1 files changed, 87 insertions, 0 deletions
diff --git a/src/main/java/com/google/devtools/build/lib/actions/FileContentsProxy.java b/src/main/java/com/google/devtools/build/lib/actions/FileContentsProxy.java
new file mode 100644
index 0000000000..b8a571bd25
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+++ b/src/main/java/com/google/devtools/build/lib/actions/FileContentsProxy.java
@@ -0,0 +1,87 @@
+// Copyright 2014 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.lib.actions;
+
+import com.google.devtools.build.lib.vfs.FileStatus;
+import java.io.IOException;
+import java.io.Serializable;
+import java.util.Objects;
+
+/**
+ * In case we can't get a fast digest from the filesystem, we store this metadata as a proxy to the
+ * file contents. Currently it is a pair of a relevant timestamp and a "node id". On Linux the
+ * former is the ctime and the latter is the inode number. We might want to add the device number in
+ * the future.
+ *
+ * <p>For a Linux example of why mtime alone is insufficient, note that 'mv' preserves timestamps.
+ * So if files 'a' and 'b' initially have the same timestamp, then we would think 'b' is unchanged
+ * after the user executes `mv a b` between two builds.
+ */
+public final class FileContentsProxy implements Serializable {
+  private final long ctime;
+  private final long nodeId;
+
+  /**
+   * Visible for serialization / deserialization. Do not use this method, but call {@link #create}
+   * instead.
+   */
+  public FileContentsProxy(long ctime, long nodeId) {
+    this.ctime = ctime;
+    this.nodeId = nodeId;
+  }
+
+  public static FileContentsProxy create(FileStatus stat) throws IOException {
+    // Note: there are file systems that return mtime for this call instead of ctime, such as the
+    // WindowsFileSystem.
+    return new FileContentsProxy(stat.getLastChangeTime(), stat.getNodeId());
+  }
+
+  /** Visible for serialization / deserialization. Do not use this method; use {@link #equals}. */
+  public long getCTime() {
+    return ctime;
+  }
+
+  /** Visible for serialization / deserialization. Do not use this method; use {@link #equals}. */
+  public long getNodeId() {
+    return nodeId;
+  }
+
+  @Override
+  public boolean equals(Object other) {
+    if (other == this) {
+      return true;
+    }
+
+    if (!(other instanceof FileContentsProxy)) {
+      return false;
+    }
+
+    FileContentsProxy that = (FileContentsProxy) other;
+    return ctime == that.ctime && nodeId == that.nodeId;
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(ctime, nodeId);
+  }
+
+  @Override
+  public String toString() {
+    return prettyPrint();
+  }
+
+  public String prettyPrint() {
+    return String.format("ctime of %d and nodeId of %d", ctime, nodeId);
+  }
+}