reftable: merge-join reftables

MergedReftable combines multiple reference tables together in a stack,
allowing higher/later tables to shadow earlier/lower tables.  This
forms the basis of a transaction system, where each transaction writes
a new reftable containing only the modified references, and readers
perform a merge on the fly to get the latest value.

Change-Id: Ic2cb750141e8c61a8b2726b2eb95195acb6ddc83

2 files changed, 699 insertions, 0 deletions

diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/MergedReftableTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/MergedReftableTest.java
new file mode 100644
index 0000000000..b6306d09e4
--- /dev/null
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/reftable/MergedReftableTest.java
@@ -0,0 +1,324 @@
+/*
+ * 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 static org.eclipse.jgit.lib.Constants.HEAD;
+import static org.eclipse.jgit.lib.Constants.OBJECT_ID_LENGTH;
+import static org.eclipse.jgit.lib.Constants.R_HEADS;
+import static org.eclipse.jgit.lib.Ref.Storage.NEW;
+import static org.eclipse.jgit.lib.Ref.Storage.PACKED;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertTrue;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+import org.eclipse.jgit.internal.storage.io.BlockSource;
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectIdRef;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.RefComparator;
+import org.junit.Test;
+
+public class MergedReftableTest {
+	@Test
+	public void noTables() throws IOException {
+		MergedReftable mr = merge(new byte[0][]);
+		try (RefCursor rc = mr.allRefs()) {
+			assertFalse(rc.next());
+		}
+		try (RefCursor rc = mr.seekRef(HEAD)) {
+			assertFalse(rc.next());
+		}
+		try (RefCursor rc = mr.seekRef(R_HEADS)) {
+			assertFalse(rc.next());
+		}
+	}
+
+	@Test
+	public void oneEmptyTable() throws IOException {
+		MergedReftable mr = merge(write());
+		try (RefCursor rc = mr.allRefs()) {
+			assertFalse(rc.next());
+		}
+		try (RefCursor rc = mr.seekRef(HEAD)) {
+			assertFalse(rc.next());
+		}
+		try (RefCursor rc = mr.seekRef(R_HEADS)) {
+			assertFalse(rc.next());
+		}
+	}
+
+	@Test
+	public void twoEmptyTables() throws IOException {
+		MergedReftable mr = merge(write(), write());
+		try (RefCursor rc = mr.allRefs()) {
+			assertFalse(rc.next());
+		}
+		try (RefCursor rc = mr.seekRef(HEAD)) {
+			assertFalse(rc.next());
+		}
+		try (RefCursor rc = mr.seekRef(R_HEADS)) {
+			assertFalse(rc.next());
+		}
+	}
+
+	@SuppressWarnings("boxing")
+	@Test
+	public void oneTableScan() throws IOException {
+		List<Ref> refs = new ArrayList<>();
+		for (int i = 1; i <= 567; i++) {
+			refs.add(ref(String.format("refs/heads/%03d", i), i));
+		}
+
+		MergedReftable mr = merge(write(refs));
+		try (RefCursor rc = mr.allRefs()) {
+			for (Ref exp : refs) {
+				assertTrue("has " + exp.getName(), rc.next());
+				Ref act = rc.getRef();
+				assertEquals(exp.getName(), act.getName());
+				assertEquals(exp.getObjectId(), act.getObjectId());
+			}
+			assertFalse(rc.next());
+		}
+	}
+
+	@Test
+	public void deleteIsHidden() throws IOException {
+		List<Ref> delta1 = Arrays.asList(
+				ref("refs/heads/apple", 1),
+				ref("refs/heads/master", 2));
+		List<Ref> delta2 = Arrays.asList(delete("refs/heads/apple"));
+
+		MergedReftable mr = merge(write(delta1), write(delta2));
+		try (RefCursor rc = mr.allRefs()) {
+			assertTrue(rc.next());
+			assertEquals("refs/heads/master", rc.getRef().getName());
+			assertEquals(id(2), rc.getRef().getObjectId());
+			assertFalse(rc.next());
+		}
+	}
+
+	@Test
+	public void twoTableSeek() throws IOException {
+		List<Ref> delta1 = Arrays.asList(
+				ref("refs/heads/apple", 1),
+				ref("refs/heads/master", 2));
+		List<Ref> delta2 = Arrays.asList(ref("refs/heads/banana", 3));
+
+		MergedReftable mr = merge(write(delta1), write(delta2));
+		try (RefCursor rc = mr.seekRef("refs/heads/master")) {
+			assertTrue(rc.next());
+			assertEquals("refs/heads/master", rc.getRef().getName());
+			assertEquals(id(2), rc.getRef().getObjectId());
+			assertFalse(rc.next());
+		}
+	}
+
+	@Test
+	public void twoTableById() throws IOException {
+		List<Ref> delta1 = Arrays.asList(
+				ref("refs/heads/apple", 1),
+				ref("refs/heads/master", 2));
+		List<Ref> delta2 = Arrays.asList(ref("refs/heads/banana", 3));
+
+		MergedReftable mr = merge(write(delta1), write(delta2));
+		try (RefCursor rc = mr.byObjectId(id(2))) {
+			assertTrue(rc.next());
+			assertEquals("refs/heads/master", rc.getRef().getName());
+			assertEquals(id(2), rc.getRef().getObjectId());
+			assertFalse(rc.next());
+		}
+	}
+
+	@SuppressWarnings("boxing")
+	@Test
+	public void fourTableScan() throws IOException {
+		List<Ref> base = new ArrayList<>();
+		for (int i = 1; i <= 567; i++) {
+			base.add(ref(String.format("refs/heads/%03d", i), i));
+		}
+
+		List<Ref> delta1 = Arrays.asList(
+				ref("refs/heads/next", 4),
+				ref(String.format("refs/heads/%03d", 55), 4096));
+		List<Ref> delta2 = Arrays.asList(
+				delete("refs/heads/next"),
+				ref(String.format("refs/heads/%03d", 55), 8192));
+		List<Ref> delta3 = Arrays.asList(
+				ref("refs/heads/master", 4242),
+				ref(String.format("refs/heads/%03d", 42), 5120),
+				ref(String.format("refs/heads/%03d", 98), 6120));
+
+		List<Ref> expected = merge(base, delta1, delta2, delta3);
+		MergedReftable mr = merge(
+				write(base),
+				write(delta1),
+				write(delta2),
+				write(delta3));
+		try (RefCursor rc = mr.allRefs()) {
+			for (Ref exp : expected) {
+				assertTrue("has " + exp.getName(), rc.next());
+				Ref act = rc.getRef();
+				assertEquals(exp.getName(), act.getName());
+				assertEquals(exp.getObjectId(), act.getObjectId());
+			}
+			assertFalse(rc.next());
+		}
+	}
+
+	@Test
+	public void scanIncludeDeletes() throws IOException {
+		List<Ref> delta1 = Arrays.asList(ref("refs/heads/next", 4));
+		List<Ref> delta2 = Arrays.asList(delete("refs/heads/next"));
+		List<Ref> delta3 = Arrays.asList(ref("refs/heads/master", 8));
+
+		MergedReftable mr = merge(write(delta1), write(delta2), write(delta3));
+		mr.setIncludeDeletes(true);
+		try (RefCursor rc = mr.allRefs()) {
+			assertTrue(rc.next());
+			Ref r = rc.getRef();
+			assertEquals("refs/heads/master", r.getName());
+			assertEquals(id(8), r.getObjectId());
+
+			assertTrue(rc.next());
+			r = rc.getRef();
+			assertEquals("refs/heads/next", r.getName());
+			assertEquals(NEW, r.getStorage());
+			assertNull(r.getObjectId());
+
+			assertFalse(rc.next());
+		}
+	}
+
+	@SuppressWarnings("boxing")
+	@Test
+	public void oneTableSeek() throws IOException {
+		List<Ref> refs = new ArrayList<>();
+		for (int i = 1; i <= 567; i++) {
+			refs.add(ref(String.format("refs/heads/%03d", i), i));
+		}
+
+		MergedReftable mr = merge(write(refs));
+		for (Ref exp : refs) {
+			try (RefCursor rc = mr.seekRef(exp.getName())) {
+				assertTrue("has " + exp.getName(), rc.next());
+				Ref act = rc.getRef();
+				assertEquals(exp.getName(), act.getName());
+				assertEquals(exp.getObjectId(), act.getObjectId());
+				assertFalse(rc.next());
+			}
+		}
+	}
+
+	private static MergedReftable merge(byte[]... table) {
+		List<Reftable> stack = new ArrayList<>(table.length);
+		for (byte[] b : table) {
+			stack.add(read(b));
+		}
+		return new MergedReftable(stack);
+	}
+
+	private static ReftableReader read(byte[] table) {
+		return new ReftableReader(BlockSource.from(table));
+	}
+
+	private static Ref ref(String name, int id) {
+		return new ObjectIdRef.PeeledNonTag(PACKED, name, id(id));
+	}
+
+	private static Ref delete(String name) {
+		return new ObjectIdRef.Unpeeled(NEW, name, null);
+	}
+
+	private static ObjectId id(int i) {
+		byte[] buf = new byte[OBJECT_ID_LENGTH];
+		buf[0] = (byte) (i & 0xff);
+		buf[1] = (byte) ((i >>> 8) & 0xff);
+		buf[2] = (byte) ((i >>> 16) & 0xff);
+		buf[3] = (byte) (i >>> 24);
+		return ObjectId.fromRaw(buf);
+	}
+
+	private byte[] write(Ref... refs) throws IOException {
+		return write(Arrays.asList(refs));
+	}
+
+	private byte[] write(Collection<Ref> refs) throws IOException {
+		ByteArrayOutputStream buffer = new ByteArrayOutputStream();
+		ReftableWriter writer = new ReftableWriter().begin(buffer);
+		for (Ref r : RefComparator.sort(refs)) {
+			writer.writeRef(r);
+		}
+		writer.finish();
+		return buffer.toByteArray();
+	}
+
+	@SafeVarargs
+	private static List<Ref> merge(List<Ref>... tables) {
+		Map<String, Ref> expect = new HashMap<>();
+		for (List<Ref> t : tables) {
+			for (Ref r : t) {
+				if (r.getStorage() == NEW && r.getObjectId() == null) {
+					expect.remove(r.getName());
+				} else {
+					expect.put(r.getName(), r);
+				}
+			}
+		}
+
+		List<Ref> expected = new ArrayList<>(expect.values());
+		Collections.sort(expected, RefComparator.INSTANCE);
+		return expected;
+	}
+}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/MergedReftable.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/MergedReftable.java
new file mode 100644
index 0000000000..71144cd30d
--- /dev/null
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/MergedReftable.java
@@ -0,0 +1,375 @@
+/*
+ * 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.util.List;
+import java.util.PriorityQueue;
+
+import org.eclipse.jgit.lib.AnyObjectId;
+import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.ReflogEntry;
+
+/**
+ * Merges multiple reference tables together.
+ * <p>
+ * A {@link MergedReftable} merge-joins multiple {@link ReftableReader} on the
+ * fly. Tables higher/later in the stack shadow lower/earlier tables, hiding
+ * references that been updated/replaced.
+ * <p>
+ * By default deleted references are skipped and not returned to the caller.
+ * {@link #setIncludeDeletes(boolean)} can be used to modify this behavior if
+ * the caller needs to preserve deletions during partial compaction.
+ * <p>
+ * A {@code MergedReftable} is not thread-safe.
+ */
+public class MergedReftable extends Reftable {
+	private final Reftable[] tables;
+
+	/**
+	 * Initialize a merged table reader.
+	 * <p>
+	 * The tables in {@code tableStack} will be closed when this
+	 * {@code MergedReftable} is closed.
+	 *
+	 * @param tableStack
+	 *            stack of tables to read from. The base of the stack is at
+	 *            index 0, the most recent should be at the top of the stack at
+	 *            {@code tableStack.size() - 1}. The top of the stack (higher
+	 *            index) shadows the base of the stack (lower index).
+	 */
+	public MergedReftable(List<Reftable> tableStack) {
+		tables = tableStack.toArray(new Reftable[0]);
+
+		// Tables must expose deletes to this instance to correctly
+		// shadow references from lower tables.
+		for (Reftable t : tables) {
+			t.setIncludeDeletes(true);
+		}
+	}
+
+	@Override
+	public RefCursor allRefs() throws IOException {
+		MergedRefCursor m = new MergedRefCursor();
+		for (int i = 0; i < tables.length; i++) {
+			m.add(new RefQueueEntry(tables[i].allRefs(), i));
+		}
+		return m;
+	}
+
+	@Override
+	public RefCursor seekRef(String name) throws IOException {
+		if (name.endsWith("/")) { //$NON-NLS-1$
+			return seekRefPrefix(name);
+		}
+		return seekSingleRef(name);
+	}
+
+	private RefCursor seekRefPrefix(String name) throws IOException {
+		MergedRefCursor m = new MergedRefCursor();
+		for (int i = 0; i < tables.length; i++) {
+			m.add(new RefQueueEntry(tables[i].seekRef(name), i));
+		}
+		return m;
+	}
+
+	private RefCursor seekSingleRef(String name) throws IOException {
+		// Walk the tables from highest priority (end of list) to lowest.
+		// As soon as the reference is found (queue not empty), all lower
+		// priority tables are irrelevant as current table shadows them.
+		MergedRefCursor m = new MergedRefCursor();
+		for (int i = tables.length - 1; i >= 0 && m.queue.isEmpty(); i--) {
+			m.add(new RefQueueEntry(tables[i].seekRef(name), i));
+		}
+		return m;
+	}
+
+	@Override
+	public RefCursor byObjectId(AnyObjectId name) throws IOException {
+		MergedRefCursor m = new MergedRefCursor();
+		for (int i = 0; i < tables.length; i++) {
+			m.add(new RefQueueEntry(tables[i].byObjectId(name), i));
+		}
+		return m;
+	}
+
+	@Override
+	public LogCursor allLogs() throws IOException {
+		MergedLogCursor m = new MergedLogCursor();
+		for (int i = 0; i < tables.length; i++) {
+			m.add(new LogQueueEntry(tables[i].allLogs(), i));
+		}
+		return m;
+	}
+
+	@Override
+	public LogCursor seekLog(String refName, long updateIdx)
+			throws IOException {
+		MergedLogCursor m = new MergedLogCursor();
+		for (int i = 0; i < tables.length; i++) {
+			m.add(new LogQueueEntry(tables[i].seekLog(refName, updateIdx), i));
+		}
+		return m;
+	}
+
+	@Override
+	public void close() throws IOException {
+		for (Reftable t : tables) {
+			t.close();
+		}
+	}
+
+	int queueSize() {
+		return Math.max(1, tables.length);
+	}
+
+	private class MergedRefCursor extends RefCursor {
+		private final PriorityQueue<RefQueueEntry> queue;
+		private RefQueueEntry head;
+		private Ref ref;
+
+		MergedRefCursor() {
+			queue = new PriorityQueue<>(queueSize(), RefQueueEntry::compare);
+		}
+
+		void add(RefQueueEntry t) throws IOException {
+			// Common case is many iterations over the same RefQueueEntry
+			// for the bottom of the stack (scanning all refs). Its almost
+			// always less than the top of the queue. Avoid the queue's
+			// O(log N) insertion and removal costs for this common case.
+			if (!t.rc.next()) {
+				t.rc.close();
+			} else if (head == null) {
+				RefQueueEntry p = queue.peek();
+				if (p == null || RefQueueEntry.compare(t, p) < 0) {
+					head = t;
+				} else {
+					head = queue.poll();
+					queue.add(t);
+				}
+			} else if (RefQueueEntry.compare(t, head) > 0) {
+				queue.add(t);
+			} else {
+				queue.add(head);
+				head = t;
+			}
+		}
+
+		@Override
+		public boolean next() throws IOException {
+			for (;;) {
+				RefQueueEntry t = poll();
+				if (t == null) {
+					return false;
+				}
+
+				ref = t.rc.getRef();
+				boolean include = includeDeletes || !t.rc.wasDeleted();
+				skipShadowedRefs(ref.getName());
+				add(t);
+				if (include) {
+					return true;
+				}
+			}
+		}
+
+		private RefQueueEntry poll() {
+			RefQueueEntry e = head;
+			if (e != null) {
+				head = null;
+				return e;
+			}
+			return queue.poll();
+		}
+
+		private void skipShadowedRefs(String name) throws IOException {
+			for (;;) {
+				RefQueueEntry t = head != null ? head : queue.peek();
+				if (t != null && name.equals(t.name())) {
+					add(poll());
+				} else {
+					break;
+				}
+			}
+		}
+
+		@Override
+		public Ref getRef() {
+			return ref;
+		}
+
+		@Override
+		public void close() {
+			while (!queue.isEmpty()) {
+				queue.remove().rc.close();
+			}
+		}
+	}
+
+	private static class RefQueueEntry {
+		static int compare(RefQueueEntry a, RefQueueEntry b) {
+			int cmp = a.name().compareTo(b.name());
+			if (cmp == 0) {
+				// higher index shadows lower index, so higher index first.
+				cmp = b.stackIdx - a.stackIdx;
+			}
+			return cmp;
+		}
+
+		final RefCursor rc;
+		final int stackIdx;
+
+		RefQueueEntry(RefCursor rc, int stackIdx) {
+			this.rc = rc;
+			this.stackIdx = stackIdx;
+		}
+
+		String name() {
+			return rc.getRef().getName();
+		}
+	}
+
+	private class MergedLogCursor extends LogCursor {
+		private final PriorityQueue<LogQueueEntry> queue;
+		private String refName;
+		private long updateIndex;
+		private ReflogEntry entry;
+
+		MergedLogCursor() {
+			queue = new PriorityQueue<>(queueSize(), LogQueueEntry::compare);
+		}
+
+		void add(LogQueueEntry t) throws IOException {
+			if (t.lc.next()) {
+				queue.add(t);
+			} else {
+				t.lc.close();
+			}
+		}
+
+		@Override
+		public boolean next() throws IOException {
+			for (;;) {
+				LogQueueEntry t = queue.poll();
+				if (t == null) {
+					return false;
+				}
+
+				refName = t.lc.getRefName();
+				updateIndex = t.lc.getUpdateIndex();
+				entry = t.lc.getReflogEntry();
+				boolean include = includeDeletes || entry != null;
+				skipShadowed(refName, updateIndex);
+				add(t);
+				if (include) {
+					return true;
+				}
+			}
+		}
+
+		private void skipShadowed(String name, long index) throws IOException {
+			for (;;) {
+				LogQueueEntry t = queue.peek();
+				if (t != null && name.equals(t.name()) && index == t.index()) {
+					add(queue.remove());
+				} else {
+					break;
+				}
+			}
+		}
+
+		@Override
+		public String getRefName() {
+			return refName;
+		}
+
+		@Override
+		public long getUpdateIndex() {
+			return updateIndex;
+		}
+
+		@Override
+		public ReflogEntry getReflogEntry() {
+			return entry;
+		}
+
+		@Override
+		public void close() {
+			while (!queue.isEmpty()) {
+				queue.remove().lc.close();
+			}
+		}
+	}
+
+	private static class LogQueueEntry {
+		static int compare(LogQueueEntry a, LogQueueEntry b) {
+			int cmp = a.name().compareTo(b.name());
+			if (cmp == 0) {
+				// higher update index sorts first.
+				cmp = Long.signum(b.index() - a.index());
+			}
+			if (cmp == 0) {
+				// higher index comes first.
+				cmp = b.stackIdx - a.stackIdx;
+			}
+			return cmp;
+		}
+
+		final LogCursor lc;
+		final int stackIdx;
+
+		LogQueueEntry(LogCursor lc, int stackIdx) {
+			this.lc = lc;
+			this.stackIdx = stackIdx;
+		}
+
+		String name() {
+			return lc.getRefName();
+		}
+
+		long index() {
+			return lc.getUpdateIndex();
+		}
+	}
+}