsTestCase(_GSSAPIKerberosTestCase):
     def setUp(self):
         super(CredsTestCase, self).setUp()
+
+        svc_princ = SERVICE_PRINCIPAL.decode("UTF-8")
+        self.realm.kinit(svc_princ, flags=['-k'])
+
         self.name = gssnames.Name(SERVICE_PRINCIPAL,
                                   gb.NameType.kerberos_principal)
 
@@ -233,6 +237,10 @@ def test_inquire_by_mech(self, str_name, kwargs):
     def test_add(self):
         self.skipTest("Not Yet Implemented")
 
+    # NB(directxman12): we don't test add_cred_from because it really requires
+    #                   multiple mechanism support, which would mean something
+    #                   like requiring NTLM libraries
+
     @_extension_test('cred_imp_ext', 'credentials import-export')
     def test_export(self):
         creds = gsscreds.Credentials(desired_name=self.name)