Add constraint package to let us reject invalid assignments.

pkg/constraint/doc.go

@@ -0,0 +1,23 @@
+/*
+Copyright 2014 Google Inc. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package constraint has functions for ensuring that collections of
+// containers are allowed to run together on a single host.
+//
+// TODO: Add resource math. Phrase this code in a way that makes it easy
+// to call from schedulers as well as from the feasiblity check that
+// apiserver performs.
+package constraint