Don't inquire about incomplete security contexts

This commit changes `SecurityContext` to handle a couple of cases where
we called `_inquire` when the context was not fully established.  A bug
in MIT krb5 prevents this from working properly.

In the case of `complete`, we simply cache the result of the last
`step` call.

Fixes #69

Author: DirectXMan12

gssapi/sec_contexts.py

@@ -110,10 +110,21 @@ def __init__(self, base=None, token=None,
 
         else:
             # we already have a context in progress, just inspect it
-            if self.locally_initiated:
-                self.usage = 'initiate'
-            else:
-                self.usage = 'accept'
+            # NB(directxman12): MIT krb5 refuses to inquire about a context
+            # if it's partially established, so we have to check here
+
+            try:
+                if self.locally_initiated:
+                    self.usage = 'initiate'
+                else:
+                    self.usage = 'accept'
+            except excs.MissingContextError:
+                msg = ("Cannot extract usage from a partially completed "
+                       "context")
+                raise excs.UnknownUsageError(msg, obj="security context")
+
+        # This is to work around an MIT krb5 bug (see the `complete` property)
+        self._complete = None
 
     # NB(directxman12): DO NOT ADD AN __del__ TO THIS CLASS -- it screws up
     #                   the garbage collector if _last_tb is still defined
@@ -422,8 +433,19 @@ def lifetime(self):
     @_utils.check_last_err
     def complete(self):
         """Whether negotiation for this context has been completed"""
+        # NB(directxman12): MIT krb5 has a bug where it refuses to
+        #                   inquire about partially completed contexts,
+        #                   so we can't just use `self._inquire` generally
         if self._started:
-            return self._inquire(complete=True).complete
+            if self._complete is None:
+                try:
+                    res = self._inquire(complete=True).complete
+                except excs.MissingContextError:
+                    return False
+                else:
+                    self._complete = res
+
+            return self._complete
         else:
             return False
 
@@ -491,6 +513,8 @@ def _acceptor_step(self, token):
 
         self.delegated_creds = Credentials(res.delegated_creds)
 
+        self._complete = not res.more_steps
+
         return res.token
 
     def _initiator_step(self, token=None):
@@ -501,6 +525,8 @@ def _initiator_step(self, token=None):
                                              self._channel_bindings,
                                              token)
 
+        self._complete = not res.more_steps
+
         return res.token
 
     # pickle protocol support

gssapi/tests/test_high_level.py

@@ -619,6 +619,19 @@ def _create_completed_contexts(self):
 
         return (client_ctx, server_ctx)
 
+    def test_complete_on_partially_completed(self):
+        client_ctx = self._create_client_ctx()
+        client_tok = client_ctx.step()
+        client_ctx.complete.should_be_false()
+
+        server_ctx = gssctx.SecurityContext(creds=self.server_creds)
+        server_tok = server_ctx.step(client_tok)
+
+        client_ctx.step(server_tok)
+
+        client_ctx.complete.should_be_true()
+        server_ctx.complete.should_be_true()
+
     def test_initiate_accept_steps(self):
         client_ctx, server_ctx = self._create_completed_contexts()