Composite accounts

[gavofyork]

This brings in the final element of my balance refactorings, namely to avoid the (pessimising) spreading of typically co-accessed account data between multiple trie keys. This is particularly annoying for nonce and balance, which for all normal transactions are always mutated and yet are split between two separate trie items.

This provides an extensible framework, via a trait StoredMap, which allows an item to be stored somewhere, without specifying whether it's stored in its own storage item or in a shared composite storage item with other (probably related) values. It is used by the system module to allow for arbitrary data to be stored alongside the account nonce.

Concretely, this merges the balances of an account with the nonce, reducing the number of trie accesses for a typical transfer from 3 to 2.

Until we know the exact tradeoff between bulking up a single key with lots of data that may possibly not tend to be co-accessed versus having multiple, smaller, trie keys, then we'll keep it lean, but benchmarking typical use-cases with "fat account items" might lead to further optimisations.

TODO

• Indices should not use chunking.
• Indices should hold a deposit.
• Reclaiming an index should not be possible before removal.
• Migration code.
• Remove index lookup (in Kusama/Polkadot).
• Test migration on live Kusama state.

Additionally...

In addition to this, it makes the system of account indices opt-in; this means that accounts need not have indexes despite being funded. Two calls for registering indices are provided, as well as a third call for giving an index up and a forth for administrating indices by root.

The upshot to this is that there no longer needs to be a separate AccountCreationFee. All transfers work in the same way.

^^^ CC @jacogr @pepyakin

Indices were originally brought in to provide for a means of enumerating all accounts (e.g. for migrations). This is no longer needed since we improved the storage interface. Their other usage was for minimising the size of a transaction, at the cost of an additional storage lookup. This additional looking is in reality an extra cost that must be borne out through conditional weighting, increasing code complexity and attack surface. The reality is that the extra bytes fee for putting an normal address is likely smaller than the fee for the extra lookup, meaning it will never be used. Its third usage is in terms of usability, allowing users to pass around smaller addresses. This is slightly dangerous in the previous usage, due to address reclaiming, and needs careful UI provisions. The cost, as it has been implemented, is an ever-growing state or significant fees.

This is mitigated in the current PR by having a single storage entry per index and a deposit, where the deposit can be reclaimed.

Upgrading Pallets & Runtimes

If you're using decl_event in your tests, you'll now need to include system<T> as an item, so
decl_event!(my_pallet<T>); would become decl_event!(system<T>, my_pallet<T>);.

system::Trait is now the place to put any OnNewAccount or OnReapAccount hooks; these will need moving from balances::Trait. You should include at least the Balances handler in the reap account, so:

impl frame_system::Trait for Runtime {
    ...
    type OnNewAccount = (...);
    type OnReapAccount = (Balances, ...);
    ...
}

Additionally, there's a new item in system::Trait called AccountData and a new item in balances::Trait called AccountStore. These can be wired together in order to let system store your account balances, which will be more efficient than balances storing them itself:

impl frame_system::Trait for Runtime {
    ...
    type AccountData = pallet_balances::AccountData<Balance>; // Or whatever your balance type is (e.g. u64 in tests)
    ...
}
impl pallet_balances::Trait for Runtime {
    ...
    type AccountStore = System;
    ...
}

Several config trait items are gone, they can be removed:

• pallet_balances::Trait::OnNewAccount (now in frame_system)
• pallet_balances::Trait::OnReapAccount (now in frame_system)
• pallet_balances::Trait::OnFreeBalanceZero (just use OnReapAccount)
• pallet_balances::Trait::CreationFee (removed)
• pallet_balances::Trait::TransferFee (removed)

[gavofyork]

logic errors fixed.

[shawntabrizi]

I am happy with this other than the final comments.

[thiolliere]

I suspect the storage version hasn't been removed from storage in kusama

[thiolliere]

But if I query on kusama the key twox128("Staking") ++ twox128("StorageVersion") == 0x5f3e4907f716ac89b6347d15ececedca308ce9615de0775a82f8a94dc3d285a1 then I get no value, and I can't understand why yet.

EDIT: I got my answer about Kusama, migration feature was off