Funds reserve when low (ethereum#850)

* Adds isReserveLow contract helper

* Checks if the reserve needs to be funded

Co-authored-by: jfoutts-celo <57463555+jfoutts-celo@users.noreply.github.com>

consensus/istanbul/backend/pos.go

@@ -170,8 +170,11 @@ func (sb *Backend) distributeCommunityRewards(header *types.Header, state *state
 	if err != nil {
 		return totalCommunityRewards, err
 	}
-	// TODO: replace 'false' with 'isReserveLow'
-	if false && reserveAddress != nil {
+	lowReserve, err := epoch_rewards.IsReserveLow(header, state)
+	if err != nil {
+		return totalCommunityRewards, err
+	}
+	if lowReserve && reserveAddress != nil {
 		state.AddBalance(*reserveAddress, communityReward)
 		totalCommunityRewards.Add(totalCommunityRewards, communityReward)
 	} else if governanceAddress != nil {

contract_comm/epoch_rewards/epoch_rewards.go

@@ -59,6 +59,20 @@ const epochRewardsABIString string = `[
       "payable": false,
       "stateMutability": "nonpayable",
       "type": "function"
+    },
+    {
+      "constant": true,
+      "inputs": [],
+      "name": "isReserveLow",
+      "outputs": [
+        {
+          "name": "",
+          "type": "bool"
+        }
+      ],
+      "payable": false,
+      "stateMutability": "view",
+      "type": "function"
     }
 ]
 `
@@ -82,3 +96,13 @@ func CalculateTargetEpochRewards(header *types.Header, state vm.StateDB) (*big.I
 	}
 	return validatorEpochReward, totalVoterRewards, totalCommunityReward, nil
 }
+
+// Determines if the reserve is below it's critical threshold
+func IsReserveLow(header *types.Header, state vm.StateDB) (bool, error) {
+	var isLow bool
+	_, err := contract_comm.MakeStaticCall(params.EpochRewardsRegistryId, epochRewardsABI, "isReserveLow", []interface{}{}, &isLow, params.MaxGasForIsReserveLow, header, state)
+	if err != nil {
+		return false, err
+	}
+	return isLow, nil
+}

params/protocol_params.go

@@ -225,4 +225,5 @@ const (
 	MaxGasForUpdateValidatorScore                  uint64 = 1 * 1000000
 	MaxGasForTotalSupply                           uint64 = 50 * 1000
 	MaxGasToReadErc20Balance                       uint64 = 100000
+	MaxGasForIsReserveLow                          uint64 = 1000000
 )