Implement ZrxVaultBackstop

contracts/staking/contracts/src/ZrxVaultBackstop.sol

@@ -0,0 +1,69 @@
+/*
+
+  Copyright 2019 ZeroEx Intl.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+*/
+
+pragma solidity ^0.5.9;
+pragma experimental ABIEncoderV2;
+
+import "@0x/contracts-utils/contracts/src/LibSafeMath.sol";
+import "./interfaces/IStructs.sol";
+import "./interfaces/IZrxVault.sol";
+import "./interfaces/IStakingProxy.sol";
+
+
+contract ZrxVaultBackstop {
+
+    using LibSafeMath for uint256;
+
+    IStakingProxy public stakingProxy;
+    IZrxVault public zrxVault;
+
+    /// @dev Constructor. Sets stakingProxy and zrxVault.
+    /// @param _stakingProxyAddress Address of stakingProxy.
+    /// @param _zrxVaultAddress Address of zrxVault.
+    constructor(
+        address payable _stakingProxyAddress,
+        address _zrxVaultAddress
+    )
+        public
+    {
+        stakingProxy = IStakingProxy(_stakingProxyAddress);
+        zrxVault = IZrxVault(_zrxVaultAddress);
+    }
+
+    /// @dev Triggers catastophic failure mode in the zrxzVault iff read-only mode
+    ///      has been continuously set for at least 40 days.
+    function enterCatastrophicFailureIfProlongedReadOnlyMode()
+        external
+    {
+        IStructs.ReadOnlyState memory readOnlyState = stakingProxy.readOnlyState();
+
+        // Ensure read-only mode is set
+        require(
+            readOnlyState.isReadOnlyModeSet,
+            "READ_ONLY_MODE_NOT_SET"
+        );
+
+        // Ensure that the stakingProxy has been in read-only mode for a long enough time
+        require(
+            block.timestamp.safeSub(readOnlyState.lastSetTimestamp) >= 40 days,
+            "READ_ONLY_MODE_LENGTH_TOO_SHORT"
+        );
+
+        zrxVault.enterCatastrophicFailure();
+    }
+}