I started this investigation/issue based on @liamaharon question
[here](https://github.com/paritytech/polkadot-sdk/pull/1801#discussion_r1410452499).
## Problem
The `pallet_balances` integrity test should correctly detect that the
runtime has correct distinct `HoldReasons` variant count. I assume the
same situation exists for RuntimeFreezeReason.
It is not a critical problem, if we set `MaxHolds` with a sufficiently
large value, everything should be ok. However, in this case, the
integrity_test check becomes less useful.
**Situation for "any" runtime:**
- `HoldReason` enums from different pallets:
```rust
/// from pallet_nis
#[pallet::composite_enum]
pub enum HoldReason {
NftReceipt,
}
/// from pallet_preimage
#[pallet::composite_enum]
pub enum HoldReason {
Preimage,
}
// from pallet_state-trie-migration
#[pallet::composite_enum]
pub enum HoldReason {
SlashForContinueMigrate,
SlashForMigrateCustomTop,
SlashForMigrateCustomChild,
}
```
- generated `RuntimeHoldReason` enum looks like:
```rust
pub enum RuntimeHoldReason {
#[codec(index = 32u8)]
Preimage(pallet_preimage::HoldReason),
#[codec(index = 38u8)]
Nis(pallet_nis::HoldReason),
#[codec(index = 42u8)]
StateTrieMigration(pallet_state_trie_migration::HoldReason),
}
```
- composite enum `RuntimeHoldReason` variant count is detected as `3`
- we set `type MaxHolds = ConstU32<3>`
- `pallet_balances::integrity_test` is ok with `3`(at least 3)
However, the real problem can occur in a live runtime where some
functionality might stop working. This is due to a total of 5 distinct
hold reasons (for pallets with multi-instance support, it is even more),
and not all of them can be used because of an incorrect `MaxHolds`,
which is deemed acceptable according to the `integrity_test`:
```
// pseudo-code - if we try to call all of these:
T::Currency::hold(&pallet_nis::HoldReason::NftReceipt.into(),
&nft_owner, deposit)?;
T::Currency::hold(&pallet_preimage::HoldReason::Preimage.into(),
&nft_owner, deposit)?;
T::Currency::hold(&pallet_state_trie_migration::HoldReason::SlashForContinueMigrate.into(),
&nft_owner, deposit)?;
// With `type MaxHolds = ConstU32<3>` these two will fail
T::Currency::hold(&pallet_state_trie_migration::HoldReason::SlashForMigrateCustomTop.into(),
&nft_owner, deposit)?;
T::Currency::hold(&pallet_state_trie_migration::HoldReason::SlashForMigrateCustomChild.into(),
&nft_owner, deposit)?;
```
## Solutions
A macro `#[pallet::*]` expansion is extended of `VariantCount`
implementation for the `#[pallet::composite_enum]` enum type. This
expansion generates the `VariantCount` implementation for pallets'
`HoldReason`, `FreezeReason`, `LockId`, and `SlashReason`. Enum variants
must be plain enum values without fields to ensure a deterministic
count.
The composite runtime enum, `RuntimeHoldReason` and
`RuntimeFreezeReason`, now sets `VariantCount::VARIANT_COUNT` as the sum
of pallets' enum `VariantCount::VARIANT_COUNT`:
```rust
#[frame_support::pallet(dev_mode)]
mod module_single_instance {
#[pallet::composite_enum]
pub enum HoldReason {
ModuleSingleInstanceReason1,
ModuleSingleInstanceReason2,
}
...
}
#[frame_support::pallet(dev_mode)]
mod module_multi_instance {
#[pallet::composite_enum]
pub enum HoldReason<I: 'static = ()> {
ModuleMultiInstanceReason1,
ModuleMultiInstanceReason2,
ModuleMultiInstanceReason3,
}
...
}
impl self::sp_api_hidden_includes_construct_runtime::hidden_include::traits::VariantCount
for RuntimeHoldReason
{
const VARIANT_COUNT: u32 = 0
+ module_single_instance::HoldReason::VARIANT_COUNT
+ module_multi_instance::HoldReason::<module_multi_instance::Instance1>::VARIANT_COUNT
+ module_multi_instance::HoldReason::<module_multi_instance::Instance2>::VARIANT_COUNT
+ module_multi_instance::HoldReason::<module_multi_instance::Instance3>::VARIANT_COUNT;
}
```
In addition, `MaxHolds` is removed (as suggested
[here](https://github.com/paritytech/polkadot-sdk/pull/2657#discussion_r1443324573))
from `pallet_balances`, and its `Holds` are now bounded to
`RuntimeHoldReason::VARIANT_COUNT`. Therefore, there is no need to let
the runtime specify `MaxHolds`.
## For reviewers
Relevant changes can be found here:
- `substrate/frame/support/procedural/src/lib.rs`
- `substrate/frame/support/procedural/src/pallet/parse/composite.rs`
- `substrate/frame/support/procedural/src/pallet/expand/composite.rs`
-
`substrate/frame/support/procedural/src/construct_runtime/expand/composite_helper.rs`
-
`substrate/frame/support/procedural/src/construct_runtime/expand/hold_reason.rs`
-
`substrate/frame/support/procedural/src/construct_runtime/expand/freeze_reason.rs`
- `substrate/frame/support/src/traits/misc.rs`
And the rest of the files is just about removed `MaxHolds` from
`pallet_balances`
## Next steps
Do the same for `MaxFreezes`
https://github.com/paritytech/polkadot-sdk/issues/2997.
---------
Co-authored-by: command-bot <>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Dónal Murray <donal.murray@parity.io>
Co-authored-by: gupnik <nikhilgupta.iitk@gmail.com>
gupnik