Add IntermediaryMedianizer for Ropsten

So that new system deployments don't require new contract authorizations
on the Maker side, an intermediate medianizer has already been deployed
to Ropsten for Maker to authorize. It is designed to have an updatable
reference to the Maker medianizer, and pass through reads to that
instance.

The already-deployed address is added to the externals file, while the
code is checked in to the price-feed directory, but not migrated as part
of the system migrations.

solidity/contracts/external/IMedianizer.sol

@@ -13,4 +13,4 @@ interface IMedianizer {
     /// @return Designated price with 18 decimal places.
     /// @return true if price is > 0, else returns false
     function peek() external view returns (uint256, bool);
-}
+}

solidity/contracts/price-feed/IntermediaryMedianizer.sol

@@ -0,0 +1,36 @@
+pragma solidity 0.5.17;
+
+import "openzeppelin-solidity/contracts/ownership/Ownable.sol";
+import "../external/IMedianizer.sol";
+
+/// @title IntermediaryMedianizer is an updatable intermediary between a real
+///        medianizer and IMedianizer users.
+/// @dev This is used in Keep testnets where Maker has deployed a Medianizer
+///      instance that needs to authorize a single consumer, to enable multiple
+///      tBTC deployments to happen in the background and be pointed at a stable
+///      medianizer instance that is authorized on the Maker contract. It allows
+///      the updating of the backing medianizer and therefore is NOT suitable
+///      for mainnet deployment.
+contract IntermediaryMedianizer is Ownable, IMedianizer {
+    IMedianizer private _realMedianizer;
+
+    constructor(IMedianizer realMedianizer) public {
+        _realMedianizer = realMedianizer;
+    }
+
+    function getMedianizer() external view returns (IMedianizer) {
+        return _realMedianizer;
+    }
+
+    function peek() external view returns (uint256, bool) {
+        return _realMedianizer.peek();
+    }
+
+    function read() external view returns (uint256) {
+        return _realMedianizer.read();
+    }
+
+    function setMedianizer(IMedianizer realMedianizer) public onlyOwner {
+        _realMedianizer = realMedianizer;
+    }
+}

solidity/migrations/3_initialize.js

@@ -10,7 +10,11 @@ const TBTCDepositToken = artifacts.require("TBTCDepositToken")
 const FeeRebateToken = artifacts.require("FeeRebateToken")
 const VendingMachine = artifacts.require("VendingMachine")
 
-const {BondedECDSAKeepVendorAddress, ETHBTCMedianizer} = require("./externals")
+const {
+  BondedECDSAKeepVendorAddress,
+  ETHBTCMedianizer,
+  RopstenETHBTCPriceFeed,
+} = require("./externals")
 
 module.exports = async function(deployer, network) {
   // Don't enact this setup during unit testing.
@@ -53,6 +57,9 @@ module.exports = async function(deployer, network) {
   if (network === "mainnet") {
     // Inject mainnet price feeds.
     await satWeiPriceFeed.initialize(tbtcSystem.address, ETHBTCMedianizer)
+  } else if (network === "ropsten") {
+    // Inject medianizer intermediary.
+    await satWeiPriceFeed.initialize(tbtcSystem.address, RopstenETHBTCPriceFeed)
   } else {
     // Inject mock price feeds.
     const mockSatWeiPriceFeed = await MockSatWeiPriceFeed.deployed()

solidity/migrations/externals.js

@@ -7,7 +7,10 @@ const BondedECDSAKeepVendorAddress = "0xZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
 // See: https://github.com/makerdao/oracles-v2#live-mainnet-oracles
 const ETHBTCMedianizer = "0xABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDE"
 
+const RopstenETHBTCPriceFeed = "0xb60844D3C84C4Db77d240bC38241C93Df5f4fbF7"
+
 module.exports = {
   BondedECDSAKeepVendorAddress,
   ETHBTCMedianizer,
+  RopstenETHBTCPriceFeed,
 }