fix: throw nested revert messages

solidity/contracts/scripts/FundingScript.sol

@@ -50,11 +50,20 @@ contract FundingScript {
         // Call the VendingMachine.
         // We could explictly encode the call to vending machine, but this would
         // involve manually parsing _extraData and allocating variables.
-        (bool success, bytes memory returnData) = address(vendingMachine).call(
-            // solium-disable-previous-line security/no-low-level-calls
-            _extraData
-        );
-        require(success, string(returnData));
+        // We capture the `returnData` in order to forward any nested revert message
+        // from the contract call.
+        (bool success, bytes memory returnData) = address(vendingMachine).call(_extraData);
+
+        string memory revertMessage;
+        assembly {
+            // A revert message is ABI-encoded as a call to Error(string).
+            // Slicing the Error() signature (4 bytes) and Data offset (4 bytes)
+            // leaves us with a pre-encoded string.
+            // We also slice off the ABI-coded length of returnData (32).
+            revertMessage := add(returnData, 0x44)
+        }
+
+        require(success, revertMessage);
 
         // Transfer the TBTC and feeRebateToken to the user.
         tbtcToken.transfer(_from, tbtcToken.balanceOf(address(this)));

solidity/contracts/scripts/RedemptionScript.sol

@@ -48,11 +48,20 @@ contract RedemptionScript {
             "Bad _extraData signature. Call must be to tbtcToBtc."
         );
 
+        // We capture the `returnData` in order to forward any nested revert message
+        // from the contract call.
+        // solium-disable-next-line security/no-low-level-calls
         (bool success, bytes memory returnData) = address(vendingMachine).call(_extraData);
-        // solium-disable-previous-line security/no-low-level-calls
-        // By default, `address.call`  will catch any revert messages.
-        // Converting the `returnData` to a string will effectively forward any revert messages.
-        // https://ethereum.stackexchange.com/questions/69133/forward-revert-message-from-low-level-solidity-call
-        require(success, string(returnData));
+
+        string memory revertMessage;
+        assembly {
+            // A revert message is ABI-encoded as a call to Error(string).
+            // Slicing the Error() signature (4 bytes) and Data offset (4 bytes)
+            // leaves us with a pre-encoded string.
+            // We also slice off the ABI-coded length of returnData (32).
+            revertMessage := add(returnData, 0x44)
+        }
+
+        require(success, revertMessage);
     }
 }

solidity/test/VendingMachineTest.js

@@ -465,6 +465,33 @@ describe("VendingMachine", async function() {
         expect(success).to.equal(true)
       })
 
+      it("forwards nested revert error messages", async () => {
+        await testDeposit.setState(states.ACTIVE)
+        await tbtcDepositToken.forceMint(vendingMachine.address, tdtId)
+        await tbtcToken.forceMint(owner, depositValue.add(signerFee))
+        await feeRebateToken.forceMint(owner, tdtId)
+        const tbtcToBtc = vendingMachine.abi.filter(
+          x => x.name == "tbtcToBtc",
+        )[0]
+        const nonexistentDeposit = "0000000000000000000000000000000000000000"
+        const calldata = web3.eth.abi.encodeFunctionCall(tbtcToBtc, [
+          nonexistentDeposit,
+          "0x1111111100000000",
+          redeemerOutputScript,
+          owner,
+        ])
+
+        await expectRevert(
+          tbtcToken.approveAndCall(
+            redemptionScript.address,
+            depositValue.add(signerFee),
+            calldata,
+            {from: owner},
+          ),
+          "tBTC Deposit Token does not exist",
+        )
+      })
+
       it("reverts for unknown function calls encoded in _extraData", async () => {
         const unknownFunctionSignature = "0xCAFEBABE"
         await tbtcToken.forceMint(owner, depositValue.add(signerFee))
@@ -565,6 +592,39 @@ describe("VendingMachine", async function() {
       expect(success).to.be.true
     })
 
+    it("forwards nested revert error messages", async () => {
+      const unqualifiedDepositToTbtcABI = vendingMachine.abi.filter(
+        x => x.name == "unqualifiedDepositToTbtc",
+      )[0]
+      const calldata = web3.eth.abi.encodeFunctionCall(
+        unqualifiedDepositToTbtcABI,
+        [
+          testDeposit.address,
+          _version,
+          _txInputVector,
+          _txOutputVector,
+          _txLocktime,
+          _fundingOutputIndex,
+          _merkleProof,
+          _txIndexInBlock,
+          _bitcoinHeaders,
+        ],
+      )
+
+      // To make the funding call fail.
+      await testDeposit.setState(states.ACTIVE)
+
+      await expectRevert(
+        tbtcDepositToken.approveAndCall.call(
+          fundingScript.address,
+          tdtId,
+          calldata,
+          {from: owner},
+        ),
+        "Not awaiting funding",
+      )
+    })
+
     it("reverts for unknown function calls encoded in _extraData", async () => {
       const unknownFunctionSignature = "0xCAFEBABE"