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Forking mainnet results in erroneous revert reason

See original GitHub issue

This issue has been noticed by the Compound team before when forking the mainnet, and can be reproduced using the tests in this commit.

Steps to Reproduce

Clone the above repository at the specified commit
Run npm install
Compile contracts and run tests with npx oz compile && npx mocha --exit --timeout 0 test/endaoment-test.js
The tests will fail with the following console output

$ npx oz compile && npx mocha --exit --timeout 0 test/endaoment-test.js
Nothing to compile, all contracts are up to date.


  Endaoment
    ✓ should see the deployed Endaoment contract
    1) should allow a membership proposal & vote


  1 passing (39s)
  1 failing

  1) Endaoment
       should allow a membership proposal & vote:
     Error: Returned error: VM Exception while processing transaction: revert Endaoment::processProposal - token transfer to guild bank failed -- Reason given: re-entered.
     at PromiEvent (node_modules/@truffle/contract/lib/promievent.js:9:30)
      at TruffleContract.processProposal (node_modules/@truffle/contract/lib/execute.js:169:26)
      at Context.it (test/endaoment-test.js:61:29)
      at process._tickCallback (internal/process/next_tick.js:68:7)

Explanation

In the file endaoment-test.js we are failing in the test called “should allow a membership proposal & vote” at the line await this.instance.processProposal(0, {from: summoner});

This line calls the function processProposal() in Endaoment.sol. The failure occurs in the following section of that function:

// transfer tokens to guild bank
require(
    guildBank.deposit(proposal.tokenTribute),
    "Endaoment::processProposal - token transfer to guild bank failed"
);

This line is calling the deposit() function in GuildBank.sol. This function is shown below for convenience:

function deposit(uint256 amount) public onlyOwner returns (bool) {
    bool transferSuccess = approvedToken.transferFrom(msg.sender, address(this), amount);
    if (!transferSuccess) {
        return false;
    }
    
    uint256 mintCode = cToken.mint(amount);
    if (0 == mintCode) {
        return false;
    }
    emit Deposit(amount);
    return true;
}

Now, the statement if (0 == mintCode) is incorrect, and really should be if (0 != mintCode). However, this mistake in the code means the function should return false and therefore the contract should error solely with the error message “Endaoment::processProposal - token transfer to guild bank failed”.

Instead, you can see the cToken error message of “re-entered” is included. This is from the nonReentrant modifier of cTokens, but because there is no reentrancy here this should not have been triggered.