[BUG] Requested types ignored if prune_schema is enabled for JSON reading

[revans2]

Describe the bug
I noticed this in some unit tests for the java APIs when I tried to enable schema pruning in CUDF by default for java JSON read APIs that explicitly do column pruning.

• cudf/java/src/test/java/ai/rapids/cudf/TableTest.java

  Lines 664 to 714 in 0b32f55

  	@Test
  	void testReadJSONNestedTypes() {
  	Schema.Builder root = Schema.builder();
  	Schema.Builder a = root.addColumn(DType.STRUCT, "a");
  	a.addColumn(DType.STRING, "b");
  	a.addColumn(DType.STRING, "c");
  	a.addColumn(DType.STRING, "missing");
  	Schema.Builder d = root.addColumn(DType.LIST, "d");
  	d.addColumn(DType.INT64, "ignored");
  	root.addColumn(DType.INT64, "also_missing");
  	Schema.Builder e = root.addColumn(DType.LIST, "e");
  	Schema.Builder eChild = e.addColumn(DType.STRUCT, "ignored");
  	eChild.addColumn(DType.INT64, "f");
  	eChild.addColumn(DType.STRING, "missing_in_list");
  	eChild.addColumn(DType.INT64, "g");
  	Schema schema = root.build();
  	JSONOptions opts = JSONOptions.builder()
  	.withLines(true)
  	.build();
  	StructType aStruct = new StructType(true,
  	new BasicType(true, DType.STRING),
  	new BasicType(true, DType.STRING),
  	new BasicType(true, DType.STRING));
  	ListType dList = new ListType(true, new BasicType(true, DType.INT64));
  	StructType eChildStruct = new StructType(true,
  	new BasicType(true, DType.INT64),
  	new BasicType(true, DType.STRING),
  	new BasicType(true, DType.INT64));
  	ListType eList = new ListType(true, eChildStruct);
  	try (Table expected = new Table.TestBuilder()
  	.column(aStruct,
  	new StructData(null, "C1", null),
  	new StructData("B2", "C2", null),
  	null,
  	null)
  	.column(dList,
  	null,
  	null,
  	Arrays.asList(1L,2L,3L),
  	new ArrayList<Long>())
  	.column((Long)null, null, null, null) // also_missing
  	.column(eList,
  	null,
  	null,
  	null,
  	Arrays.asList(new StructData(null, null, 1L), new StructData(2L, null, null), new StructData(3L, null, 4L)))
  	.build();
  	Table table = Table.readJSON(schema, opts, NESTED_JSON_DATA_BUFFER)) {
  	assertTablesAreEqual(expected, table);
  	}
  	}

  which fails because column d is being returned as a LIST<INT8> instead of a LIST<INT64> which is what it was requested to be, and which is what is returned for column d if pruning is disabled.

• cudf/java/src/test/java/ai/rapids/cudf/TableTest.java

  Lines 743 to 790 in 0b32f55

  	@Test
  	void testReadJSONNestedTypesDataSource() {
  	Schema.Builder root = Schema.builder();
  	Schema.Builder a = root.addColumn(DType.STRUCT, "a");
  	a.addColumn(DType.STRING, "b");
  	a.addColumn(DType.STRING, "c");
  	a.addColumn(DType.STRING, "missing");
  	Schema.Builder d = root.addColumn(DType.LIST, "d");
  	d.addColumn(DType.INT64, "ignored");
  	root.addColumn(DType.INT64, "also_missing");
  	Schema.Builder e = root.addColumn(DType.LIST, "e");
  	Schema.Builder eChild = e.addColumn(DType.STRUCT, "ignored");
  	eChild.addColumn(DType.INT64, "g");
  	Schema schema = root.build();
  	JSONOptions opts = JSONOptions.builder()
  	.withLines(true)
  	.build();
  	StructType aStruct = new StructType(true,
  	new BasicType(true, DType.STRING),
  	new BasicType(true, DType.STRING),
  	new BasicType(true, DType.STRING));
  	ListType dList = new ListType(true, new BasicType(true, DType.INT64));
  	StructType eChildStruct = new StructType(true,
  	new BasicType(true, DType.INT64));
  	ListType eList = new ListType(true, eChildStruct);
  	try (Table expected = new Table.TestBuilder()
  	.column(aStruct,
  	new StructData(null, "C1", null),
  	new StructData("B2", "C2", null),
  	null,
  	null)
  	.column(dList,
  	null,
  	null,
  	Arrays.asList(1L,2L,3L),
  	new ArrayList<Long>())
  	.column((Long)null, null, null, null) // also_missing
  	.column(eList,
  	null,
  	null,
  	null,
  	Arrays.asList(new StructData(1L), new StructData((Long)null), new StructData(4L)))
  	.build();
  	MultiBufferDataSource source = sourceFrom(NESTED_JSON_DATA_BUFFER);
  	Table table = Table.readJSON(schema, opts, source)) {
  	assertTablesAreEqual(expected, table);
  	}
  	}

  is failing for the same reason as the above one. column d is the wrong type.

• cudf/java/src/test/java/ai/rapids/cudf/TableTest.java

  Lines 716 to 741 in 0b32f55

  	@Test
  	void testReadJSONNestedTypesVerySmallChanges() {
  	Schema.Builder root = Schema.builder();
  	Schema.Builder e = root.addColumn(DType.LIST, "e");
  	Schema.Builder eChild = e.addColumn(DType.STRUCT, "ignored");
  	eChild.addColumn(DType.INT64, "g");
  	eChild.addColumn(DType.INT64, "f");
  	Schema schema = root.build();
  	JSONOptions opts = JSONOptions.builder()
  	.withLines(true)
  	.build();
  	StructType eChildStruct = new StructType(true,
  	new BasicType(true, DType.INT64),
  	new BasicType(true, DType.INT64));
  	ListType eList = new ListType(true, eChildStruct);
  	try (Table expected = new Table.TestBuilder()
  	.column(eList,
  	null,
  	null,
  	null,
  	Arrays.asList(new StructData(1L, null), new StructData(null, 2L), new StructData(4L, 3L)))
  	.build();
  	Table table = Table.readJSON(schema, opts, NESTED_JSON_DATA_BUFFER)) {
  	assertTablesAreEqual(expected, table);
  	}
  	}

  is failing because column e was requested to be a LIST<STRUCT>, but it was returned as a LIST<INT8> column.

Steps/Code to reproduce bug
If you want to reproduce this you can take #16796 and enable column pruning for the tests that are listed as failing. The third test is the scariest one, and it appears to return totally invalid results where the data column is empty despite the there being offsets pointing into it.

If I need to create a C++ repro case I am happy to do it

Expected behavior
I would expect the types in the schema to be honored at least in the same way that it is for the non pruning use case.

[revans2]

I think this is fixed if the experimental feature is enabled.