Fix TreeWalk bug comparing DirCache and WorkingTree with ANY_DIFF

When comparing a DirCache and a WorkingTree using ANY_DIFF we
sometimes didn't recursive into a subtree of both sides gave us
zeroId() back for the identity of a subtree.  This happens when the
DirCache doesn't have a valid cache tree for the subtree, as then
it uses zeroId() for the ObjectId of the subtree, which then appears
to be equal to the zeroId() of the WorkingTreeIterator's subtree.

We work around this by adding a hasId() method that returns true
only if this iterator has a valid ObjectId.  The idEquals method
on TreeWalk than only performs a compare between two iterators if
both iterators have a valid id.

Change-Id: I695f7fafbeb452e8c0703a05c02921fae0822d3f
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCacheIterator.java

@@ -45,7 +45,6 @@
 package org.eclipse.jgit.dircache;
 
 import java.io.IOException;
-import java.util.Arrays;
 
 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.lib.Constants;
@@ -141,10 +140,17 @@ public class DirCacheIterator extends AbstractTreeIterator {
 		return new EmptyTreeIterator(this, n, pathLen + 1);
 	}
 
+	@Override
+	public boolean hasId() {
+		if (currentSubtree != null)
+			return currentSubtree.isValid();
+		return currentEntry != null;
+	}
+
 	@Override
 	public byte[] idBuffer() {
 		if (currentSubtree != null)
-			return subtreeId;
+			return currentSubtree.isValid() ? subtreeId : zeroid;
 		if (currentEntry != null)
 			return currentEntry.idBuffer();
 		return zeroid;
@@ -218,8 +224,6 @@ public class DirCacheIterator extends AbstractTreeIterator {
 
 				if (s.isValid())
 					s.getObjectId().copyRawTo(subtreeId, 0);
-				else
-					Arrays.fill(subtreeId, (byte) 0);
 				mode = FileMode.TREE.getBits();
 				path = cep;
 				pathLen = pathOffset + s.nameLength();

org.eclipse.jgit/src/org/eclipse/jgit/treewalk/AbstractTreeIterator.java

@@ -369,6 +369,9 @@ public abstract class AbstractTreeIterator {
 				otherIterator.idBuffer(), otherIterator.idOffset());
 	}
 
+	/** @return true if the entry has a valid ObjectId. */
+	public abstract boolean hasId();
+
 	/**
 	 * Get the object id of the current entry.
 	 *

org.eclipse.jgit/src/org/eclipse/jgit/treewalk/CanonicalTreeParser.java

@@ -233,6 +233,11 @@ public class CanonicalTreeParser extends AbstractTreeIterator {
 		return createSubtreeIterator(reader, new MutableObjectId());
 	}
 
+	@Override
+	public boolean hasId() {
+		return true;
+	}
+
 	@Override
 	public byte[] idBuffer() {
 		return raw;

org.eclipse.jgit/src/org/eclipse/jgit/treewalk/EmptyTreeIterator.java

@@ -92,6 +92,11 @@ public class EmptyTreeIterator extends AbstractTreeIterator {
 		return new EmptyTreeIterator(this);
 	}
 
+	@Override
+	public boolean hasId() {
+		return false;
+	}
+
 	@Override
 	public ObjectId getEntryObjectId() {
 		return ObjectId.zeroId();

org.eclipse.jgit/src/org/eclipse/jgit/treewalk/TreeWalk.java

@@ -683,8 +683,6 @@ public class TreeWalk {
 		final AbstractTreeIterator ch = currentHead;
 		final AbstractTreeIterator a = trees[nthA];
 		final AbstractTreeIterator b = trees[nthB];
-		if (a.matches == ch && b.matches == ch)
-			return a.idEqual(b);
 		if (a.matches != ch && b.matches != ch) {
 			// If neither tree matches the current path node then neither
 			// tree has this entry. In such case the ObjectId is zero(),
@@ -692,6 +690,10 @@ public class TreeWalk {
 			//
 			return true;
 		}
+		if (!a.hasId() || !b.hasId())
+			return false;
+		if (a.matches == ch && b.matches == ch)
+			return a.idEqual(b);
 		return false;
 	}
 

org.eclipse.jgit/src/org/eclipse/jgit/treewalk/WorkingTreeIterator.java

@@ -194,6 +194,13 @@ public abstract class WorkingTreeIterator extends AbstractTreeIterator {
 		ignoreNode = new RootIgnoreNode(entry, repo);
 	}
 
+	@Override
+	public boolean hasId() {
+		if (contentIdFromPtr == ptr)
+			return true;
+		return (mode & FileMode.TYPE_MASK) == FileMode.TYPE_FILE;
+	}
+
 	@Override
 	public byte[] idBuffer() {
 		if (contentIdFromPtr == ptr)