pallet ranked collective: max member count per rank (paritytech#4807)

Configuration for the maximum member count per rank, with the option for
no limit.

cumulus/parachains/runtimes/collectives/collectives-westend/src/ambassador/mod.rs

@@ -117,6 +117,7 @@ impl pallet_ranked_collective::Config<AmbassadorCollectiveInstance> for Runtime
 	type MinRankOfClass = sp_runtime::traits::Identity;
 	type MemberSwappedHandler = (crate::AmbassadorCore, crate::AmbassadorSalary);
 	type VoteWeight = pallet_ranked_collective::Linear;
+	type MaxMemberCount = ();
 	#[cfg(feature = "runtime-benchmarks")]
 	type BenchmarkSetup = (crate::AmbassadorCore, crate::AmbassadorSalary);
 }

cumulus/parachains/runtimes/collectives/collectives-westend/src/fellowship/mod.rs

@@ -150,6 +150,7 @@ impl pallet_ranked_collective::Config<FellowshipCollectiveInstance> for Runtime
 	type MinRankOfClass = tracks::MinRankOfClass;
 	type MemberSwappedHandler = (crate::FellowshipCore, crate::FellowshipSalary);
 	type VoteWeight = pallet_ranked_collective::Geometric;
+	type MaxMemberCount = ();
 	#[cfg(feature = "runtime-benchmarks")]
 	type BenchmarkSetup = (crate::FellowshipCore, crate::FellowshipSalary);
 }

polkadot/runtime/rococo/src/governance/fellowship.rs

@@ -356,6 +356,7 @@ impl pallet_ranked_collective::Config<FellowshipCollectiveInstance> for Runtime
 	type MinRankOfClass = sp_runtime::traits::Identity;
 	type MemberSwappedHandler = ();
 	type VoteWeight = pallet_ranked_collective::Geometric;
+	type MaxMemberCount = ();
 	#[cfg(feature = "runtime-benchmarks")]
 	type BenchmarkSetup = ();
 }

prdoc/pr_4807.prdoc

@@ -0,0 +1,11 @@
+title: "pallet ranked collective: max member count per rank"
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Configuration for the maximum member count per rank, with the option for no limit.
+
+crates:
+  - name: pallet-ranked-collective
+    bump: major
+

substrate/bin/node/runtime/src/lib.rs

@@ -1038,6 +1038,7 @@ impl pallet_ranked_collective::Config for Runtime {
 	type MinRankOfClass = traits::Identity;
 	type VoteWeight = pallet_ranked_collective::Geometric;
 	type MemberSwappedHandler = (CoreFellowship, Salary);
+	type MaxMemberCount = ();
 	#[cfg(feature = "runtime-benchmarks")]
 	type BenchmarkSetup = (CoreFellowship, Salary);
 }

substrate/frame/core-fellowship/src/tests/integration.rs

@@ -157,6 +157,7 @@ impl pallet_ranked_collective::Config for Test {
 	type MinRankOfClass = MinRankOfClass<MinRankOfClassDelta>;
 	type MemberSwappedHandler = CoreFellowship;
 	type VoteWeight = Geometric;
+	type MaxMemberCount = ();
 	#[cfg(feature = "runtime-benchmarks")]
 	type BenchmarkSetup = CoreFellowship;
 }

substrate/frame/ranked-collective/src/lib.rs

@@ -379,6 +379,7 @@ pub mod pallet {
 	use super::*;
 	use frame_support::{pallet_prelude::*, storage::KeyLenOf};
 	use frame_system::pallet_prelude::*;
+	use sp_runtime::traits::MaybeConvert;
 
 	#[pallet::pallet]
 	pub struct Pallet<T, I = ()>(PhantomData<(T, I)>);
@@ -431,6 +432,14 @@ pub mod pallet {
 		/// in the poll.
 		type VoteWeight: Convert<Rank, Votes>;
 
+		/// The maximum number of members for a given rank in the collective.
+		///
+		/// The member at rank `x` contributes to the count at rank `x` and all ranks below it.
+		/// Therefore, the limit `m` at rank `x` sets the maximum total member count for rank `x`
+		/// and all ranks above.
+		/// The `None` indicates no member count limit for the given rank.
+		type MaxMemberCount: MaybeConvert<Rank, MemberIndex>;
+
 		/// Setup a member for benchmarking.
 		#[cfg(feature = "runtime-benchmarks")]
 		type BenchmarkSetup: BenchmarkSetup<Self::AccountId>;
@@ -511,6 +520,8 @@ pub mod pallet {
 		NoPermission,
 		/// The new member to exchange is the same as the old member
 		SameMember,
+		/// The max member count for the rank has been reached.
+		TooManyMembers,
 	}
 
 	#[pallet::call]
@@ -758,6 +769,9 @@ pub mod pallet {
 			ensure!(!Members::<T, I>::contains_key(&who), Error::<T, I>::AlreadyMember);
 			let index = MemberCount::<T, I>::get(0);
 			let count = index.checked_add(1).ok_or(Overflow)?;
+			if let Some(max) = T::MaxMemberCount::maybe_convert(0) {
+				ensure!(count <= max, Error::<T, I>::TooManyMembers);
+			}
 
 			Members::<T, I>::insert(&who, MemberRecord { rank: 0 });
 			IdToIndex::<T, I>::insert(0, &who, index);
@@ -784,6 +798,11 @@ pub mod pallet {
 				ensure!(max_rank >= rank, Error::<T, I>::NoPermission);
 			}
 			let index = MemberCount::<T, I>::get(rank);
+			let count = index.checked_add(1).ok_or(Overflow)?;
+			if let Some(max) = T::MaxMemberCount::maybe_convert(rank) {
+				ensure!(count <= max, Error::<T, I>::TooManyMembers);
+			}
+
 			MemberCount::<T, I>::insert(rank, index.checked_add(1).ok_or(Overflow)?);
 			IdToIndex::<T, I>::insert(rank, &who, index);
 			IndexToId::<T, I>::insert(rank, index, &who);

substrate/frame/ranked-collective/src/tests.rs

@@ -27,7 +27,7 @@ use frame_support::{
 };
 use sp_core::Get;
 use sp_runtime::{
-	traits::{ReduceBy, ReplaceWithDefault},
+	traits::{MaybeConvert, ReduceBy, ReplaceWithDefault},
 	BuildStorage,
 };
 
@@ -148,6 +148,17 @@ impl<Delta: Get<Rank>> Convert<Class, Rank> for MinRankOfClass<Delta> {
 	}
 }
 
+pub struct MaxMemberCount;
+impl MaybeConvert<Rank, MemberIndex> for MaxMemberCount {
+	fn maybe_convert(a: Rank) -> Option<MemberIndex> {
+		if a == 11 {
+			Some(2)
+		} else {
+			None
+		}
+	}
+}
+
 parameter_types! {
 	pub static MinRankOfClassDelta: Rank = 0;
 }
@@ -179,6 +190,7 @@ impl Config for Test {
 	type MinRankOfClass = MinRankOfClass<MinRankOfClassDelta>;
 	type MemberSwappedHandler = ();
 	type VoteWeight = Geometric;
+	type MaxMemberCount = MaxMemberCount;
 	#[cfg(feature = "runtime-benchmarks")]
 	type BenchmarkSetup = ();
 }
@@ -645,3 +657,32 @@ fn exchange_member_same_noops() {
 		);
 	});
 }
+
+#[test]
+fn max_member_count_works() {
+	ExtBuilder::default().build_and_execute(|| {
+		assert_ok!(Club::do_add_member_to_rank(1, 10, false));
+		assert_ok!(Club::do_add_member_to_rank(2, 10, false));
+		assert_ok!(Club::do_add_member_to_rank(3, 10, false));
+		assert_eq!(member_count(10), 3);
+		assert_eq!(member_count(11), 0);
+
+		assert_ok!(Club::promote_member(RuntimeOrigin::root(), 1));
+		assert_ok!(Club::promote_member(RuntimeOrigin::root(), 2));
+		assert_noop!(Club::promote_member(RuntimeOrigin::root(), 3), Error::<Test>::TooManyMembers);
+		assert_eq!(member_count(10), 3);
+		assert_eq!(member_count(11), 2);
+
+		assert_ok!(Club::demote_member(RuntimeOrigin::root(), 1));
+		assert_ok!(Club::promote_member(RuntimeOrigin::root(), 3));
+		assert_eq!(member_count(10), 3);
+		assert_eq!(member_count(11), 2);
+
+		assert_ok!(Club::promote_member(RuntimeOrigin::root(), 2));
+		assert_ok!(Club::promote_member(RuntimeOrigin::root(), 3));
+		assert_noop!(Club::promote_member(RuntimeOrigin::root(), 1), Error::<Test>::TooManyMembers);
+		assert_eq!(member_count(10), 3);
+		assert_eq!(member_count(11), 2);
+		assert_eq!(member_count(12), 2);
+	});
+}

substrate/frame/salary/src/tests/integration.rs

@@ -180,6 +180,7 @@ impl pallet_ranked_collective::Config for Test {
 	type MinRankOfClass = MinRankOfClass<MinRankOfClassDelta>;
 	type MemberSwappedHandler = Salary;
 	type VoteWeight = Geometric;
+	type MaxMemberCount = ();
 	#[cfg(feature = "runtime-benchmarks")]
 	type BenchmarkSetup = Salary;
 }