Add extra input validations to the AllocationExchange

Author: abarmat

contracts/statechannels/AllocationExchange.sol

@@ -4,6 +4,7 @@ pragma solidity ^0.7.3;
 pragma experimental ABIEncoderV2;
 
 import "@openzeppelin/contracts/cryptography/ECDSA.sol";
+import "@openzeppelin/contracts/utils/Address.sol";
 
 import "../governance/Governed.sol";
 import "../staking/IStaking.sol";
@@ -54,6 +55,7 @@ contract AllocationExchange is Governed {
         address _governor,
         address _authority
     ) {
+        require(_governor != address(0), "Exchange: governor must be set");
         Governed._initialize(_governor);
 
         graphToken = _graphToken;
@@ -97,6 +99,12 @@ contract AllocationExchange is Governed {
      */
     function _setAuthority(address _authority) private {
         require(_authority != address(0), "Exchange: empty authority");
+        // This will help catch some operational errors but not all.
+        // The validation will fail under the following conditions:
+        // - a contract in construction
+        // - an address where a contract will be created
+        // - an address where a contract lived, but was destroyed
+        require(!Address.isContract(_authority), "Exchange: authority must be EOA");
         authority = _authority;
         emit AuthoritySet(authority);
     }