Utils.convertNulls mutates its input, so a nested evaluate in a custom function corrupts JSON nulls in the outer expression #124

Utils.convertNulls replaces Jsonata.NULL_VALUE with Java null in place. Because Jsonata.evaluate calls it on its result (when output null conversion is on, the default), any nested evaluate inside a custom function rewrites data that the outer expression is still using. After that, JSON nulls in the outer data are treated as undefined.

Reproduce

var expr = Jsonata.jsonata("( $t := {'p':[null, 1]}; $echo($t); $t.p.$type($) )");
expr.registerFunction("echo", (Object x) -> Jsonata.jsonata("$").evaluate(x));
expr.evaluate(null);

Expected: ["null", "number"]
Actual: "number". The nested evaluate turned $t.p into [null, 1] (Java null), so the first element is treated as undefined.

Without the $echo($t) call, the result is correct.

On 0.9.10 (before 49df9f8, "Preserve JSON null when indexing into an array") the effect is also visible with indexing. For example, $exists($t.tests[0].parameters[0]) flips from true to false after the list was passed to such a custom function.

Cause

• Utils.convertNulls(Object) and its Map/List overloads modify the structure passed to them (Utils.java, ~L207-243).
• Jsonata.evaluate calls Utils.convertNulls(it) on the result (Jsonata.java, ~L2728). That result can share lists and maps with the caller's input or with another expression's live data.

Suggested fix

Have convertNulls return a converted copy, or copy only the containers that contain NULL_VALUE, instead of mutating its argument.

Tests

Two failing tests that cover this:

@Test
public void testConvertNullsDoesNotModifyInput() {
    List<Object> list = new ArrayList<>(Arrays.asList(NULL_VALUE, 1));
    Map<String, Object> map = new java.util.HashMap<>(Map.of("p", list));
    Object res = Utils.convertNulls((Object) map);
    Assertions.assertEquals(Arrays.asList(null, 1), ((Map)res).get("p"));
    Assertions.assertEquals(Arrays.asList(NULL_VALUE, 1), list);
}

@Test
public void testNestedEvaluateInCustomFunctionKeepsJsonNull() {
    var expr = jsonata("( $t := {'p':[null, 1]}; $echo($t); $t.p.$type($) )");
    expr.registerFunction("echo", (Object x) -> jsonata("$").evaluate(x));
    Assertions.assertEquals(List.of("null", "number"), expr.evaluate(null));
}