Allow zero copy put by reserving space directly in LMDB, #51

lmdbjni/src/main/java/org/fusesource/lmdbjni/Database.java

@@ -22,6 +22,7 @@
 import org.fusesource.hawtjni.runtime.Callback;
 import org.fusesource.lmdbjni.EntryIterator.IteratorType;
 
+import java.nio.ByteBuffer;
 import java.util.Comparator;
 
 import static org.fusesource.lmdbjni.JNI.*;
@@ -345,6 +346,25 @@ public int put(Transaction tx, DirectBuffer key, DirectBuffer value, int flags)
     return rc;
   }
 
+  /**
+   * Just reserve space for data in the database, don't copy it. Return a pointer to the reserved space.
+   */
+  public DirectBuffer reserve(Transaction tx, DirectBuffer key, int size) {
+    checkArgNotNull(key, "key");
+    long address = tx.getBufferAddress();
+    Unsafe.putLong(address, 0, key.capacity());
+    Unsafe.putLong(address, 1, key.addressOffset());
+    Unsafe.putLong(address, 2, size);
+
+    int rc = mdb_put_address(tx.pointer(), pointer(), address, address + 2 * Unsafe.ADDRESS_SIZE, Constants.RESERVE);
+    checkErrorCode(rc);
+    int valSize = (int) Unsafe.getLong(address, 2);
+    long valAddress = Unsafe.getAddress(address, 3);
+    DirectBuffer empty = new DirectBuffer(0, 0);
+    empty.wrap(valAddress, valSize);
+    return empty;
+  }
+
   /**
    * @see org.fusesource.lmdbjni.Database#put(Transaction, byte[], byte[], int)
    */

lmdbjni/src/test/java/org/fusesource/lmdbjni/ZeroCopyTest.java

@@ -8,12 +8,14 @@
 import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 
 import static org.fusesource.lmdbjni.Constants.FIRST;
 import static org.fusesource.lmdbjni.Constants.NEXT;
 import static org.fusesource.lmdbjni.LMDBException.NOTFOUND;
 import static org.hamcrest.core.Is.is;
+import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertThat;
 
 public class ZeroCopyTest {
@@ -159,5 +161,24 @@ public void testIteration() {
     assertThat(result.size(), is(2));
     assertThat(result.get(0), is(18L));
     assertThat(result.get(1), is(20L));
- }
+  }
+
+  @Test
+  public void testReserve() {
+    byte[] key = new byte[] {1,2,3};
+    byte[] val = new byte[] {3,2,1};
+
+    try (Transaction tx = env.createWriteTransaction()) {
+      DirectBuffer keyBuf = new DirectBuffer(ByteBuffer.allocateDirect(key.length));
+      keyBuf.putBytes(0, key);
+      DirectBuffer valBuf = db.reserve(tx, keyBuf, val.length);
+      valBuf.putBytes(0, val);
+      tx.commit();
+    }
+
+    try (Transaction tx = env.createReadTransaction()) {
+      byte[] result = db.get(tx, key);
+      assertArrayEquals(result, val);
+    }
+  }
 }