types: prevent temporary power overflows on validator updates (#4165)

Closes #4164

types/validator_set.go

@@ -399,13 +399,14 @@ func verifyUpdates(
 			// Updated validator, add the difference in power to the total.
 			updatedTotalVotingPower += valUpdate.VotingPower - val.VotingPower
 		}
-		overflow := updatedTotalVotingPower > MaxTotalVotingPower
-		if overflow {
-			err = fmt.Errorf(
-				"failed to add/update validator %v, total voting power would exceed the max allowed %v",
-				valUpdate, MaxTotalVotingPower)
-			return 0, 0, err
-		}
+	}
+
+	overflow := updatedTotalVotingPower > MaxTotalVotingPower
+	if overflow {
+		err = fmt.Errorf(
+			"failed to add/update validator, total voting power would exceed the max allowed %v",
+			MaxTotalVotingPower)
+		return 0, 0, err
 	}
 
 	return updatedTotalVotingPower, numNewValidators, nil

types/validator_set_test.go

@@ -354,6 +354,21 @@ func TestValidatorSetTotalVotingPowerPanicsOnOverflow(t *testing.T) {
 	assert.Panics(t, shouldPanic)
 }
 
+func TestValidatorSetShouldNotErrorOnTemporalOverflow(t *testing.T) {
+	// Updating the validator set might trigger an Overflow error during the update process
+	valSet := NewValidatorSet([]*Validator{
+		{Address: []byte("b"), VotingPower: MaxTotalVotingPower - 1, ProposerPriority: 0},
+		{Address: []byte("a"), VotingPower: 1, ProposerPriority: 0},
+	})
+
+	err := valSet.UpdateWithChangeSet([]*Validator{
+		{Address: []byte("b"), VotingPower: 1, ProposerPriority: 0},
+		{Address: []byte("a"), VotingPower: MaxTotalVotingPower - 1, ProposerPriority: 0},
+	})
+
+	assert.NoError(t, err)
+}
+
 func TestAvgProposerPriority(t *testing.T) {
 	// Create Validator set without calling IncrementProposerPriority:
 	tcs := []struct {