Fix wasNull state across toString() calls in RowStruct (#3276)

This PR fixes the bug related to `wasNull` state not being preserved
across `toString()` calls in a `RowStruct`. For instance,

If a struct is defined as `{a -> null, b -> 1}`, `getObject(1)` should
set the `wasNull` to `true` which should be uneffected if a call to
`toString()` is made.

Also adds tests around the same.

Author: g31pranjal

fdb-relational-api/src/main/java/com/apple/foundationdb/relational/api/RelationalStruct.java

@@ -23,6 +23,7 @@
 import com.apple.foundationdb.relational.api.exceptions.ErrorCode;
 import com.apple.foundationdb.relational.api.exceptions.InvalidColumnReferenceException;
 
+import java.sql.ResultSet;
 import java.sql.SQLException;
 import java.sql.Struct;
 import java.sql.Wrapper;
@@ -80,6 +81,9 @@ public interface RelationalStruct extends Struct, Wrapper {
 
     UUID getUUID(String fieldName) throws SQLException;
 
+    /**
+     * Reports whether the last column read had a value of SQL NULL. See {@link ResultSet#wasNull()}
+     */
     boolean wasNull() throws SQLException;
 
     @Override

fdb-relational-core/src/main/java/com/apple/foundationdb/relational/api/RowStruct.java

@@ -335,6 +335,9 @@ private int getZeroBasedPosition(int oneBasedPosition) {
 
     @Override
     public String toString() {
+        // We want to preserve the last wasNull state and not let it get effected by toString() since this too uses
+        // the getObject() call to fetch values.
+        final var preservedWasNullValue = wasNull();
         final var builder = new StringBuilder();
         try {
             final var colCount = metaData.getColumnCount();
@@ -351,8 +354,10 @@ public String toString() {
             builder.append("} ");
         } catch (SQLException e) {
             throw new UncheckedRelationalException(new RelationalException(e));
+        } finally {
+            // restore wasNull
+            wasNull = preservedWasNullValue;
         }
-
         return builder.toString();
     }
 }

fdb-relational-core/src/test/java/com/apple/foundationdb/relational/recordlayer/RowStructTest.java

@@ -0,0 +1,69 @@
+/*
+ * RowStructTest.java
+ *
+ * This source file is part of the FoundationDB open source project
+ *
+ * Copyright 2015-2025 Apple Inc. and the FoundationDB project authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.apple.foundationdb.relational.recordlayer;
+
+import com.apple.foundationdb.relational.api.FieldDescription;
+import com.apple.foundationdb.relational.api.ImmutableRowStruct;
+import com.apple.foundationdb.relational.api.MutableRowStruct;
+import com.apple.foundationdb.relational.api.RelationalStructMetaData;
+import com.apple.foundationdb.relational.api.RowStruct;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+
+import java.sql.DatabaseMetaData;
+import java.sql.SQLException;
+import java.sql.Types;
+
+public class RowStructTest {
+
+    @ParameterizedTest
+    @ValueSource(booleans = {true, false})
+    void wasNullWorks(boolean mutable) throws SQLException {
+        final var struct = createStruct(mutable);
+        struct.getObject(1);
+        Assertions.assertTrue(struct.wasNull());
+    }
+
+    @ParameterizedTest
+    @ValueSource(booleans = {true, false})
+    void wasNullWorksWithToString(boolean mutable) throws SQLException {
+        final var struct = createStruct(mutable);
+        struct.getObject(1);
+        Assertions.assertFalse(struct.toString().isEmpty());
+        Assertions.assertTrue(struct.wasNull());
+    }
+
+    private static RowStruct createStruct(boolean mutable) {
+        final var metadata = new RelationalStructMetaData(
+                FieldDescription.primitive("fInt", Types.INTEGER, DatabaseMetaData.columnNullable),
+                FieldDescription.primitive("fLong", Types.BIGINT, DatabaseMetaData.columnNullable)
+        );
+        final var row = new ArrayRow(null, 1L);
+        if (mutable) {
+            final var toReturn = new MutableRowStruct(metadata);
+            toReturn.setRow(row);
+            return toReturn;
+        } else {
+            return new ImmutableRowStruct(row, metadata);
+        }
+    }
+}