fix(java): row format buffer recycling leaves offset and size for null values (#2540)

Author: stevenschlansker

java/fory-format/src/main/java/org/apache/fory/format/row/binary/writer/BinaryRowWriter.java

@@ -128,6 +128,12 @@ public void write(int ordinal, BigDecimal value) {
     writeDecimal(ordinal, value, (ArrowType.Decimal) schema.getFields().get(ordinal).getType());
   }
 
+  @Override
+  public void setNullAt(int ordinal) {
+    super.setNullAt(ordinal);
+    write(ordinal, 0L);
+  }
+
   public BinaryRow getRow() {
     BinaryRow row = new BinaryRow(schema);
     int size = size();

java/fory-format/src/main/java/org/apache/fory/format/row/binary/writer/BinaryWriter.java

@@ -121,10 +121,10 @@ protected final void zeroOutPaddingBytes(int numBytes) {
   }
 
   /**
-   * Since writer is used for one-pass writer, same field won't be writer twice. There is no need to
-   * put zero into the corresponding field when set null.
+   * Writer might recycle buffers, so implementations should clear data from a previous not-null
+   * write when setting null to avoid information leaks.
    */
-  public final void setNullAt(int ordinal) {
+  public void setNullAt(int ordinal) {
     BitUtils.set(buffer, startIndex + bytesBeforeBitMap, ordinal);
   }
 

java/fory-format/src/test/java/org/apache/fory/format/encoder/RowEncoderTest.java

@@ -118,4 +118,21 @@ public void testPrivateBean() {
     Assert.assertEquals(s1.f1, s.f1);
     Assert.assertEquals(s1.f2, s.f2);
   }
+
+  @Test
+  public void testNullClearOffset() {
+    RowEncoder<Bar> encoder = Encoders.bean(Bar.class);
+    Bar bar = new Bar();
+    bar.f1 = 42;
+    bar.f2 = null;
+    byte[] nullBefore = encoder.encode(bar);
+
+    bar.f2 = "not null";
+    // write offset and size
+    encoder.encode(bar);
+
+    bar.f2 = null;
+    byte[] nullAfter = encoder.encode(bar);
+    Assert.assertEquals(nullAfter, nullBefore);
+  }
 }