Add MdbmLocker courtesy of Bhopesh Bassi (@bbhopesh)

Fix a bug in the iteration that came up courtesy of Bhopesh Bassi (@bbhopesh)

Author: Allen Reese

AUTHORS

@@ -61,3 +61,8 @@ Based on:
        Bhagyashri Mahule, Yahoo!, Inc.
        Lakshmanan Suryanarayanan, Yahoo!, Inc.
 
+  Modifications for:
+       . Java JNI wrappers
+  By
+       Allen Reese, Yahoo!, Inc.
+       Manu Bassi, Yahoo!, Inc. (MdbmLocker code, bug fixes around iteration)

src/java/com_yahoo_db_mdbm_internal_NativeMdbmAccess.cc

@@ -340,7 +340,7 @@ class ScopedKvPair {
 
         GET_CACHED_CLASS(jenv, mdbmKvPairClass);
         GET_CACHED_METHOD_ID(jenv, mdbmKvPairDatumCtorId);
-        if (NULL == mdbmDatumClass || NULL == mdbmKvPairDatumCtorId) {
+        if (NULL == mdbmKvPairClass || NULL == mdbmKvPairDatumCtorId) {
             // valid is set to false so bail.
             return;
         }

src/java/src/main/java/com/yahoo/db/mdbm/MdbmLocker.java

@@ -0,0 +1,30 @@
+/* Copyright 2017 Yahoo! Inc. */
+/* Licensed under the terms of the 3-Clause BSD license. See LICENSE file in the project root for details. */
+
+package com.yahoo.db.mdbm;
+
+import com.yahoo.db.mdbm.exceptions.MdbmException;
+
+/**
+ * This class takes an MdbmInterface as it's constructor, and locks the mdbm. The use case is for locking during
+ * iteration via try-with resources.
+ * 
+ * Credit to bbhopesh for noticing this was missing.
+ *
+ */
+public class MdbmLocker implements AutoCloseable {
+    private MdbmInterface m;
+
+    public MdbmLocker(MdbmInterface m) throws MdbmException {
+        this.m = m;
+        m.lock();
+    }
+
+    @Override
+    public void close() throws MdbmException {
+        if (null != m) {
+            m.unlock();
+            m = null;
+        }
+    }
+}