Ensures that using a cursor after close has no effect.

src/main/java/com/amazon/ion/impl/IonCursorBinary.java

@@ -92,7 +92,7 @@ private static class RefillableState {
          * READY state). This enables the reader to complete the previous IonCursor API invocation if it returned a
          * NEEDS_DATA event.
          */
-        State state = State.READY;
+        State state;
 
         /**
          * The number of bytes that still need to be consumed from the input during a fill or seek operation.
@@ -124,14 +124,20 @@ private static class RefillableState {
          */
         int individualBytesSkippedWithoutBuffering = 0;
 
-        RefillableState(InputStream inputStream, int capacity, int maximumBufferSize) {
+        RefillableState(InputStream inputStream, int capacity, int maximumBufferSize, State initialState) {
             this.inputStream = inputStream;
             this.capacity = capacity;
             this.maximumBufferSize = maximumBufferSize;
+            this.state = initialState;
         }
 
     }
 
+    /**
+     * Dummy state that indicates the cursor has been terminated and that additional API calls will have no effect.
+     */
+    private static final RefillableState TERMINATED_STATE = new RefillableState(null, -1, -1, State.TERMINATED);
+
     /**
      * Stack to hold container info. Stepping into a container results in a push; stepping out results in a pop.
      */
@@ -249,7 +255,7 @@ private static class RefillableState {
     /**
      * Holds information necessary for reading from refillable input. Null if the cursor is byte-backed.
      */
-    private final RefillableState refillableState;
+    private RefillableState refillableState;
 
     /**
      * Describes the byte at the `checkpoint` index.
@@ -455,7 +461,8 @@ private static IonBufferConfiguration getFixedSizeConfigurationFor(
         refillableState = new RefillableState(
             inputStream,
             configuration.getInitialBufferSize(),
-            configuration.getMaximumBufferSize()
+            configuration.getMaximumBufferSize(),
+            State.READY
         );
         registerOversizedValueHandler(configuration.getOversizedValueHandler());
     }
@@ -1898,12 +1905,15 @@ private boolean slowIsAwaitingMoreData() {
      * exceeds the maximum buffer size.
      */
     void terminate() {
-        refillableState.state = State.TERMINATED;
+        refillableState = TERMINATED_STATE;
+        // Use a unified code path for all cursors after termination. This path forces a termination check before
+        // accessing the input stream or buffer.
+        isSlowMode = true;
     }
 
     @Override
     public void close() {
-        if (refillableState != null) {
+        if (refillableState != null && refillableState.inputStream != null) {
             try {
                 refillableState.inputStream.close();
             } catch (IOException e) {
@@ -1913,6 +1923,7 @@ public void close() {
         buffer = null;
         containerStack = null;
         byteBuffer = null;
+        terminate();
     }
 
     /* ---- End: version-agnostic parsing, utility, and public API methods ---- */

src/test/java/com/amazon/ion/impl/IonCursorBinaryTest.java

@@ -1,6 +1,5 @@
 // Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
-
 package com.amazon.ion.impl;
 
 import com.amazon.ion.IonBufferConfiguration;
@@ -31,6 +30,7 @@
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.containsString;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 public class IonCursorBinaryTest {
@@ -427,4 +427,22 @@ public void expectValueLargerThanMaxArraySizeToFailCleanly() {
 
         cursor.close();
     }
+
+    @ParameterizedTest(name = "constructFromBytes={0}")
+    @ValueSource(booleans = {true, false})
+    public void expectUseAfterCloseToHaveNoEffect(boolean constructFromBytes) {
+        // Using the cursor after close should not fail with an obscure unchecked
+        // exception like NullPointerException, ArrayIndexOutOfBoundsException, etc.
+        IonCursorBinary cursor = initializeCursor(
+            constructFromBytes,
+            0xE0, 0x01, 0x00, 0xEA,
+            0x20
+        );
+        cursor.close();
+        assertEquals(IonCursor.Event.NEEDS_DATA, cursor.nextValue());
+        assertNull(cursor.getValueMarker().typeId);
+        assertEquals(IonCursor.Event.NEEDS_DATA, cursor.nextValue());
+        assertNull(cursor.getValueMarker().typeId);
+        cursor.close();
+    }
 }

src/test/java/com/amazon/ion/impl/IonReaderContinuableTopLevelBinaryTest.java

@@ -4009,4 +4009,22 @@ public void corruptEachByteThrowsIonException(boolean constructFromBytes) {
         corruptEachByteThrowsIonException(constructFromBytes, false, true);
         corruptEachByteThrowsIonException(constructFromBytes, false, false);
     }
+
+    @ParameterizedTest(name = "constructFromBytes={0}")
+    @ValueSource(booleans = {true, false})
+    public void expectUseAfterCloseToHaveNoEffect(boolean constructFromBytes) throws Exception {
+        // Using the cursor after close should not fail with an obscure unchecked
+        // exception like NullPointerException, ArrayIndexOutOfBoundsException, etc.
+        reader = readerFor(
+            constructFromBytes,
+            0xE0, 0x01, 0x00, 0xEA,
+            0x20
+        );
+        reader.close();
+        assertNull(reader.next());
+        assertNull(reader.getType());
+        assertNull(reader.next());
+        assertNull(reader.getType());
+        reader.close();
+    }
 }