fix

spark/src/main/scala/org/apache/comet/CometExecIterator.scala

@@ -74,6 +74,7 @@ class CometExecIterator(
   }
 
   private var nextBatch: Option[ColumnarBatch] = None
+  private var prevBatch: ColumnarBatch = null
   private var currentBatch: ColumnarBatch = null
   private var closed: Boolean = false
 
@@ -95,6 +96,14 @@ class CometExecIterator(
       return true
     }
 
+    // Close previous batch if any.
+    // This is to guarantee safety at the native side before we overwrite the buffer memory
+    // shared across batches in the native side.
+    if (prevBatch != null) {
+      prevBatch.close()
+      prevBatch = null
+    }
+
     nextBatch = getNextBatch()
 
     if (nextBatch.isEmpty) {
@@ -117,6 +126,7 @@ class CometExecIterator(
     }
 
     currentBatch = nextBatch.get
+    prevBatch = currentBatch
     nextBatch = None
     currentBatch
   }