add candidacy un-bonding period

pallets/collator-selection/src/lib.rs

@@ -265,11 +265,17 @@ pub mod pallet {
     #[pallet::event]
     #[pallet::generate_deposit(pub(super) fn deposit_event)]
     pub enum Event<T: Config> {
+        /// New invulnerables candidates were set.
         NewInvulnerables(Vec<T::AccountId>),
+        /// The number of desired candidates was set.
         NewDesiredCandidates(u32),
+        /// The candidacy bond was set.
         NewCandidacyBond(BalanceOf<T>),
+        /// A new candidate joined.
         CandidateAdded(T::AccountId, BalanceOf<T>),
+        /// A candidate was removed.
         CandidateRemoved(T::AccountId),
+        /// A candidate was slashed.
         CandidateSlashed(T::AccountId),
     }
 
@@ -557,7 +563,7 @@ pub mod pallet {
                     Self::slash_non_candidate(&who);
                 }
             }
-            (count, count - Self::candidates().len() as u32)
+            (count, count.saturating_sub(Self::candidates().len() as u32))
         }
 
         /// Check whether an account is a candidate.

pallets/collator-selection/src/tests.rs

@@ -16,7 +16,7 @@
 // limitations under the License.
 
 use crate as collator_selection;
-use crate::{mock::*, CandidateInfo, Error, NonCandidates};
+use crate::{mock::*, CandidateInfo, Error, LastAuthoredBlock, NonCandidates};
 use frame_support::{
     assert_noop, assert_ok,
     traits::{Currency, OnInitialize},
@@ -520,7 +520,7 @@ fn slash_mechanism_for_unbonding_candidates() {
         initialize_to_block(20);
         assert_eq!(CollatorSelection::candidates().len(), 1);
         assert_eq!(SessionChangeBlock::get(), 20);
-        assert_eq!(CollatorSelection::last_authored_block(3), 0);
+        assert_eq!(LastAuthoredBlock::<Test>::contains_key(3), false);
         assert_eq!(CollatorSelection::last_authored_block(4), 20);
 
         // slashed, remaining bond was refunded