[JGIT PATCH 1/2] Introduce ThreeWayMerge API

[JGIT PATCH 1/2] Introduce ThreeWayMerge API

[Shawn O. Pearce]

A 3-way merge (2 trees, and one common ancestor) is a specialized
type of merge which has a tighter restriction on its inputs.  This
change refactors the merge API slightly to introduce this special
type, so we can add 3-way specific operations on the API.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
---

 This is a quick two patch series to teach JGit enough to enable an
 application to perform simple cherry-picks, much like we already
 support doing simple merges.

 I'm already using it in Gerrit Code Review to enable projects to
 have a more "format-patch | am" style workflow over a "push; pull"
 style of workflow, if they choose to do that.


 .../src/org/spearce/jgit/merge/MergeStrategy.java  |    2 +-
 .../src/org/spearce/jgit/merge/Merger.java         |    2 +-
 .../jgit/merge/StrategySimpleTwoWayInCore.java     |   11 +--
 .../spearce/jgit/merge/ThreeWayMergeStrategy.java  |   46 ++++++++++
 .../src/org/spearce/jgit/merge/ThreeWayMerger.java |   89 ++++++++++++++++++++
 5 files changed, 141 insertions(+), 9 deletions(-)
 create mode 100644 org.spearce.jgit/src/org/spearce/jgit/merge/ThreeWayMergeStrategy.java
 create mode 100644 org.spearce.jgit/src/org/spearce/jgit/merge/ThreeWayMerger.java

diff --git a/org.spearce.jgit/src/org/spearce/jgit/merge/MergeStrategy.java b/org.spearce.jgit/src/org/spearce/jgit/merge/MergeStrategy.java
index 66bd86c..5439e5c 100644
--- a/org.spearce.jgit/src/org/spearce/jgit/merge/MergeStrategy.java
+++ b/org.spearce.jgit/src/org/spearce/jgit/merge/MergeStrategy.java
@@ -55,7 +55,7 @@
 	public static final MergeStrategy THEIRS = new StrategyOneSided("theirs", 1);
 
 	/** Simple strategy to merge paths, without simultaneous edits. */
-	public static final MergeStrategy SIMPLE_TWO_WAY_IN_CORE = StrategySimpleTwoWayInCore.INSTANCE;
+	public static final ThreeWayMergeStrategy SIMPLE_TWO_WAY_IN_CORE = StrategySimpleTwoWayInCore.INSTANCE;
 
 	private static final HashMap<String, MergeStrategy> STRATEGIES = new HashMap<String, MergeStrategy>();
 
diff --git a/org.spearce.jgit/src/org/spearce/jgit/merge/Merger.java b/org.spearce.jgit/src/org/spearce/jgit/merge/Merger.java
index 100cc38..db3e329 100644
--- a/org.spearce.jgit/src/org/spearce/jgit/merge/Merger.java
+++ b/org.spearce.jgit/src/org/spearce/jgit/merge/Merger.java
@@ -122,7 +122,7 @@ public ObjectWriter getObjectWriter() {
 	 *             one or more sources could not be read, or outputs could not
 	 *             be written to the Repository.
 	 */
-	public final boolean merge(final AnyObjectId[] tips) throws IOException {
+	public boolean merge(final AnyObjectId[] tips) throws IOException {
 		sourceObjects = new RevObject[tips.length];
 		for (int i = 0; i < tips.length; i++)
 			sourceObjects[i] = walk.parseAny(tips[i]);
diff --git a/org.spearce.jgit/src/org/spearce/jgit/merge/StrategySimpleTwoWayInCore.java b/org.spearce.jgit/src/org/spearce/jgit/merge/StrategySimpleTwoWayInCore.java
index 3ebe397..9807644 100644
--- a/org.spearce.jgit/src/org/spearce/jgit/merge/StrategySimpleTwoWayInCore.java
+++ b/org.spearce.jgit/src/org/spearce/jgit/merge/StrategySimpleTwoWayInCore.java
@@ -60,8 +60,8 @@
  * trees will cause a merge conflict, as this strategy does not attempt to merge
  * file contents.
  */
-public class StrategySimpleTwoWayInCore extends MergeStrategy {
-	static final MergeStrategy INSTANCE = new StrategySimpleTwoWayInCore();
+public class StrategySimpleTwoWayInCore extends ThreeWayMergeStrategy {
+	static final ThreeWayMergeStrategy INSTANCE = new StrategySimpleTwoWayInCore();
 
 	/** Create a new instance of the strategy. */
 	protected StrategySimpleTwoWayInCore() {
@@ -74,11 +74,11 @@ public String getName() {
 	}
 
 	@Override
-	public Merger newMerger(final Repository db) {
+	public ThreeWayMerger newMerger(final Repository db) {
 		return new InCoreMerger(db);
 	}
 
-	private static class InCoreMerger extends Merger {
+	private static class InCoreMerger extends ThreeWayMerger {
 		private static final int T_BASE = 0;
 
 		private static final int T_OURS = 1;
@@ -101,9 +101,6 @@ InCoreMerger(final Repository local) {
 
 		@Override
 		protected boolean mergeImpl() throws IOException {
-			if (sourceTrees.length != 2)
-				return false;
-
 			tw.reset();
 			tw.addTree(mergeBase(0, 1));
 			tw.addTree(sourceTrees[0]);
diff --git a/org.spearce.jgit/src/org/spearce/jgit/merge/ThreeWayMergeStrategy.java b/org.spearce.jgit/src/org/spearce/jgit/merge/ThreeWayMergeStrategy.java
new file mode 100644
index 0000000..848e80b
--- /dev/null
+++ b/org.spearce.jgit/src/org/spearce/jgit/merge/ThreeWayMergeStrategy.java
@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2009, Google Inc.
+ *
+ * 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 Git Development Community 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.spearce.jgit.merge;
+
+import org.spearce.jgit.lib.Repository;
+
+/** A merge strategy to merge 2 trees, using a common base ancestor tree. */
+public abstract class ThreeWayMergeStrategy extends MergeStrategy {
+	@Override
+	public abstract ThreeWayMerger newMerger(Repository db);
+}
diff --git a/org.spearce.jgit/src/org/spearce/jgit/merge/ThreeWayMerger.java b/org.spearce.jgit/src/org/spearce/jgit/merge/ThreeWayMerger.java
new file mode 100644
index 0000000..9d1621d
--- /dev/null
+++ b/org.spearce.jgit/src/org/spearce/jgit/merge/ThreeWayMerger.java
@@ -0,0 +1,89 @@
+/*
+ * Copyright (C) 2009, Google Inc.
+ *
+ * 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 Git Development Community 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.spearce.jgit.merge;
+
+import java.io.IOException;
+
+import org.spearce.jgit.errors.IncorrectObjectTypeException;
+import org.spearce.jgit.lib.AnyObjectId;
+import org.spearce.jgit.lib.Repository;
+
+/** A merge of 2 trees, using a common base ancestor tree. */
+public abstract class ThreeWayMerger extends Merger {
+	/**
+	 * Create a new merge instance for a repository.
+	 * 
+	 * @param local
+	 *            the repository this merger will read and write data on.
+	 */
+	protected ThreeWayMerger(final Repository local) {
+		super(local);
+	}
+
+	/**
+	 * Merge together two tree-ish objects.
+	 * <p>
+	 * Any tree-ish may be supplied as inputs. Commits and/or tags pointing at
+	 * trees or commits may be passed as input objects.
+	 * 
+	 * @param a
+	 *            source tree to be combined together.
+	 * @param b
+	 *            source tree to be combined together.
+	 * @return true if the merge was completed without conflicts; false if the
+	 *         merge strategy cannot handle this merge or there were conflicts
+	 *         preventing it from automatically resolving all paths.
+	 * @throws IncorrectObjectTypeException
+	 *             one of the input objects is not a commit, but the strategy
+	 *             requires it to be a commit.
+	 * @throws IOException
+	 *             one or more sources could not be read, or outputs could not
+	 *             be written to the Repository.
+	 */
+	public boolean merge(final AnyObjectId a, final AnyObjectId b)
+			throws IOException {
+		return merge(new AnyObjectId[] { a, b });
+	}
+
+	@Override
+	public boolean merge(final AnyObjectId[] tips) throws IOException {
+		if (tips.length != 2)
+			return false;
+		return super.merge(tips);
+	}
+}
-- 
1.6.2.1.471.g682837

[JGIT PATCH 2/2] Add cherry picking and revert support to JGit

[Shawn O. Pearce]

A cherry-pick can be performed by a 3 way merge, where the base is
the parent of the commit you are cherry-picking, instead of using
the derived base from the commit graph.

By allowing the caller of a ThreeWayMerger to set the merge base
prior to invoking the merge method, we allow the caller to direct
us to execute a cherry-pick, or a revert.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
---
 .../tst/org/spearce/jgit/merge/CherryPickTest.java |  149 ++++++++++++++++++++
 .../src/org/spearce/jgit/merge/Merger.java         |   18 +++-
 .../jgit/merge/StrategySimpleTwoWayInCore.java     |    2 +-
 .../src/org/spearce/jgit/merge/ThreeWayMerger.java |   41 ++++++
 .../spearce/jgit/treewalk/CanonicalTreeParser.java |    2 +-
 5 files changed, 209 insertions(+), 3 deletions(-)
 create mode 100644 org.spearce.jgit.test/tst/org/spearce/jgit/merge/CherryPickTest.java

diff --git a/org.spearce.jgit.test/tst/org/spearce/jgit/merge/CherryPickTest.java b/org.spearce.jgit.test/tst/org/spearce/jgit/merge/CherryPickTest.java
new file mode 100644
index 0000000..78a5553
--- /dev/null
+++ b/org.spearce.jgit.test/tst/org/spearce/jgit/merge/CherryPickTest.java
@@ -0,0 +1,149 @@
+/*
+ * Copyright (C) 2009, Google Inc.
+ * Copyright (C) 2008, Robin Rosenberg
+ *
+ * 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 Git Development Community 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.spearce.jgit.merge;
+
+import java.io.ByteArrayInputStream;
+
+import org.spearce.jgit.dircache.DirCache;
+import org.spearce.jgit.dircache.DirCacheBuilder;
+import org.spearce.jgit.dircache.DirCacheEntry;
+import org.spearce.jgit.lib.Commit;
+import org.spearce.jgit.lib.Constants;
+import org.spearce.jgit.lib.FileMode;
+import org.spearce.jgit.lib.ObjectId;
+import org.spearce.jgit.lib.ObjectWriter;
+import org.spearce.jgit.lib.PersonIdent;
+import org.spearce.jgit.lib.RepositoryTestCase;
+import org.spearce.jgit.treewalk.TreeWalk;
+
+public class CherryPickTest extends RepositoryTestCase {
+	public void testPick() throws Exception {
+		// B---O
+		// \----P---T
+		//
+		// Cherry-pick "T" onto "O". This shouldn't introduce "p-fail", which
+		// was created by "P", nor should it modify "a", which was done by "P".
+		//
+		final DirCache treeB = DirCache.read(db);
+		final DirCache treeO = DirCache.read(db);
+		final DirCache treeP = DirCache.read(db);
+		final DirCache treeT = DirCache.read(db);
+		{
+			final DirCacheBuilder b = treeB.builder();
+			final DirCacheBuilder o = treeO.builder();
+			final DirCacheBuilder p = treeP.builder();
+			final DirCacheBuilder t = treeT.builder();
+
+			b.add(makeEntry("a", FileMode.REGULAR_FILE));
+
+			o.add(makeEntry("a", FileMode.REGULAR_FILE));
+			o.add(makeEntry("o", FileMode.REGULAR_FILE));
+
+			p.add(makeEntry("a", FileMode.REGULAR_FILE, "q"));
+			p.add(makeEntry("p-fail", FileMode.REGULAR_FILE));
+
+			t.add(makeEntry("a", FileMode.REGULAR_FILE));
+			t.add(makeEntry("t", FileMode.REGULAR_FILE));
+
+			b.finish();
+			o.finish();
+			p.finish();
+			t.finish();
+		}
+
+		final ObjectWriter ow = new ObjectWriter(db);
+		final ObjectId B = commit(ow, treeB, new ObjectId[] {});
+		final ObjectId O = commit(ow, treeO, new ObjectId[] { B });
+		final ObjectId P = commit(ow, treeP, new ObjectId[] { B });
+		final ObjectId T = commit(ow, treeT, new ObjectId[] { P });
+
+		ThreeWayMerger twm = MergeStrategy.SIMPLE_TWO_WAY_IN_CORE.newMerger(db);
+		twm.setBase(P);
+		boolean merge = twm.merge(new ObjectId[] { O, T });
+		assertTrue(merge);
+
+		final TreeWalk tw = new TreeWalk(db);
+		tw.setRecursive(true);
+		tw.reset(twm.getResultTreeId());
+
+		assertTrue(tw.next());
+		assertEquals("a", tw.getPathString());
+		assertCorrectId(treeO, tw);
+
+		assertTrue(tw.next());
+		assertEquals("o", tw.getPathString());
+		assertCorrectId(treeO, tw);
+
+		assertTrue(tw.next());
+		assertEquals("t", tw.getPathString());
+		assertCorrectId(treeT, tw);
+
+		assertFalse(tw.next());
+	}
+
+	private void assertCorrectId(final DirCache treeT, final TreeWalk tw) {
+		assertEquals(treeT.getEntry(tw.getPathString()).getObjectId(), tw
+				.getObjectId(0));
+	}
+
+	private ObjectId commit(final ObjectWriter ow, final DirCache treeB,
+			final ObjectId[] parentIds) throws Exception {
+		final Commit c = new Commit(db);
+		c.setTreeId(treeB.writeTree(ow));
+		c.setAuthor(new PersonIdent("A U Thor", "a.u.thor", 1L, 0));
+		c.setCommitter(c.getAuthor());
+		c.setParentIds(parentIds);
+		c.setMessage("Tree " + c.getTreeId().name());
+		return ow.writeCommit(c);
+	}
+
+	private DirCacheEntry makeEntry(final String path, final FileMode mode)
+			throws Exception {
+		return makeEntry(path, mode, path);
+	}
+
+	private DirCacheEntry makeEntry(final String path, final FileMode mode,
+			final String content) throws Exception {
+		final DirCacheEntry ent = new DirCacheEntry(path);
+		ent.setFileMode(mode);
+		final byte[] contentBytes = Constants.encode(content);
+		ent.setObjectId(new ObjectWriter(db).computeBlobSha1(
+				contentBytes.length, new ByteArrayInputStream(contentBytes)));
+		return ent;
+	}
+}
diff --git a/org.spearce.jgit/src/org/spearce/jgit/merge/Merger.java b/org.spearce.jgit/src/org/spearce/jgit/merge/Merger.java
index db3e329..2d5d44f 100644
--- a/org.spearce.jgit/src/org/spearce/jgit/merge/Merger.java
+++ b/org.spearce.jgit/src/org/spearce/jgit/merge/Merger.java
@@ -179,9 +179,25 @@ protected AbstractTreeIterator mergeBase(final int aIdx, final int bIdx)
 					+ sourceCommits[bIdx].name() + "found:" + "\n  "
 					+ base.name() + "\n  " + base2.name());
 		}
+		return openTree(base.getTree());
+	}
+
+	/**
+	 * Open an iterator over a tree.
+	 * 
+	 * @param treeId
+	 *            the tree to scan; must be a tree (not a treeish).
+	 * @return an iterator for the tree.
+	 * @throws IncorrectObjectTypeException
+	 *             the input object is not a tree.
+	 * @throws IOException
+	 *             the tree object is not found or cannot be read.
+	 */
+	protected AbstractTreeIterator openTree(final AnyObjectId treeId)
+			throws IncorrectObjectTypeException, IOException {
 		final WindowCursor curs = new WindowCursor();
 		try {
-			return new CanonicalTreeParser(null, db, base.getTree(), curs);
+			return new CanonicalTreeParser(null, db, treeId, curs);
 		} finally {
 			curs.release();
 		}
diff --git a/org.spearce.jgit/src/org/spearce/jgit/merge/StrategySimpleTwoWayInCore.java b/org.spearce.jgit/src/org/spearce/jgit/merge/StrategySimpleTwoWayInCore.java
index 9807644..5d4447c 100644
--- a/org.spearce.jgit/src/org/spearce/jgit/merge/StrategySimpleTwoWayInCore.java
+++ b/org.spearce.jgit/src/org/spearce/jgit/merge/StrategySimpleTwoWayInCore.java
@@ -102,7 +102,7 @@ InCoreMerger(final Repository local) {
 		@Override
 		protected boolean mergeImpl() throws IOException {
 			tw.reset();
-			tw.addTree(mergeBase(0, 1));
+			tw.addTree(mergeBase());
 			tw.addTree(sourceTrees[0]);
 			tw.addTree(sourceTrees[1]);
 
diff --git a/org.spearce.jgit/src/org/spearce/jgit/merge/ThreeWayMerger.java b/org.spearce.jgit/src/org/spearce/jgit/merge/ThreeWayMerger.java
index 9d1621d..6f041c1 100644
--- a/org.spearce.jgit/src/org/spearce/jgit/merge/ThreeWayMerger.java
+++ b/org.spearce.jgit/src/org/spearce/jgit/merge/ThreeWayMerger.java
@@ -40,11 +40,16 @@
 import java.io.IOException;
 
 import org.spearce.jgit.errors.IncorrectObjectTypeException;
+import org.spearce.jgit.errors.MissingObjectException;
 import org.spearce.jgit.lib.AnyObjectId;
 import org.spearce.jgit.lib.Repository;
+import org.spearce.jgit.revwalk.RevTree;
+import org.spearce.jgit.treewalk.AbstractTreeIterator;
 
 /** A merge of 2 trees, using a common base ancestor tree. */
 public abstract class ThreeWayMerger extends Merger {
+	private RevTree baseTree;
+
 	/**
 	 * Create a new merge instance for a repository.
 	 * 
@@ -56,6 +61,29 @@ protected ThreeWayMerger(final Repository local) {
 	}
 
 	/**
+	 * Set the common ancestor tree.
+	 * 
+	 * @param id
+	 *            common base treeish; null to automatically compute the common
+	 *            base from the input commits during
+	 *            {@link #merge(AnyObjectId, AnyObjectId)}.
+	 * @throws IncorrectObjectTypeException
+	 *             the object is not a treeish.
+	 * @throws MissingObjectException
+	 *             the object does not exist.
+	 * @throws IOException
+	 *             the object could not be read.
+	 */
+	public void setBase(final AnyObjectId id) throws MissingObjectException,
+			IncorrectObjectTypeException, IOException {
+		if (id != null) {
+			baseTree = walk.parseTree(id);
+		} else {
+			baseTree = null;
+		}
+	}
+
+	/**
 	 * Merge together two tree-ish objects.
 	 * <p>
 	 * Any tree-ish may be supplied as inputs. Commits and/or tags pointing at
@@ -86,4 +114,17 @@ public boolean merge(final AnyObjectId[] tips) throws IOException {
 			return false;
 		return super.merge(tips);
 	}
+
+	/**
+	 * Create an iterator to walk the merge base.
+	 * 
+	 * @return an iterator over the caller-specified merge base, or the natural
+	 *         merge base of the two input commits.
+	 * @throws IOException
+	 */
+	protected AbstractTreeIterator mergeBase() throws IOException {
+		if (baseTree != null)
+			return openTree(baseTree);
+		return mergeBase(0, 1);
+	}
 }
diff --git a/org.spearce.jgit/src/org/spearce/jgit/treewalk/CanonicalTreeParser.java b/org.spearce.jgit/src/org/spearce/jgit/treewalk/CanonicalTreeParser.java
index 7f89cff..ec1cf10 100644
--- a/org.spearce.jgit/src/org/spearce/jgit/treewalk/CanonicalTreeParser.java
+++ b/org.spearce.jgit/src/org/spearce/jgit/treewalk/CanonicalTreeParser.java
@@ -91,7 +91,7 @@ public CanonicalTreeParser() {
 	 *             a loose object or pack file could not be read.
 	 */
 	public CanonicalTreeParser(final byte[] prefix, final Repository repo,
-			final ObjectId treeId, final WindowCursor curs)
+			final AnyObjectId treeId, final WindowCursor curs)
 			throws IncorrectObjectTypeException, IOException {
 		super(prefix);
 		reset(repo, treeId, curs);
-- 
1.6.2.1.471.g682837