fix!: x/staking - remove delegation with amount is zero

CHANGELOG.md

@@ -143,6 +143,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### State Machine Breaking
 
+* (x/staking) [#10254](https://github.com/cosmos/cosmos-sdk/pull/10254) Instead of using the shares to determine if a delegation should be removed, use the truncated (token) amount.
 * (x/auth)[\#9596](https://github.com/cosmos/cosmos-sdk/pull/9596) Enable creating periodic vesting accounts with a transactions instead of requiring them to be created in genesis.
 * (x/bank) [\#9611](https://github.com/cosmos/cosmos-sdk/pull/9611) Introduce a new index to act as a reverse index between a denomination and address allowing to query for
   token holders of a specific denomination. `DenomOwners` is updated to use the new reverse index.

x/staking/keeper/delegation.go

@@ -680,8 +680,11 @@ func (k Keeper) Unbond(
 		validator = k.mustGetValidator(ctx, validator.GetOperator())
 	}
 
-	// remove the delegation
-	if delegation.Shares.IsZero() {
+	// Remove the delegation if the resulting shares yield a truncated zero amount
+	// of tokens in the delegation. Note, in this this case, the shares themselves
+	// may not be zero, but rather a small fractional amount. Otherwise, we update
+	// the delegation object.
+	if validator.TokensFromShares(delegation.Shares).TruncateInt().IsZero() {
 		err = k.RemoveDelegation(ctx, delegation)
 	} else {
 		k.SetDelegation(ctx, delegation)

x/staking/keeper/invariants.go

@@ -137,13 +137,11 @@ func PositiveDelegationInvariant(k Keeper) sdk.Invariant {
 		for _, delegation := range delegations {
 			if delegation.Shares.IsNegative() {
 				count++
-
 				msg += fmt.Sprintf("\tdelegation with negative shares: %+v\n", delegation)
 			}
 
 			if delegation.Shares.IsZero() {
 				count++
-
 				msg += fmt.Sprintf("\tdelegation with zero shares: %+v\n", delegation)
 			}
 		}