Make HasDigest serializable.
package com.google.devtools.build.lib.actions;
import com.google.protobuf.ByteString;
+import java.io.Serializable;
/** A marker interface for objects which can return a byte[] digest. */
-public interface HasDigest {
+public interface HasDigest extends Serializable {
byte[] getDigest();
public static final HasDigest EMPTY = new ByteStringDigest(new byte[] {});
