Add util for atomic file writing

Given a particular OS or drive combination, atomic rename may not be available,
but we do the overall write as atomically as possible. The method takes a
function for writing the file contents, rather than a string, so that we can
support streaming to the file, which is more efficient.

src/main/java/edu/umich/soar/visualsoar/files/Util.java

@@ -0,0 +1,49 @@
+package edu.umich.soar.visualsoar.files;
+
+import java.io.FileOutputStream;
+import java.io.OutputStream;
+import java.nio.file.AtomicMoveNotSupportedException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.logging.Logger;
+
+import static java.nio.file.StandardCopyOption.ATOMIC_MOVE;
+import static java.nio.file.StandardCopyOption.REPLACE_EXISTING;
+
+public class Util {
+  private static final Logger LOGGER = Logger.getLogger(Util.class.getName());
+
+  @FunctionalInterface
+  public interface Writer {
+    void write(OutputStream out);
+  }
+
+  /**
+   * Write to the destination file from {@code writer} as atomically as possible.
+   */
+  public void Save(Path destination, Writer writer) throws java.io.IOException {
+    // Write to a temp file first, then make the final changes via a rename,
+    // which can often be done atomically. This prevents issues such as overwriting a file with a
+    // incomplete data, as could be the case if we hit an IO exception in the middle of writing the
+    // file (especially relevant for folks using network drives!)
+    String tempFilename = destination.toAbsolutePath() + ".temp";
+    Path tempPath = Paths.get(tempFilename);
+
+    try (FileOutputStream output = new FileOutputStream(tempPath.toFile())) {
+      writer.write(output);
+    }
+
+    try {
+      Files.move(tempPath, destination, REPLACE_EXISTING, ATOMIC_MOVE);
+    } catch (AtomicMoveNotSupportedException e) {
+      LOGGER.warning(
+          "Cannot write "
+              + destination
+              + " atomically  ("
+              + e.getMessage()
+              + "); falling back to non-atomic write");
+      Files.move(tempPath, destination, REPLACE_EXISTING);
+    }
+  }
+}