Allow DfsReader to be subclassed

Necessary if a DFS implementation wants to override close()
to record DfsReaderIoStats.

Change-Id: I144575f9bf1abf2c1fd72030550c4f0795fcf44d

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/dfs/DfsGarbageCollectorTest.java

@@ -674,7 +674,7 @@ public class DfsGarbageCollectorTest {
 
 	private boolean isObjectInPack(AnyObjectId id, DfsPackFile pack)
 			throws IOException {
-		try (DfsReader reader = new DfsReader(odb)) {
+		try (DfsReader reader = odb.newReader()) {
 			return pack.hasObject(reader, id);
 		}
 	}

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsInserter.java

@@ -530,7 +530,7 @@ public class DfsInserter extends ObjectInserter {
 	}
 
 	private class Reader extends ObjectReader {
-		private final DfsReader ctx = new DfsReader(db);
+		private final DfsReader ctx = db.newReader();
 
 		@Override
 		public ObjectReader newReader() {
@@ -647,7 +647,7 @@ public class DfsInserter extends ObjectInserter {
 
 		@Override
 		public ObjectStream openStream() throws IOException {
-			final DfsReader ctx = new DfsReader(db);
+			final DfsReader ctx = db.newReader();
 			if (srcPack != packKey) {
 				try {
 					// Post DfsInserter.flush() use the normal code path.

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsObjDatabase.java

@@ -170,7 +170,7 @@ public abstract class DfsObjDatabase extends ObjectDatabase {
 	}
 
 	@Override
-	public ObjectReader newReader() {
+	public DfsReader newReader() {
 		return new DfsReader(this);
 	}
 

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsReader.java

@@ -95,7 +95,7 @@ import org.eclipse.jgit.util.BlockList;
  * See the base {@link ObjectReader} documentation for details. Notably, a
  * reader is not thread safe.
  */
-public final class DfsReader extends ObjectReader implements ObjectReuseAsIs {
+public class DfsReader extends ObjectReader implements ObjectReuseAsIs {
 	private static final int MAX_RESOLVE_MATCHES = 256;
 
 	/** Temporary buffer large enough for at least one raw object id. */
@@ -112,7 +112,13 @@ public final class DfsReader extends ObjectReader implements ObjectReuseAsIs {
 	private DfsPackFile last;
 	private boolean avoidUnreachable;
 
-	DfsReader(DfsObjDatabase db) {
+	/**
+	 * Initialize a new DfsReader
+	 *
+	 * @param db
+	 *            parent DfsObjDatabase.
+	 */
+	protected DfsReader(DfsObjDatabase db) {
 		this.db = db;
 		this.streamFileThreshold = db.getReaderOptions().getStreamFileThreshold();
 	}
@@ -129,7 +135,7 @@ public final class DfsReader extends ObjectReader implements ObjectReuseAsIs {
 
 	@Override
 	public ObjectReader newReader() {
-		return new DfsReader(db);
+		return db.newReader();
 	}
 
 	@Override

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/LargePackedWholeObject.java

@@ -99,7 +99,7 @@ final class LargePackedWholeObject extends ObjectLoader {
 
 	@Override
 	public ObjectStream openStream() throws MissingObjectException, IOException {
-		DfsReader ctx = new DfsReader(db);
+		DfsReader ctx = db.newReader();
 		InputStream in;
 		try {
 			in = new PackInputStream(pack, objectOffset + headerLength, ctx);