reftable: compact merged tables

A compaction of reftables is just copying the results of a
MergedReftable into a ReftableWriter.  Wrap this up into a utility.

Change-Id: I6f5677d923e9628993a2d8b4b007a9b8662c9045

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/MergedReftableTest.java

@@ -261,6 +261,33 @@ public class MergedReftableTest {
 		}
 	}
 
+	@Test
+	public void compaction() throws IOException {
+		List<Ref> delta1 = Arrays.asList(
+				ref("refs/heads/next", 4),
+				ref("refs/heads/master", 1));
+		List<Ref> delta2 = Arrays.asList(delete("refs/heads/next"));
+		List<Ref> delta3 = Arrays.asList(ref("refs/heads/master", 8));
+
+		ReftableCompactor compactor = new ReftableCompactor();
+		compactor.addAll(Arrays.asList(
+				read(write(delta1)),
+				read(write(delta2)),
+				read(write(delta3))));
+		ByteArrayOutputStream out = new ByteArrayOutputStream();
+		compactor.compact(out);
+		byte[] table = out.toByteArray();
+
+		ReftableReader reader = read(table);
+		try (RefCursor rc = reader.allRefs()) {
+			assertTrue(rc.next());
+			Ref r = rc.getRef();
+			assertEquals("refs/heads/master", r.getName());
+			assertEquals(id(8), r.getObjectId());
+			assertFalse(rc.next());
+		}
+	}
+
 	private static MergedReftable merge(byte[]... table) {
 		List<Reftable> stack = new ArrayList<>(table.length);
 		for (byte[] b : table) {

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableCompactor.java

@@ -0,0 +1,223 @@
+/*
+ * 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.reftable;
+
+import java.io.IOException;
+import java.io.OutputStream;
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.List;
+
+import org.eclipse.jgit.lib.PersonIdent;
+import org.eclipse.jgit.internal.storage.reftable.ReftableWriter.Stats;
+import org.eclipse.jgit.lib.ReflogEntry;
+
+/**
+ * Merges reftables and compacts them into a single output.
+ * <p>
+ * For a partial compaction callers should {@link #setIncludeDeletes(boolean)}
+ * to {@code true} to ensure the new reftable continues to use a delete marker
+ * to shadow any lower reftable that may have the reference present.
+ * <p>
+ * By default all log entries within the range defined by
+ * {@link #setMinUpdateIndex(long)} and {@link #setMaxUpdateIndex(long)} are
+ * copied, even if no references in the output file match the log records.
+ * Callers may truncate the log to a more recent time horizon with
+ * {@link #setOldestReflogTimeMillis(long)}, or disable the log altogether with
+ * {@code setOldestReflogTimeMillis(Long.MAX_VALUE)}.
+ */
+public class ReftableCompactor {
+	private final ReftableWriter writer = new ReftableWriter();
+	private final ArrayDeque<Reftable> tables = new ArrayDeque<>();
+
+	private boolean includeDeletes;
+	private long minUpdateIndex;
+	private long maxUpdateIndex;
+	private long oldestReflogTimeMillis;
+	private Stats stats;
+
+	/**
+	 * @param cfg
+	 *            configuration for the reftable.
+	 * @return {@code this}
+	 */
+	public ReftableCompactor setConfig(ReftableConfig cfg) {
+		writer.setConfig(cfg);
+		return this;
+	}
+
+	/**
+	 * @param deletes
+	 *            {@code true} to include deletions in the output, which may be
+	 *            necessary for partial compaction.
+	 * @return {@code this}
+	 */
+	public ReftableCompactor setIncludeDeletes(boolean deletes) {
+		includeDeletes = deletes;
+		return this;
+	}
+
+	/**
+	 * @param min
+	 *            the minimum update index for log entries that appear in the
+	 *            compacted reftable. This should be 1 higher than the prior
+	 *            reftable's {@code maxUpdateIndex} if this table will be used
+	 *            in a stack.
+	 * @return {@code this}
+	 */
+	public ReftableCompactor setMinUpdateIndex(long min) {
+		minUpdateIndex = min;
+		return this;
+	}
+
+	/**
+	 * @param max
+	 *            the maximum update index for log entries that appear in the
+	 *            compacted reftable. This should be at least 1 higher than the
+	 *            prior reftable's {@code maxUpdateIndex} if this table will be
+	 *            used in a stack.
+	 * @return {@code this}
+	 */
+	public ReftableCompactor setMaxUpdateIndex(long max) {
+		maxUpdateIndex = max;
+		return this;
+	}
+
+	/**
+	 * @param timeMillis
+	 *            oldest log time to preserve. Entries whose timestamps are
+	 *            {@code >= timeMillis} will be copied into the output file. Log
+	 *            entries that predate {@code timeMillis} will be discarded.
+	 *            Specified in Java standard milliseconds since the epoch.
+	 * @return {@code this}
+	 */
+	public ReftableCompactor setOldestReflogTimeMillis(long timeMillis) {
+		oldestReflogTimeMillis = timeMillis;
+		return this;
+	}
+
+	/**
+	 * Add all of the tables, in the specified order.
+	 *
+	 * @param readers
+	 *            tables to compact. Tables should be ordered oldest first/most
+	 *            recent last so that the more recent tables can shadow the
+	 *            older results. Caller is responsible for closing the readers.
+	 */
+	public void addAll(List<? extends Reftable> readers) {
+		tables.addAll(readers);
+	}
+
+	/**
+	 * Write a compaction to {@code out}.
+	 *
+	 * @param out
+	 *            stream to write the compacted tables to. Caller is responsible
+	 *            for closing {@code out}.
+	 * @throws IOException
+	 *             if tables cannot be read, or cannot be written.
+	 */
+	public void compact(OutputStream out) throws IOException {
+		MergedReftable mr = new MergedReftable(new ArrayList<>(tables));
+		mr.setIncludeDeletes(includeDeletes);
+
+		writer.setMinUpdateIndex(minUpdateIndex);
+		writer.setMaxUpdateIndex(maxUpdateIndex);
+		writer.begin(out);
+		mergeRefs(mr);
+		mergeLogs(mr);
+		writer.finish();
+		stats = writer.getStats();
+	}
+
+	/** @return statistics of the last written reftable. */
+	public Stats getStats() {
+		return stats;
+	}
+
+	private void mergeRefs(MergedReftable mr) throws IOException {
+		try (RefCursor rc = mr.allRefs()) {
+			while (rc.next()) {
+				writer.writeRef(rc.getRef());
+			}
+		}
+	}
+
+	private void mergeLogs(MergedReftable mr) throws IOException {
+		if (oldestReflogTimeMillis == Long.MAX_VALUE) {
+			return;
+		}
+
+		try (LogCursor lc = mr.allLogs()) {
+			while (lc.next()) {
+				long updateIndex = lc.getUpdateIndex();
+				if (updateIndex < minUpdateIndex
+						|| updateIndex > maxUpdateIndex) {
+					// Cannot merge log records outside the header's range.
+					continue;
+				}
+
+				String refName = lc.getRefName();
+				ReflogEntry log = lc.getReflogEntry();
+				if (log == null) {
+					if (includeDeletes) {
+						writer.deleteLog(refName, updateIndex);
+					}
+					continue;
+				}
+
+				PersonIdent who = log.getWho();
+				if (who.getWhen().getTime() >= oldestReflogTimeMillis) {
+					writer.writeLog(
+							refName,
+							updateIndex,
+							who,
+							log.getOldId(),
+							log.getNewId(),
+							log.getComment());
+				}
+			}
+		}
+	}
+}