[JNI] Adds HostColumnVector.EventHandler for spillability checks

java/src/main/java/ai/rapids/cudf/HostColumnVector.java

@@ -39,12 +39,31 @@
  * and call incRefCount to increment the reference count.
  */
 public final class HostColumnVector extends HostColumnVectorCore {
+  /**
+   * Interface to handle events for this HostColumnVector. Only invoked during
+   * close, hence `onClosed` is the only event.
+   */
+  public interface EventHandler {
+    /**
+     * `onClosed` is invoked with the updated `refCount` during `close`.
+     * The last invocation of `onClosed` will be with `refCount=0`.
+     *
+     * @note the callback is invoked with this `HostColumnVector`'s lock held.
+     *
+     * @param cv - a reference to the HostColumnVector we are closing
+     * @param refCount - the updated ref count for this HostColumnVector at
+     *                   the time of invocation
+     */
+    void onClosed(HostColumnVector cv, int refCount);
+  }
+
   /**
    * The size in bytes of an offset entry
    */
   static final int OFFSET_SIZE = DType.INT32.getSizeInBytes();
 
   private int refCount;
+  private EventHandler eventHandler;
 
   /**
    * Create a new column vector with data populated on the host.
@@ -93,6 +112,27 @@ public HostColumnVector(DType type, long rows, Optional<Long> nullCount,
     incRefCountInternal(true);
   }
 
+  /**
+   * Set an event handler for this host vector. This method can be invoked with
+   * null to unset the handler.
+   *
+   * @param newHandler - the EventHandler to use from this point forward
+   * @return the prior event handler, or null if not set.
+   */
+  public synchronized EventHandler setEventHandler(EventHandler newHandler) {
+    EventHandler prev = this.eventHandler;
+    this.eventHandler = newHandler;
+    return prev;
+  }
+
+  /**
+   * Returns the current event handler for this HostColumnVector or null if no
+   * handler is associated.
+   */
+  public synchronized EventHandler getEventHandler() {
+    return this.eventHandler;
+  }
+
   /**
    * This is a really ugly API, but it is possible that the lifecycle of a column of
    * data may not have a clear lifecycle thanks to java and GC. This API informs the leak
@@ -110,6 +150,9 @@ public void noWarnLeakExpected() {
   public synchronized void close() {
     refCount--;
     offHeap.delRef();
+    if (eventHandler != null) {
+      eventHandler.onClosed(this, refCount);
+    }
     if (refCount == 0) {
       offHeap.clean(false);
       for( HostColumnVectorCore child : children) {

java/src/test/java/ai/rapids/cudf/ColumnVectorTest.java

@@ -6791,6 +6791,18 @@ public void testEventHandlerIsCalledForEachClose() {
     assertEquals(1, onClosedWasCalled.get());
   }
 
+  @Test
+  public void testHostEventHandlerIsCalledForEachClose() {
+    final AtomicInteger onClosedWasCalled = new AtomicInteger(0);
+    try (HostColumnVector cv = HostColumnVector.fromInts(1,2,3,4)) {
+      cv.setEventHandler((col, refCount) -> {
+        assertEquals(cv, col);
+        onClosedWasCalled.incrementAndGet();
+      });
+    }
+    assertEquals(1, onClosedWasCalled.get());
+  }
+
   @Test
   public void testEventHandlerIsNotCalledIfNotSet() {
     final AtomicInteger onClosedWasCalled = new AtomicInteger(0);
@@ -6808,6 +6820,23 @@ public void testEventHandlerIsNotCalledIfNotSet() {
     assertEquals(0, onClosedWasCalled.get());
   }
 
+  @Test
+  public void testHostEventHandlerIsNotCalledIfNotSet() {
+    final AtomicInteger onClosedWasCalled = new AtomicInteger(0);
+    try (HostColumnVector cv = HostColumnVector.fromInts(1,2,3,4)) {
+      assertNull(cv.getEventHandler());
+    }
+    assertEquals(0, onClosedWasCalled.get());
+
+    try (HostColumnVector cv = HostColumnVector.fromInts(1,2,3,4)) {
+      cv.setEventHandler((col, refCount) -> {
+        onClosedWasCalled.incrementAndGet();
+      });
+      cv.setEventHandler(null);
+    }
+    assertEquals(0, onClosedWasCalled.get());
+  }
+
   /**
    * Test that the ColumnView with unknown null-counts still returns
    * the correct null-count when queried.