Bank example truffle tests

Repo contains truffle tests for contract from Simple Bank Example

Let's add method to check bank's balances to be able write assertments in tests:

function getBalance(address aAddress) public constant returns(uint256) {
  return balances[aAddress];
}

Initial contract code is available in contracts/Bank_0.sol

Execute truffle test to see failing tests.

Problem 0. Non-payable saveAccount method (not found by sbuilder-eth)

Before fixing logical errors in the contract we have to fix saveAccount() method. Method saveAccount() is not payable so msg.value doesn't exist in the context.

• Warning throwed by Remix IDE

browser/Bank.sol:10:53: Warning: "msg.value" used in non-payable function. Do you want to add the "payable" modifier to this function?
      balances[msg.sender] = balances[msg.sender] + msg.value;
                                                    ^-------^

• Validated by test test/Bank_0_saveAccount_not_payable.js

Solution: add payable modifier to the method

Problem 1. Non-payable consturctor (not found by sbuilder-eth)

Accordingly to section 3.4. Run 2 for setup3 – Find 1st Bug sbuilder-eth indicates that it is possible to call constructor and transfer some amount of ether. Unfortunately it is impossible as constructor is not payable.

• Remix IDE gives the same warning:

browser/Bank.sol:8:30: Warning: "msg.value" used in non-payable function. Do you want to add the "payable" modifier to this function?
      balances[msg.sender] = msg.value;
                             ^-------^

• Validated by migrations/1516447849_deploy__bank_1.js. Uncomment line 4 and run truffle test and as a result migration will fail with error: Cannot send value to non-payable constructor

Solution: add payable modifier to the constructor

Problem 2. this.balance != Sum(this.balances) (found by sbuilder-eth)

Important: If problem 1. is not solved this problem doesn't exist!

In the same section 3.4. Run 2 for setup3 – Find 1st Bug stated:

Error analysis: constructor does not update balances variable, and invariant Bank_promise is violated in Bank() constructor.

There is internal variable for every contract called balance. It is updated every time somebody sends funds to a contract. So actual balance of the contract will be updated but this transaction will not be represented in the balances map

Validated by migration migrations/1516449292_deploy__bank_2.js which sends funds to contract during deployment and test test/Bank_2_constructor_records_transaction.js

Solution: add balances[msg.sender] = msg.value; to the constructor.

Problem 3. Imposibility to fake origin address of transaction (not found by sbuilder-eth)

Section 4.2 Run 3 for setup3 – Find 2nd Bug states that it is possible to call saveAccount() with bank address. It is impossible as to use bank (contract) address we have to unlock bank (contract) account but we have no key and password to do so. Only contract can call saveAccount() with it's own address.

Validated by test/Bank_3_saveAccount_recursive_funding_0.js

Solution: add method recursiveTransfer(uint amount) to the contract which will transfer funds. Important notice: Remix IDE gives two warnings about the method:

• Gas requirement of function Bank_4.recursiveTransfer() high: infinite. If the gas requirement of a function is higher than the block gas limit, it cannot be executed. Please avoid loops in your functions or actions that modify large areas of storage (this includes clearing or copying arrays in storage)
• browser/Bank_4.sol:18:5:Use of "this" for local functions: Never use this to call functions in the same contract, it only consumes more gas than normal local calls.

Problem 4. Possibility to send funds "recursively" (found by sbuilder-eth)

Important: If problem 3. is not solved this problem doesn't exist.

Validated by test/Bank_4_saveAccount_recursive_funding_1.js

Summary

Final version of contract availabe in contracts/Bank_Final.sol and tests are in test/Bank_Final_saveAccount_no_recursive_funding.js.

Simplified but still valid version presented in contracts/Bank_Final_Simple.sol. The only difference between simplified version and initial contract is payable modifier for saveAccount() method.