Optional slice causes panic

[Pryz]

Using optional on a slice causes the following panic:

panic: reflect: call of reflect.Value.Field on slice Value [recovered]
        panic: reflect: call of reflect.Value.Field on slice Value

goroutine 4 [running]:
testing.tRunner.func1.2({0x104afcc40, 0x1400000eb10})
        /usr/local/go/src/testing/testing.go:1396 +0x1c8
testing.tRunner.func1()
        /usr/local/go/src/testing/testing.go:1399 +0x378
panic({0x104afcc40, 0x1400000eb10})
        /usr/local/go/src/runtime/panic.go:884 +0x204
reflect.Value.Field({0x104aca6c0?, 0x140001f8e70?, 0x104afb6e0?}, 0x70?)
        /usr/local/go/src/reflect/value.go:1266 +0xec
github.com/segmentio/parquet-go.(*structField).Value(0x140000cf600, {0x104aca6c0?, 0x140001f8e70?, 0x10455b72c?})
        /Users/julienfabre/segment/code/github.com/segmentio/parquet-go/schema.go:440 +0x2a8
github.com/segmentio/parquet-go.reconstructFuncOfGroup.func1({0x104aca6c0?, 0x140001f8e70?, 0x140000c1ab8?}, {0x3c?, 0xc0?, 0x87?}, {0x1400000eaf8, 0x1, 0x1})
        /Users/julienfabre/segment/code/github.com/segmentio/parquet-go/row.go:729 +0x340
github.com/segmentio/parquet-go.reconstructFuncOfOptional.func1({0x104aca6c0?, 0x140001f8e70?, 0x140001f8e70?}, {0x18?, 0x0?, 0x0?}, {0x1400000eaf8?, 0x1, 0x1})
        /Users/julienfabre/segment/code/github.com/segmentio/parquet-go/row.go:571 +0x208
github.com/segmentio/parquet-go.reconstructFuncOfGroup.func1({0x104b25120?, 0x140001f8e70?, 0x0?}, {0x70?, 0xfa?, 0xbc?}, {0x1400000eaf8, 0x1, 0x1})
        /Users/julienfabre/segment/code/github.com/segmentio/parquet-go/row.go:729 +0x374
github.com/segmentio/parquet-go.(*Schema).Reconstruct(0x140000942a0, {0x104ac0ae0?, 0x140001f8e70?}, {0x1400000eaf8, 0x1, 0x1})
        /Users/julienfabre/segment/code/github.com/segmentio/parquet-go/schema.go:264 +0x22c
github.com/segmentio/parquet-go.(*GenericReader[...]).readRows(0x140000ad180, {0x140001f8e70, 0x2, 0x0})
        /Users/julienfabre/segment/code/github.com/segmentio/parquet-go/reader_go18.go:142 +0x148
github.com/segmentio/parquet-go.(*GenericReader[...]).Read(0x104bbe120?, {0x140001f8e70?, 0x0?, 0x51b?})
        /Users/julienfabre/segment/code/github.com/segmentio/parquet-go/reader_go18.go:107 +0x38
github.com/segmentio/parquet-go_test.TestIssueXXX(0x140000fe340)
        /Users/julienfabre/segment/code/github.com/segmentio/parquet-go/reader_go18_test.go:184 +0x22c
testing.tRunner(0x140000fe340, 0x104bb5af0)
        /usr/local/go/src/testing/testing.go:1446 +0x10c
created by testing.(*T).Run
        /usr/local/go/src/testing/testing.go:1493 +0x300

Reproduction:

    type B struct {
        Name string
    }
    type A struct {
        B []B `parquet:",optional"`
    }

    b := new(bytes.Buffer)
    w := parquet.NewGenericWriter[A](b)
    expect := []A{
        {
            B: []B{
                {
                    Name: "foo",
                },
            },
        },
    }
    _, err := w.Write(expect)
    if err != nil {
        t.Fatal(err)
    }
    if err = w.Close(); err != nil {
        t.Fatal(err)
    }

    bufReader := bytes.NewReader(b.Bytes())
    r := parquet.NewGenericReader[A](bufReader)
    values := make([]A, 2)
    _, err = r.Read(values)
    if err != nil {
        t.Fatal(err)
    }

[achille-roussel]

Copying context from internal discussions:

In parquet, columns can only be required, optional, or repeated. Slice fields in Go structs are naturally represented as repeated columns, so adding the optional tag needs to be interpreted as each element of the slice might be optional instead of the slice itself; which is what triggers the panic currently.