fix assert error while db.getDefaultColumnFamily().getDescriptor() (#6006)

Summary:
Threw assert error at assert(isOwningHandle()) in ColumnFamilyHandle.getDescriptor(),
because default CF don't own a handle, due to [RocksDB.getDefaultColumnFamily()](3a408eeae9/java/src/main/java/org/rocksdb/RocksDB.java (L3702)) called cfHandle.disOwnNativeHandle().
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6006

Differential Revision: D19031448

fbshipit-source-id: 2420c45e835bda0e552e919b1b63708472b91538

java/src/main/java/org/rocksdb/ColumnFamilyHandle.java

@@ -32,6 +32,7 @@ public class ColumnFamilyHandle extends RocksObject {
    * @throws RocksDBException if an error occurs whilst retrieving the name.
    */
   public byte[] getName() throws RocksDBException {
+    assert(isOwningHandle() || isDefaultColumnFamily());
     return getName(nativeHandle_);
   }
 
@@ -41,6 +42,7 @@ public class ColumnFamilyHandle extends RocksObject {
    * @return the ID of the Column Family.
    */
   public int getID() {
+    assert(isOwningHandle() || isDefaultColumnFamily());
     return getID(nativeHandle_);
   }
 
@@ -59,7 +61,7 @@ public class ColumnFamilyHandle extends RocksObject {
    *     descriptor.
    */
   public ColumnFamilyDescriptor getDescriptor() throws RocksDBException {
-    assert(isOwningHandle());
+    assert(isOwningHandle() || isDefaultColumnFamily());
     return getDescriptor(nativeHandle_);
   }
 
@@ -91,6 +93,10 @@ public class ColumnFamilyHandle extends RocksObject {
     }
   }
 
+  protected boolean isDefaultColumnFamily() {
+    return nativeHandle_ == rocksDB_.getDefaultColumnFamily().nativeHandle_;
+  }
+
   /**
    * <p>Deletes underlying C++ iterator pointer.</p>
    *

java/src/test/java/org/rocksdb/ColumnFamilyTest.java

@@ -75,6 +75,7 @@ public class ColumnFamilyTest {
 
         assertThat(cfh.getName()).isEqualTo("default".getBytes(UTF_8));
         assertThat(cfh.getID()).isEqualTo(0);
+        assertThat(cfh.getDescriptor().getName()).isEqualTo("default".getBytes(UTF_8));
 
         final byte[] key = "key".getBytes();
         final byte[] value = "value".getBytes();