dfs: helper to open multiple reftables

ReftableStack maintains multiple open reftables
in an AutoCloseable format, making it easier for
higher level code to handle multiple files.

Change-Id: I7ac35c18e67b7e771fb3de29169d1ee50fab62ca

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/ReftableStack.java

@@ -0,0 +1,108 @@
+/*
+ * Copyright (C) 2017, Google Inc.
+ * and other copyright owners as documented in the project's IP log.
+ *
+ * This program and the accompanying materials are made available
+ * under the terms of the Eclipse Distribution License v1.0 which
+ * accompanies this distribution, is reproduced below, and is
+ * available at http://www.eclipse.org/org/documents/edl-v10.php
+ *
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or
+ * without modification, are permitted provided that the following
+ * conditions are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ *
+ * - Redistributions in binary form must reproduce the above
+ *   copyright notice, this list of conditions and the following
+ *   disclaimer in the documentation and/or other materials provided
+ *   with the distribution.
+ *
+ * - Neither the name of the Eclipse Foundation, Inc. nor the
+ *   names of its contributors may be used to endorse or promote
+ *   products derived from this software without specific prior
+ *   written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+ * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+package org.eclipse.jgit.internal.storage.dfs;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+import org.eclipse.jgit.internal.storage.reftable.Reftable;
+
+/** Tracks multiple open {@link Reftable} instances. */
+public class ReftableStack implements AutoCloseable {
+	/**
+	 * Opens a stack of tables for reading.
+	 *
+	 * @param ctx
+	 *            context to read the tables with. This {@code ctx} will be
+	 *            retained by the stack and each of the table readers.
+	 * @param tables
+	 *            the tables to open.
+	 * @return stack reference to close the tables.
+	 * @throws IOException
+	 *             a table could not be opened
+	 */
+	public static ReftableStack open(DfsReader ctx, List<DfsReftable> tables)
+			throws IOException {
+		ReftableStack stack = new ReftableStack(tables.size());
+		boolean close = true;
+		try {
+			for (DfsReftable t : tables) {
+				stack.tables.add(t.open(ctx));
+			}
+			close = false;
+			return stack;
+		} finally {
+			if (close) {
+				stack.close();
+			}
+		}
+	}
+
+	private final List<Reftable> tables;
+
+	private ReftableStack(int tableCnt) {
+		this.tables = new ArrayList<>(tableCnt);
+	}
+
+	/**
+	 * @return unmodifiable list of tables, in the same order the files were
+	 *         passed to {@link #open(DfsReader, List)}.
+	 */
+	public List<Reftable> readers() {
+		return Collections.unmodifiableList(tables);
+	}
+
+	@Override
+	public void close() {
+		for (Reftable t : tables) {
+			try {
+				t.close();
+			} catch (IOException e) {
+				// Ignore close failures.
+			}
+		}
+	}
+}