fix: source wasNull from validity buffer in VarChar/Binary accessors

jdbc-core/build.gradle.kts

@@ -44,3 +44,11 @@ dependencies {
 tasks.named("compileJava") {
     dependsOn(":jdbc-grpc:compileJava")
 }
+
+// Iceberg sets -Darrow.enable_null_check_for_get=false on the JVM. With that flag off, Arrow's
+// VarCharVector/VarBinaryVector/FixedSizeBinaryVector .get(int) skip the validity check and return
+// stale buffer bytes for null rows instead of null. Run the test suite under that condition so
+// the existing null-handling assertions cover the Iceberg scenario.
+tasks.test {
+    systemProperty("arrow.enable_null_check_for_get", "false")
+}

jdbc-core/src/main/java/com/salesforce/datacloud/jdbc/core/accessor/impl/BaseListVectorAccessor.java

@@ -32,6 +32,8 @@ public Class<?> getObjectClass() {
     }
 
     protected List<?> getListObject(VectorProvider vectorProvider) throws SQLException {
+        // ListVector/LargeListVector.getObject is validity-correct (not gated by
+        // arrow.enable_null_check_for_get), so a null return reliably indicates a null row.
         List<?> object = vectorProvider.getObject(getCurrentRow());
         this.wasNull = object == null;
         return object;

jdbc-core/src/main/java/com/salesforce/datacloud/jdbc/core/accessor/impl/BinaryVectorAccessor.java

@@ -6,6 +6,7 @@
 
 import com.salesforce.datacloud.jdbc.core.accessor.QueryJDBCAccessor;
 import java.nio.charset.StandardCharsets;
+import java.util.function.IntPredicate;
 import java.util.function.IntSupplier;
 import org.apache.arrow.vector.FixedSizeBinaryVector;
 import org.apache.arrow.vector.LargeVarBinaryVector;
@@ -18,30 +19,37 @@ private interface ByteArrayGetter {
     }
 
     private final ByteArrayGetter getter;
+    private final IntPredicate nullChecker;
 
     public BinaryVectorAccessor(FixedSizeBinaryVector vector, IntSupplier currentRowSupplier) {
-        this(vector::get, currentRowSupplier);
+        this(vector::get, vector::isNull, currentRowSupplier);
     }
 
     public BinaryVectorAccessor(VarBinaryVector vector, IntSupplier currentRowSupplier) {
-        this(vector::get, currentRowSupplier);
+        this(vector::get, vector::isNull, currentRowSupplier);
     }
 
     public BinaryVectorAccessor(LargeVarBinaryVector vector, IntSupplier currentRowSupplier) {
-        this(vector::get, currentRowSupplier);
+        this(vector::get, vector::isNull, currentRowSupplier);
     }
 
-    private BinaryVectorAccessor(ByteArrayGetter getter, IntSupplier currentRowSupplier) {
+    private BinaryVectorAccessor(ByteArrayGetter getter, IntPredicate nullChecker, IntSupplier currentRowSupplier) {
         super(currentRowSupplier);
         this.getter = getter;
+        this.nullChecker = nullChecker;
     }
 
     @Override
     public byte[] getBytes() {
-        byte[] bytes = getter.get(getCurrentRow());
-        this.wasNull = bytes == null;
-
-        return bytes;
+        // Arrow's vector.get(int) skips the isSet check when arrow.enable_null_check_for_get=false
+        // (e.g. set by Iceberg on the JVM), so a null entry returns garbage rather than null.
+        // Check the validity buffer explicitly via isNull(int).
+        final int row = getCurrentRow();
+        this.wasNull = nullChecker.test(row);
+        if (this.wasNull) {
+            return null;
+        }
+        return getter.get(row);
     }
 
     @Override

jdbc-core/src/main/java/com/salesforce/datacloud/jdbc/core/accessor/impl/DecimalVectorAccessor.java

@@ -25,6 +25,8 @@ public Class<?> getObjectClass() {
 
     @Override
     public BigDecimal getBigDecimal() {
+        // DecimalVector.getObject is validity-correct (not gated by arrow.enable_null_check_for_get),
+        // so a null return reliably indicates a null row even when Iceberg disables that flag.
         final BigDecimal value = vector.getObject(getCurrentRow());
         this.wasNull = value == null;
         return value;

jdbc-core/src/main/java/com/salesforce/datacloud/jdbc/core/accessor/impl/VarCharVectorAccessor.java

@@ -6,6 +6,7 @@
 
 import com.salesforce.datacloud.jdbc.core.accessor.QueryJDBCAccessor;
 import java.nio.charset.StandardCharsets;
+import java.util.function.IntPredicate;
 import java.util.function.IntSupplier;
 import org.apache.arrow.vector.LargeVarCharVector;
 import org.apache.arrow.vector.VarCharVector;
@@ -18,18 +19,20 @@ interface Getter {
     }
 
     private final Getter getter;
+    private final IntPredicate nullChecker;
 
     public VarCharVectorAccessor(VarCharVector vector, IntSupplier currenRowSupplier) {
-        this(vector::get, currenRowSupplier);
+        this(vector::get, vector::isNull, currenRowSupplier);
     }
 
     public VarCharVectorAccessor(LargeVarCharVector vector, IntSupplier currenRowSupplier) {
-        this(vector::get, currenRowSupplier);
+        this(vector::get, vector::isNull, currenRowSupplier);
     }
 
-    VarCharVectorAccessor(Getter getter, IntSupplier currentRowSupplier) {
+    VarCharVectorAccessor(Getter getter, IntPredicate nullChecker, IntSupplier currentRowSupplier) {
         super(currentRowSupplier);
         this.getter = getter;
+        this.nullChecker = nullChecker;
     }
 
     @Override
@@ -39,9 +42,15 @@ public Class<?> getObjectClass() {
 
     @Override
     public byte[] getBytes() {
-        final byte[] bytes = this.getter.get(getCurrentRow());
-        this.wasNull = bytes == null;
-        return this.getter.get(getCurrentRow());
+        // Arrow's vector.get(int) skips the isSet check when arrow.enable_null_check_for_get=false
+        // (e.g. set by Iceberg on the JVM), so a null entry returns an empty byte[] rather than null.
+        // Check the validity buffer explicitly via isNull(int).
+        final int row = getCurrentRow();
+        this.wasNull = nullChecker.test(row);
+        if (this.wasNull) {
+            return null;
+        }
+        return this.getter.get(row);
     }
 
     @Override