storage_key_iter does return invalid second keys for StorageDoubleMap

[sea212]

Is there an existing issue?

• I have searched the existing issues

Experiencing problems? Have you tried our Stack Exchange first?

• This is not a support question.

Description of bug

When using storage_key_iter on a StorageDoubleMap with a ReversibleStorageHasher other than Identity, the second keys that are returned contain wrong values.

I am not quite sure if storage_key_iter was ever designed to be used with StorageDoubleMaps. In that case, a note in the documentation is crucial, otherwise devs might mess up their storage by assuming that it will work.

Steps to reproduce

I created a repository that contains a rust crate with a minimal example and three tests that can be used as a reference.

1. Create StorageDoubleMap using Blake2_128Concat or Twox64Concat hasher. The key types should be i32, the value should be u32
2. Insert an example value in the storage, like: (-12, -34) = 56
3. Use storage_key_iter to iterate over the keys and values. The second key in the retrieved value should not match the second key in the inserted value.

[bkchr]

I checked this and the code is actually working, but storage_key_iter is only for storage maps and not double maps.

For a double map you actually need to do something like this:

let key_material = something;
let mut key_one_and_rest = K1Hasher::reverse(&key_material);
let k1 = K1::decode(&mut key_one_and_rest)?;
let key_two_material = K2Hasher::reverse(&key_one_and_rest);
let key_two = K2::decode(&mut &key_two_material[..])?'

But you can not control the key decoding from the function you are calling. I propose that you switch to generate_storage_alias! or to the new storage_alias (when it is merged). Then you don't need to use storage_key_iter.

[sea212]

@bkchr Thanks for the clarification and for providing an alternative approach. I suggest to document that storage_key_iter should only be used for StorageMap (I can create the PR if desired).

[bkchr]

Yeah a pr would be nice @sea212 :)