@0x/contracts-exchange: Refactor out EIP1271_MAGIC_VALUE into a LibEIP1271 contract and inherit from that.

`@0x/contracts-exchange`: Use `LibBytes.readBytes4()` to validate EIP1271 return values and be more permissive of shorter return values.
`@0x/contracts-exchange`: Use `abi.decode()` for `EIP1271OrderWallet` in `TestSignatureValidator.sol`.
`@0x/contracts-exchange`: Correct minor formatting changes in contracts.

contracts/exchange/contracts/src/MixinSignatureValidator.sol

@@ -20,6 +20,7 @@ pragma solidity ^0.5.9;
 pragma experimental ABIEncoderV2;
 
 import "@0x/contracts-utils/contracts/src/LibBytes.sol";
+import "@0x/contracts-utils/contracts/src/LibEIP1271.sol";
 import "@0x/contracts-utils/contracts/src/ReentrancyGuard.sol";
 import "@0x/contracts-utils/contracts/src/RichErrors.sol";
 import "@0x/contracts-exchange-libs/contracts/src/LibOrder.sol";
@@ -36,14 +37,12 @@ contract MixinSignatureValidator is
     MixinExchangeRichErrors,
     ReentrancyGuard,
     LibOrder,
+    LibEIP1271,
     ISignatureValidator,
     MixinTransactions
 {
     using LibBytes for bytes;
 
-    // Magic bytes returned by EIP1271 wallets on success.
-    bytes4 constant public EIP1271_MAGIC_VALUE = 0x20c13b0b;
-
     // Mapping of hash => signer => signed
     mapping (bytes32 => mapping (address => bool)) public preSigned;
 
@@ -385,8 +384,8 @@ contract MixinSignatureValidator is
         // Static call the verification function.
         (bool didSucceed, bytes memory returnData) = walletAddress.staticcall(callData);
         // Return data should be the `EIP1271_MAGIC_VALUE`.
-        if (didSucceed && returnData.length == 32) {
+        if (didSucceed && returnData.length <= 32) {
-            return bytes4(returnData.readBytes32(0)) == EIP1271_MAGIC_VALUE;
+            return returnData.readBytes4(0) == EIP1271_MAGIC_VALUE;
         }
         // Static call to verifier failed.
         _rrevert(SignatureWalletError(
@@ -544,8 +543,8 @@ contract MixinSignatureValidator is
         // Static call the verification function.
         (bool didSucceed, bytes memory returnData) = walletAddress.staticcall(callData);
         // Return data should be the `EIP1271_MAGIC_VALUE`.
-        if (didSucceed && returnData.length == 32) {
+        if (didSucceed && returnData.length <= 32) {
-            return bytes4(returnData.readBytes32(0)) == EIP1271_MAGIC_VALUE;
+            return returnData.readBytes4(0) == EIP1271_MAGIC_VALUE;
         }
         // Static call to verifier failed.
         _rrevert(SignatureOrderWalletError(

contracts/exchange/contracts/src/interfaces/IEIP1271Wallet.sol

@@ -1,6 +1,6 @@
 /*
 
-  Copyright 2018 ZeroEx Intl.
+  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.
@@ -18,12 +18,11 @@
 
 pragma solidity ^0.5.9;
 
+import "@0x/contracts-utils/contracts/src/LibEIP1271.sol";
 
-contract IEIP1271Wallet {
+contract IEIP1271Wallet is
-
+    LibEIP1271
-    // Magic bytes returned by EIP1271 wallets on success.
+{
-    bytes4 constant public EIP1271_MAGIC_VALUE = 0x20c13b0b;
-
     /// @dev Verifies that a signature is valid.
     /// @param data Arbitrary data.
     /// @param signature Signature of `data`.

contracts/exchange/contracts/test/TestValidatorWallet.sol

@@ -22,6 +22,7 @@ pragma experimental ABIEncoderV2;
 import "@0x/contracts-exchange-libs/contracts/src/LibOrder.sol";
 import "@0x/contracts-erc20/contracts/src/interfaces/IERC20Token.sol";
 import "@0x/contracts-utils/contracts/src/LibBytes.sol";
+import "@0x/contracts-utils/contracts/src/LibEIP1271.sol";
 
 
 interface ISimplifiedExchange {
@@ -33,13 +34,13 @@ interface ISimplifiedExchange {
 
 
 // solhint-disable no-unused-vars
-contract TestValidatorWallet {
+contract TestValidatorWallet is
+    LibEIP1271
+{
     using LibBytes for bytes;
 
     // Revert reason for `Revert` `ValidatorAction`.
     string constant public REVERT_REASON = "you shall not pass";
-    // Magic bytes returned by EIP1271 wallets on success.
-    bytes4 constant public EIP1271_MAGIC_VALUE = 0x20c13b0b;
 
     enum ValidatorAction {
         // Return false (default)
@@ -281,16 +282,7 @@ contract TestValidatorWallet {
         returns (LibOrder.Order memory order)
     {
         require(data.length > 32, "INVALID_EIP1271_ORDER_DATA_LENGTH");
-        assembly {
+        return abi.decode(data, (LibOrder.Order));
-            // Skip past the length to find the first parameter.
-            let argsStart := add(data, 32)
-            order := add(argsStart, mload(argsStart))
-            // Destructively point the asset data fields to absolute locations.
-            for {let o := 0x140} lt(o, 0x1C0) {o := add(o, 0x20)} {
-                let arg := add(order, o)
-                mstore(arg, add(argsStart, add(mload(arg), 0x20)))
-            }
-        }
     }
 
 }