Bug with SnapshotMap remove when value is `()`

[kevzzsk]

When value removing () value, somehow the value is not stored in the changelog. so when calling may_load_at_height, the old value returned is None when it should be Some(()). Example below, illustrate it better.

This code below illustrate the problem.

type Firstname = String;
const PEOPLE: SnapshotMap<Firstname, ()> = SnapshotMap::new(
    "people",
    "people_checkpoint",
    "people_changelog",
    Strategy::EveryBlock,
);

let mut deps = mock_dependencies();
let mut env = mock_env();

let firstname = "john".to_string();

PEOPLE
    .save(&mut deps.storage, firstname.clone(), &(), env.block.height)
    .unwrap();

// assert that john exists
let res = PEOPLE.may_load(&deps.storage, firstname.clone()).unwrap();
assert_eq!(res, Some(()));

let res = PEOPLE
    .may_load_at_height(&deps.storage, firstname.clone(), env.block.height + 1)
    .unwrap();
assert_eq!(res, Some(()));

// block advances
env.block.height += 10;

// remove john
PEOPLE
    .remove(&mut deps.storage, firstname.clone(), env.block.height)
    .unwrap();

// assert that john does not exist
let res = PEOPLE.may_load(&deps.storage, firstname.clone()).unwrap();
assert_eq!(res, None);
let res = PEOPLE
    .may_load_at_height(&deps.storage, firstname.clone(), env.block.height + 1)
    .unwrap();
assert_eq!(res, None);

// assert that john used to exists in previous block height
let res = PEOPLE
    .may_load_at_height(&deps.storage, firstname.clone(), env.block.height - 1)
    .unwrap();
assert_eq!(res, Some(()));  // ❌ ERROR: res is None

When the value stored is change from () to bool, it is able to retrieve correctly.

type Firstname = String;
const PEOPLE2: SnapshotMap<Firstname, bool> = SnapshotMap::new(
    "people",
    "people_checkpoint",
    "people_changelog",
    Strategy::EveryBlock,
);

let mut deps = mock_dependencies();
let mut env = mock_env();

let firstname = "john".to_string();

PEOPLE2
    .save(
        &mut deps.storage,
        firstname.clone(),
        &true,
        env.block.height,
    )
    .unwrap();

// assert that john exists
let res = PEOPLE2.may_load(&deps.storage, firstname.clone()).unwrap();
assert_eq!(res, Some(true));

let res = PEOPLE2
    .may_load_at_height(&deps.storage, firstname.clone(), env.block.height + 1)
    .unwrap();
assert_eq!(res, Some(true));

// block advances
env.block.height += 10;

// remove john
PEOPLE2
    .remove(&mut deps.storage, firstname.clone(), env.block.height)
    .unwrap();

// assert that john does not exist
let res = PEOPLE2.may_load(&deps.storage, firstname.clone()).unwrap();
assert_eq!(res, None);
let res = PEOPLE2
    .may_load_at_height(&deps.storage, firstname.clone(), env.block.height + 1)
    .unwrap();
assert_eq!(res, None);

// assert that john used to exists in previous block height
let res = PEOPLE2
    .may_load_at_height(&deps.storage, firstname.clone(), env.block.height - 1)
    .unwrap();
assert_eq!(res, Some(true)); // ✅Passes

[fuxingloh]

#[test]
fn empty_struct_some_equal_none() {
    let a: &Option<()> = &Some(());
    let b: &Option<()> = &None;

    assert_eq!(to_json_vec(a), to_json_vec(b));
}

It seems like it's due to how the underlying serialization encodes? via to_json_vec.

Some(()) and None both get encoded to null due to JSON rules.