fix(lottery): drawings fail when restaking unstaked funds

pallets/pallet-lottery/src/lib.rs

@@ -330,6 +330,8 @@ pub mod pallet {
         /// FATAL: Assigning/Transferring winning claims
         /// would **remove** user deposited funds from pallet
         PotBalanceTooLow,
+        /// FATAL: Can't stake the requested amount with available funds
+        PotBalanceTooLowToStake,
         /// Pallet balance is lower than the needed gas fee buffer
         PotBalanceBelowGasReserve,
         /// Pallet balance is too low to submit a needed transaction

pallets/pallet-lottery/src/staking/mod.rs

@@ -219,9 +219,17 @@ impl<T: Config> Pallet<T> {
     ) -> DispatchResult {
         log::trace!(function_name!());
         // preconditions
-        if Self::surplus_funds().is_zero() {
-            return Err(Error::<T>::PotBalanceTooLow.into());
+        // funds eligible for staking:
+        // - newly deposited funds > min_deposit
+        // - unstaked-but-not-needed-for-withdrawals funds > min_deposit
+        if amount < Self::min_deposit() {
+            return Err(Error::<T>::DepositBelowMinAmount.into());
         }
+        if amount > Self::surplus_funds() && amount > Self::unlocked_unstaking_funds() {
+            // we can't handle this withdrawal from new deposits or unstaked funds
+            return Err(Error::<T>::PotBalanceTooLowToStake.into());
+        }
+        // collator exists
         let candidate_delegation_count;
         if let Some(info) = pallet_parachain_staking::Pallet::<T>::candidate_info(&collator) {
             candidate_delegation_count = info.delegation_count;