min collator check

pallets/collator-selection/src/lib.rs

@@ -41,6 +41,9 @@
 //! The current implementation resolves congestion of [`Candidates`] in a first-come-first-serve
 //! manner.
 //!
+//! Candidates will not be allowed to get kicked or leave_intent if the total number of candidates
+//! fall below MinCandidates. This is for potential disaster recovery scenarios.
+//!
 //! ### Rewards
 //!
 //! The Collator Selection pallet maintains an on-chain account (the "Pot"). In each block, the
@@ -127,6 +130,12 @@ pub mod pallet {
 		/// This does not take into account the invulnerables.
 		type MaxCandidates: Get<u32>;
 
+		/// Minimum number of candidates that we should have. This is used for disaster recovery.
+		///
+		/// This does not take into account the invulnerables.
+		type MinCandidates: Get<u32>;
+
+
 		/// Maximum number of invulnerables.
 		///
 		/// Used only for benchmarking.
@@ -239,6 +248,7 @@ pub mod pallet {
 	#[pallet::error]
 	pub enum Error<T> {
 		TooManyCandidates,
+		TooFewCandidates,
 		Unknown,
 		Permission,
 		AlreadyCandidate,
@@ -323,7 +333,7 @@ pub mod pallet {
 		#[pallet::weight(T::WeightInfo::leave_intent(T::MaxCandidates::get()))]
 		pub fn leave_intent(origin: OriginFor<T>) -> DispatchResultWithPostInfo {
 			let who = ensure_signed(origin)?;
-
+			ensure!(Self::candidates().len() as u32 > T::MinCandidates::get(), Error::<T>::TooFewCandidates);
 			let current_count = Self::try_remove_candidate(&who)?;
 
 			Ok(Some(T::WeightInfo::leave_intent(current_count as u32)).into())
@@ -365,7 +375,7 @@ pub mod pallet {
 			let new_candidates = candidates.into_iter().filter_map(|c| {
 				let last_block = <LastAuthoredBlock<T>>::get(c.who.clone());
 				let since_last = now.saturating_sub(last_block);
-				if since_last < kick_threshold {
+				if since_last < kick_threshold || Self::candidates().len() <= T::MinCandidates::get() {
 					Some(c.who)
 				} else {
 					let outcome = Self::try_remove_candidate(&c.who);

pallets/collator-selection/src/mock.rs

@@ -188,6 +188,7 @@ parameter_types! {
 	pub const PotId: PalletId = PalletId(*b"PotStake");
 	pub const MaxCandidates: u32 = 20;
 	pub const MaxInvulnerables: u32 = 20;
+	pub const MinCandidates: usize = 1;
 }
 
 impl Config for Test {
@@ -196,6 +197,7 @@ impl Config for Test {
 	type UpdateOrigin = EnsureSignedBy<RootAccount, u64>;
 	type PotId = PotId;
 	type MaxCandidates = MaxCandidates;
+	type MinCandidates = MinCandidates;
 	type MaxInvulnerables = MaxInvulnerables;
 	type KickThreshold = Period;
 	type WeightInfo = ();

pallets/collator-selection/src/tests.rs

@@ -106,6 +106,21 @@ fn cannot_register_candidate_if_too_many() {
 	})
 }
 
+#[test]
+fn cannot_unregister_candidate_if_too_few() {
+	new_test_ext().execute_with(|| {
+		// reset desired candidates:
+		<crate::DesiredCandidates<Test>>::put(1);
+		assert_ok!(CollatorSelection::register_as_candidate(Origin::signed(4)));
+
+		// can not remove too few
+		assert_noop!(
+			CollatorSelection::leave_intent(Origin::signed(4)),
+			Error::<Test>::TooFewCandidates,
+		);
+	})
+}
+
 #[test]
 fn cannot_register_as_candidate_if_invulnerable() {
 	new_test_ext().execute_with(|| {
@@ -184,6 +199,10 @@ fn leave_intent() {
 		assert_ok!(CollatorSelection::register_as_candidate(Origin::signed(3)));
 		assert_eq!(Balances::free_balance(3), 90);
 
+		// register too so can leave above min candidates
+		assert_ok!(CollatorSelection::register_as_candidate(Origin::signed(5)));
+		assert_eq!(Balances::free_balance(5), 90);
+
 		// cannot leave if not candidate.
 		assert_noop!(
 			CollatorSelection::leave_intent(Origin::signed(4)),
@@ -318,6 +337,34 @@ fn kick_mechanism() {
 	});
 }
 
+#[test]
+fn should_not_kick_mechanism_too_few() {
+	new_test_ext().execute_with(|| {
+		// add a new collator
+		assert_ok!(CollatorSelection::register_as_candidate(Origin::signed(3)));
+		assert_ok!(CollatorSelection::register_as_candidate(Origin::signed(5)));
+		initialize_to_block(10);
+		assert_eq!(CollatorSelection::candidates().len(), 2);
+		initialize_to_block(20);
+		assert_eq!(SessionChangeBlock::get(), 20);
+		// 4 authored this block, 5 gets to stay too few 3 was kicked
+		assert_eq!(CollatorSelection::candidates().len(), 1);
+		// 3 will be kicked after 1 session delay
+		assert_eq!(SessionHandlerCollators::get(), vec![1, 2, 3, 5]);
+		let collator = CandidateInfo {
+			who: 5,
+			deposit: 10,
+		};
+		assert_eq!(CollatorSelection::candidates(), vec![collator]);
+		assert_eq!(CollatorSelection::last_authored_block(4), 20);
+		initialize_to_block(30);
+		// 3 gets kicked after 1 session delay
+		assert_eq!(SessionHandlerCollators::get(), vec![1, 2, 5]);
+		// kicked collator gets funds back
+		assert_eq!(Balances::free_balance(3), 100);
+	});
+}
+
 
 #[test]
 #[should_panic = "duplicate invulnerables in genesis."]

polkadot-parachains/statemine-runtime/src/lib.rs

@@ -638,6 +638,7 @@ impl pallet_aura::Config for Runtime {
 parameter_types! {
 	pub const PotId: PalletId = PalletId(*b"PotStake");
 	pub const MaxCandidates: u32 = 1000;
+	pub const MinCandidates: u32 = 5;
 	pub const SessionLength: BlockNumber = 6 * HOURS;
 	pub const MaxInvulnerables: u32 = 100;
 }
@@ -655,6 +656,7 @@ impl pallet_collator_selection::Config for Runtime {
 	type UpdateOrigin = CollatorSelectionUpdateOrigin;
 	type PotId = PotId;
 	type MaxCandidates = MaxCandidates;
+	type MinCandidates = MinCandidates;
 	type MaxInvulnerables = MaxInvulnerables;
 	// should be a multiple of session or things will get inconsistent
 	type KickThreshold = Period;

polkadot-parachains/statemint-common/src/impls.rs

@@ -149,6 +149,7 @@ mod tests {
 		pub const PotId: PalletId = PalletId(*b"PotStake");
 		pub const MaxCandidates: u32 = 20;
 		pub const MaxInvulnerables: u32 = 20;
+		pub const MinInvulnerables: u32 = 1;
 	}
 
 	impl pallet_collator_selection::Config for Test {
@@ -157,6 +158,7 @@ mod tests {
 		type UpdateOrigin = EnsureRoot<AccountId>;
 		type PotId = PotId;
 		type MaxCandidates = MaxCandidates;
+		type MinCandidates = MinCandidates;
 		type MaxInvulnerables = MaxInvulnerables;
 		type KickThreshold = ();
 		type WeightInfo = ();

polkadot-parachains/statemint-runtime/src/lib.rs

@@ -579,6 +579,7 @@ impl pallet_aura::Config for Runtime {
 parameter_types! {
 	pub const PotId: PalletId = PalletId(*b"PotStake");
 	pub const MaxCandidates: u32 = 1000;
+	pub const MinCandidates: u32 = 5;
 	pub const SessionLength: BlockNumber = 6 * HOURS;
 	pub const MaxInvulnerables: u32 = 100;
 }
@@ -596,6 +597,7 @@ impl pallet_collator_selection::Config for Runtime {
 	type UpdateOrigin = CollatorSelectionUpdateOrigin;
 	type PotId = PotId;
 	type MaxCandidates = MaxCandidates;
+	type MinCandidates = MinCandidates;
 	type MaxInvulnerables = MaxInvulnerables;
 	// should be a multiple of session or things will get inconsistent
 	type KickThreshold = Period;

polkadot-parachains/westmint-runtime/src/lib.rs

@@ -576,6 +576,7 @@ impl pallet_aura::Config for Runtime {
 parameter_types! {
 	pub const PotId: PalletId = PalletId(*b"PotStake");
 	pub const MaxCandidates: u32 = 1000;
+	pub const MinCandidates: u32 = 1;
 	pub const SessionLength: BlockNumber = 6 * HOURS;
 	pub const MaxInvulnerables: u32 = 100;
 }
@@ -586,6 +587,7 @@ impl pallet_collator_selection::Config for Runtime {
 	type UpdateOrigin = EnsureRoot<AccountId>;
 	type PotId = PotId;
 	type MaxCandidates = MaxCandidates;
+	type MinCandidates = MinCandidates;
 	type MaxInvulnerables = MaxInvulnerables;
 	// should be a multiple of session or things will get inconsistent
 	type KickThreshold = Period;