DirCache: Add helper to read from a tree

Application code sometimes wants to read a DirCache from an ObjectId,
but its confusing how to do this because its buried inside the
DirCacheBuilder.

Use this utility in a few places within JGit that also want to read
a DirCache from a tree's ObjectId.

Change-Id: I578b7e18e58753d154937f4ab835012b09e5adca

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

@@ -71,9 +71,11 @@ import org.eclipse.jgit.events.IndexChangedListener;
 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.internal.storage.file.FileSnapshot;
 import org.eclipse.jgit.internal.storage.file.LockFile;
+import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectInserter;
+import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.treewalk.FileTreeIterator;
 import org.eclipse.jgit.treewalk.TreeWalk;
@@ -146,6 +148,28 @@ public class DirCache {
 		return new DirCache(null, null);
 	}
 
+	/**
+	 * Create a new in memory index read from the contents of a tree.
+	 *
+	 * @param reader
+	 *            reader to access the tree objects from a repository.
+	 * @param treeId
+	 *            tree to read. Must identify a tree, not a tree-ish.
+	 * @return a new cache which has no backing store file, but contains the
+	 *         contents of {@code treeId}.
+	 * @throws IOException
+	 *             one or more trees not available from the ObjectReader.
+	 * @since 4.2
+	 */
+	public static DirCache read(ObjectReader reader, AnyObjectId treeId)
+			throws IOException {
+		DirCache d = newInCore();
+		DirCacheBuilder b = d.builder();
+		b.addTree(null, DirCacheEntry.STAGE_0, reader, treeId);
+		b.finish();
+		return d;
+	}
+
 	/**
 	 * Create a new in-core index representation and read an index from disk.
 	 * <p>

org.eclipse.jgit/src/org/eclipse/jgit/merge/RecursiveMerger.java

@@ -57,8 +57,6 @@ import java.util.List;
 import java.util.TimeZone;
 
 import org.eclipse.jgit.dircache.DirCache;
-import org.eclipse.jgit.dircache.DirCacheBuilder;
-import org.eclipse.jgit.dircache.DirCacheEntry;
 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.NoMergeBaseException;
 import org.eclipse.jgit.internal.JGitText;
@@ -70,7 +68,6 @@ import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.revwalk.filter.RevFilter;
 import org.eclipse.jgit.treewalk.AbstractTreeIterator;
 import org.eclipse.jgit.treewalk.EmptyTreeIterator;
-import org.eclipse.jgit.treewalk.TreeWalk;
 import org.eclipse.jgit.treewalk.WorkingTreeIterator;
 
 /**
@@ -181,7 +178,7 @@ public class RecursiveMerger extends ResolveMerger {
 		WorkingTreeIterator oldWTreeIt = workingTreeIterator;
 		workingTreeIterator = null;
 		try {
-			dircache = dircacheFromTree(currentBase.getTree());
+			dircache = DirCache.read(reader, currentBase.getTree());
 			inCore = true;
 
 			List<RevCommit> parents = new ArrayList<RevCommit>();
@@ -256,30 +253,4 @@ public class RecursiveMerger extends ResolveMerger {
 				new Date((time + 1) * 1000L),
 				TimeZone.getTimeZone("GMT+0000")); //$NON-NLS-1$
 	}
-
-	/**
-	 * Create a new in memory dircache which has the same content as a given
-	 * tree.
-	 *
-	 * @param treeId
-	 *            the tree which should be used to fill the dircache
-	 * @return a new in memory dircache
-	 * @throws IOException
-	 */
-	private DirCache dircacheFromTree(ObjectId treeId) throws IOException {
-		DirCache ret = DirCache.newInCore();
-		DirCacheBuilder aBuilder = ret.builder();
-		try (TreeWalk atw = new TreeWalk(reader)) {
-			atw.addTree(treeId);
-			atw.setRecursive(true);
-			while (atw.next()) {
-				DirCacheEntry e = new DirCacheEntry(atw.getRawPath());
-				e.setFileMode(atw.getFileMode(0));
-				e.setObjectId(atw.getObjectId(0));
-				aBuilder.add(e);
-			}
-		}
-		aBuilder.finish();
-		return ret;
-	}
 }

org.eclipse.jgit/src/org/eclipse/jgit/transport/PushCertificateStore.java

@@ -65,7 +65,6 @@ import java.util.Map;
 import java.util.NoSuchElementException;
 
 import org.eclipse.jgit.dircache.DirCache;
-import org.eclipse.jgit.dircache.DirCacheBuilder;
 import org.eclipse.jgit.dircache.DirCacheEditor;
 import org.eclipse.jgit.dircache.DirCacheEditor.PathEdit;
 import org.eclipse.jgit.dircache.DirCacheEntry;
@@ -448,13 +447,10 @@ public class PushCertificateStore implements AutoCloseable {
 	}
 
 	private DirCache newDirCache() throws IOException {
-		DirCache dc = DirCache.newInCore();
 		if (commit != null) {
-			DirCacheBuilder b = dc.builder();
-			b.addTree(new byte[0], DirCacheEntry.STAGE_0, reader, commit.getTree());
-			b.finish();
+			return DirCache.read(reader, commit.getTree());
 		}
-		return dc;
+		return DirCache.newInCore();
 	}
 
 	private ObjectId saveCert(ObjectInserter inserter, DirCache dc,