Fix List's missing children metadata in JSON writer

python/cudf/cudf/_lib/json.pyx

@@ -259,9 +259,10 @@ cdef _set_col_children_metadata(Column col,
                 child_col, col_meta.children[i]
             )
     elif is_list_dtype(col):
-        _set_col_children_metadata(
-            col.children[0],
-            col_meta.children[0]
-        )
+        for i, child_col in enumerate(col.children):
+            col_meta.children.push_back(child_info)
+            _set_col_children_metadata(
+                child_col, col_meta.children[i]
+            )
     else:
         return

python/cudf/cudf/tests/test_json.py

@@ -215,6 +215,69 @@ def test_cudf_json_writer_read(gdf_writer_types):
     assert_eq(pdf2, gdf2)
 
 
+@pytest.mark.parametrize(
+    "jsonl_string, expected",
+    [
+        # fixed width
+        ("""{"a":10, "b":1.1}\n {"a":20, "b":2.1}\n""", None),
+        # simple list
+        ("""{"a":[1, 2, 3], "b":1.1}\n {"a":[]}\n""", None),
+        # simple struct
+        ("""{"a":{"c": 123 }, "b":1.1}\n {"a": {"c": 456}}\n""", None),
+        # list of lists
+        ("""{"a":[[], [1, 2], [3, 4]], "b":1.1}\n""", None),
+        ("""{"a":[null, [1, 2], [null, 4]], "b":1.1}\n""", None),
+        # list of structs
+        # error ("""{"a":[null, {}], "b":1.1}\n""", None),
+        (
+            """{"a":[null, {"L": 123}], "b":1.0}\n {"b":1.1}\n {"b":2.1}\n""",
+            None,
+        ),
+        (
+            """{"a":[{"L": 123}, null], "b":1.0}\n {"b":1.1}\n {"b":2.1}\n""",
+            None,
+        ),
+        # struct of lists
+        (
+            """{"a":{"L": [1, 2, 3]}, "b":1.1}\n {"a": {"L": [4, 5, 6]}}\n""",
+            None,
+        ),
+        ("""{"a":{"L": [1, 2, null]}, "b":1.1}\n {"a": {"L": []}}\n""", None),
+        # struct of structs
+        (
+            """{"a":{"L": {"M": 123}}, "b":1.1}
+               {"a": {"L": {"M": 456}}}\n""",
+            None,
+        ),
+        (
+            """{"a":{"L": {"M": null}}, "b":1.1}\n {"a": {"L": {}}}\n""",
+            """{"a":{"L": {}}, "b":1.1}\n {"a": {"L": {}}}\n""",
+        ),
+        # list of structs of lists
+        ("""{"a":[{"L": [1, 2, 3]}, {"L": [4, 5, 6]}], "b":1.1}\n""", None),
+        ("""{"a":[{"L": [1, 2, null]}, {"L": []}], "b":1.1}\n""", None),
+        # struct of lists of structs
+        ("""{"a":{"L": [{"M": 123}, {"M": 456}]}, "b":1.1}\n""", None),
+        (
+            """{"a":{"L": [{"M": null}, {}]}, "b":1.1}\n""",
+            """{"a":{"L": [{}, {}]}, "b":1.1}\n""",
+        ),
+    ],
+)
+def test_cudf_json_roundtrip(jsonl_string, expected):
+    gdf = cudf.read_json(
+        StringIO(jsonl_string),
+        lines=True,
+        engine="cudf",
+        # dtype=dict(dtypes),
+    )
+    expected = jsonl_string if expected is None else expected
+    gdf_string = gdf.to_json(
+        orient="records", lines=True, engine="cudf", include_nulls=False
+    )
+    assert_eq(gdf_string, expected.replace(" ", ""))
+
+
 @pytest.mark.parametrize("sink", ["string", "file"])
 def test_cudf_json_writer_sinks(sink, tmp_path_factory):
     df = cudf.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
@@ -1185,7 +1248,7 @@ def test_json_array_of_arrays(data, lines):
         # simple list with mixed types
         """{"a":[123, {}], "b":1.1}""",
         """{"a":[123, {"0": 123}], "b":1.0}\n {"b":1.1}\n {"b":2.1}""",
-        """{"a":[{"0": 123}, 123], "b":1.0}\n {"b":1.1}\n {"b":2.1}""",
+        """{"a":[{"L": 123}, 123], "b":1.0}\n {"b":1.1}\n {"b":2.1}""",
         """{"a":[123, {"0": 123}, 12.3], "b":1.0}\n {"b":1.1}\n {"b":2.1}""",
         """{"a":[123, {"0": 123}, null], "b":1.0}\n {"b":1.1}\n {"b":2.1}""",
         """{"a":["123", {"0": 123}], "b":1.0}\n {"b":1.1}\n {"b":2.1}""",