Make helper return a valid CoW Swap signature

src/ConstantProductHelper.sol

@@ -66,6 +66,7 @@ contract ConstantProductHelper is ICOWAMMPoolHelper, LegacyHelper {
             revert InvalidArrayLength();
         }
 
+        bytes memory eip1271sig;
         if (!isLegacy(pool)) {
             // Standalone CoW AMMs (**non-Gnosis Safe Wallets**)
             if (!isCanonical(pool)) {
@@ -93,7 +94,7 @@ contract ConstantProductHelper is ICOWAMMPoolHelper, LegacyHelper {
                 })
             );
 
-            sig = abi.encode(_order);
+            eip1271sig = abi.encode(_order);
 
             preInteractions = new GPv2Interaction.Data[](1);
             preInteractions[0] = GPv2Interaction.Data({
@@ -102,8 +103,12 @@ contract ConstantProductHelper is ICOWAMMPoolHelper, LegacyHelper {
                 callData: abi.encodeCall(ConstantProduct.commit, (_order.hash(SETTLEMENT.domainSeparator())))
             });
         } else {
-            (_order, preInteractions, postInteractions, sig) = legacyOrder(pool, prices);
+            (_order, preInteractions, postInteractions, eip1271sig) = legacyOrder(pool, prices);
         }
+
+        // A ERC-1271 signature on CoW Protocol is composed of two parts: the
+        // signer address and the valid ERC-1271 signature data for that signer.
+        sig = abi.encodePacked(pool, eip1271sig);
     }
 
     /// @dev Take advantage of the mapping on the factory that is set to the owner's address for canonical pools.

src/interfaces/ICOWAMMPoolHelper.sol

@@ -65,7 +65,8 @@ interface ICOWAMMPoolHelper {
      * @return order The CoW Protocol JIT order
      * @return preInteractions The array array for any **PRE** interactions (empty if none)
      * @return postInteractions The array array for any **POST** interactions (empty if none)
-     * @return sig The ERC-1271 signature for the order
+     * @return sig A valid CoW-Protocol signature for the resulting order using
+     * the ERC-1271 signature scheme.
      */
     function order(address pool, uint256[] calldata prices)
         external

test/fork/ConstantProductHelper.t.sol

@@ -87,12 +87,6 @@ contract ConstantProductHelperForkedTest is ForkedTest {
         GPv2Interaction.Data[] memory postInteractions;
         bytes memory sig;
         (ammOrder, preInteractions, postInteractions, sig) = helper.order(wethUsdcAmm, prices);
-        // The signature is valid for the contract but not for the
-        // settlement contract. We need to prepend the verifying contract
-        // address.
-        // TODO: update the helper so that it already includes the address in
-        // the signature.
-        sig = abi.encodePacked(wethUsdcAmm, sig);
         trades[0] = orderToFullTrade(ammOrder, tokens, GPv2Signing.Scheme.Eip1271, sig);
 
         // We expect a commit interaction in both pre and post interactions