feat: kick collators without sufficient escrow balance

crates/collator-selection/src/lib.rs

@@ -428,17 +428,24 @@ pub mod pallet {
         }
 
         /// Kicks out candidates that did not produce a block in the kick threshold
-        /// and refund their deposits.
+        /// or whose free balance drops below the minimum and refund their deposits.
         pub fn kick_stale_candidates(candidates: Vec<CandidateInfo<T::AccountId, BalanceOf<T>>>) -> Vec<T::AccountId> {
-            // TODO: also kick candidates when escrow balance drops
             let now = frame_system::Pallet::<T>::block_number();
             let kick_threshold = T::KickThreshold::get();
+            let candidacy_bond = Self::candidacy_bond();
             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 || Self::candidates().len() as u32 <= T::MinCandidates::get() {
+                    // total balance excludes reserved
+                    let balance = T::StakingCurrency::total_balance(&c.who);
+
+                    // don't kick if not enough candidates
+                    let surplus_candidates = Self::candidates().len() as u32 > T::MinCandidates::get();
+                    // kick if stale OR balance is not sufficient
+                    let is_good_collator = since_last < kick_threshold && balance >= candidacy_bond;
+                    if is_good_collator || !surplus_candidates {
                         Some(c.who)
                     } else {
                         let outcome = Self::try_remove_candidate(&c.who);

crates/collator-selection/src/tests.rs

@@ -13,8 +13,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use crate as collator_selection;
-use crate::{mock::*, CandidateInfo, Error};
+use crate::{self as collator_selection, mock::*, CandidateInfo, Error};
 use frame_support::{
     assert_noop, assert_ok,
     traits::{Currency, GenesisBuild, OnInitialize},
@@ -377,6 +376,47 @@ fn should_not_kick_mechanism_too_few() {
     });
 }
 
+#[test]
+fn should_kick_mechanism_below_balance() {
+    new_test_ext().execute_with(|| {
+        // add two candidates
+        assert_ok!(CollatorSelection::register_as_candidate(Origin::signed(3)));
+        assert_ok!(CollatorSelection::register_as_candidate(Origin::signed(5)));
+
+        // first session change
+        initialize_to_block(Period::get());
+        assert_eq!(SessionChangeBlock::get(), Period::get());
+
+        assert_eq!(CollatorSelection::candidates().len(), 2);
+        <crate::LastAuthoredBlock<Test>>::insert(3, 17);
+        <crate::LastAuthoredBlock<Test>>::insert(5, 19);
+
+        // need to decrease reference counter (from candidate bond)
+        frame_system::Pallet::<Test>::dec_consumers(&3);
+        // workaround to emulate escrow total balance decreasing
+        assert_ok!(Balances::set_balance(Origin::root(), 3, 0, 0));
+
+        // second session change
+        initialize_to_block(Period::get() * 2);
+        assert_eq!(SessionChangeBlock::get(), Period::get() * 2);
+
+        // 3 & 5 authored recently but 3 balance too low
+        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]);
+
+        // third session change
+        initialize_to_block(Period::get() * 3);
+        assert_eq!(SessionChangeBlock::get(), Period::get() * 3);
+
+        // 3 gets kicked after 1 session delay
+        assert_eq!(SessionHandlerCollators::get(), vec![1, 2, 5]);
+    });
+}
+
 #[test]
 #[should_panic = "duplicate invulnerables in genesis."]
 fn cannot_set_genesis_value_twice() {