fixes based on review

Author: hmottestad

core/queryalgebra/evaluation/src/main/java/org/eclipse/rdf4j/query/algebra/evaluation/iterator/GroupIterator.java

@@ -479,6 +479,10 @@ private void operate(BindingSet bs, Predicate<?> predicate, Object t) {
 				return createNAryCustomAggregate(ge, operator, aggOperator, nAryFactory.get());
 			}
 			if (unaryFactory.isPresent()) {
+				if (argumentCount != 1) {
+					throw new QueryEvaluationException("Custom unary aggregate function '" + aggOperator.getIRI()
+							+ "' expects exactly 1 argument, got " + argumentCount);
+				}
 				return createUnaryCustomAggregate(ge, operator, aggOperator, unaryFactory.get());
 			}
 

core/queryalgebra/evaluation/src/test/java/org/eclipse/rdf4j/query/algebra/evaluation/iterator/GroupIteratorTest.java

@@ -274,6 +274,29 @@ public void testCustomAggregateFunction_WrongIri() throws QueryEvaluationExcepti
 		}
 	}
 
+	@Test
+	public void testCustomAggregateFunction_MultipleArgsRejected() throws QueryEvaluationException {
+		BindingSetAssignment assignment = new BindingSetAssignment();
+		var list = new ArrayList<BindingSet>();
+		for (int i = 1; i < 10; i++) {
+			var bindings = new QueryBindingSet();
+			bindings.addBinding("a", VF.createLiteral(i));
+			bindings.addBinding("b", VF.createLiteral(i * 2));
+			list.add(bindings);
+		}
+		assignment.setBindingSets(Collections.unmodifiableList(list));
+
+		Group group = new Group(assignment);
+		group.addGroupElement(new GroupElem("customSum",
+				new AggregateFunctionCall(List.of(Var.of("a"), Var.of("b")), AGGREGATE_FUNCTION_FACTORY.getIri(),
+						false)));
+		try (GroupIterator gi = new GroupIterator(EVALUATOR, group, EmptyBindingSet.getInstance(), CONTEXT)) {
+			assertThatExceptionOfType(QueryEvaluationException.class)
+					.isThrownBy(() -> gi.next().getBinding("customSum").getValue())
+					.withMessageContaining("expects exactly 1 argument");
+		}
+	}
+
 	@Test
 	public void testCustomNAryAggregateFunction_Nonempty() throws QueryEvaluationException {
 		AggregateNAryFunctionFactory nAryFactory = new FakeAggregateNAryFunctionFactory();

core/queryparser/sparql/src/main/java/org/eclipse/rdf4j/query/parser/sparql/aggregate/AggregateNAryFunction.java

@@ -8,6 +8,7 @@
  *
  * SPDX-License-Identifier: BSD-3-Clause
  *******************************************************************************/
+// Some portions generated by Codex
 package org.eclipse.rdf4j.query.parser.sparql.aggregate;
 
 import java.util.List;
@@ -40,9 +41,13 @@ protected AggregateNAryFunction(BiFunction<Integer, BindingSet, Value> evaluatio
 	 * Process an aggregate with tuple-level distinctness for n-ary functions.
 	 *
 	 * @param bindingSet    the current binding set
-	 * @param distinctTuple predicate to check if the tuple of argument values is distinct. the tuple may contain
+	 * @param distinctTuple predicate to check if the tuple of argument values is distinct. The tuple may contain an
 	 *                      arbitrary amount of arguments, therefore if necessary single argument distinctness can be
-	 *                      checked inside the predicate. Mixing argument sizes of tuples is not recommended.
+	 *                      checked inside the predicate. Mixing argument sizes of tuples is not recommended. Note: do
+	 *                      not mutate (or reuse) a mutable {@link List} instance after passing it to
+	 *                      {@code distinctTuple.test(...)} because the predicate may keep it in a hash-based set. If
+	 *                      you construct tuples using a mutable list, pass an immutable snapshot (e.g.
+	 *                      {@code List.copyOf(tuple)}).
 	 * @param agv           the aggregate collector
 	 * @throws QueryEvaluationException if evaluation fails
 	 */