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diff --git a/src/main/java/com/google/devtools/build/skyframe/FunctionHermeticity.java b/src/main/java/com/google/devtools/build/skyframe/FunctionHermeticity.java
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+// Copyright 2018 The Bazel Authors. 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 com.google.devtools.build.skyframe;
+
+/**
+ * Hermeticity of a {@link SkyFunction}, meaning whether it accesses external state untracked by
+ * Skyframe during its evaluation. A classic example is a {@link SkyFunction} that consumes a file
+ * on a filesystem: that state is untracked by Skyframe. Skyframe must be more conservative when
+ * using values generated by a non-hermetic function: for instance, a non-hermetic function may need
+ * to be re-run even if all its Skyframe dependencies are unchanged: such a node may be explicitly
+ * dirtied due to outside changes.
+ *
+ * <p>Note that Skyframe does <i>not</i> explicitly re-evaluate non-hermetic functions on every
+ * build: it just relaxes some of its graph-pruning logic to be more conservative with such nodes.
+ */
+public enum FunctionHermeticity {
+  HERMETIC,
+  NONHERMETIC
+}