Fix the returnPartition function

[satyamakgec]

No description provided.

[tintinweb]

argument _partition is required to be UNLOCKED in _isValidPartition but it is not used in _balanceOfByPartition which instead hardcodes LOCKED. toPartition will always be UNLOCKED because of a bug in _returnPartition. finally an event gets emitted with the unused _partition (which needs to be UNLOCKED from the function call args.

I am struggling to understand the changes. @satyamakgec I would appreciate if you could elaborate on how this is working.

[satyamakgec]

So the Idea is that tokenHolder only allows transacting the UNLOCKED token. _isValidPartition() is used to validate whether the partition is UNLOCKED or not. Then function checks the earlier LOCKED balance of the _to and after transfer, it will check the new LOCKED balance of _to so _returnPartition checks if the difference between both locked balance is equal to the value that is being transferred then it means all value is transferred to LOCKED partition. If not then it means some(or full) value transfferd to UNLOCKED partition of the receiver(_to).

[satyamakgec]

And I agree on the 542 line change I should add the else statement.

[tintinweb]

Hi @satyamakgec,

1. _returnPartition: if line 542 sets toPartition to LOCKED it is always overridden at line 544 to be UNLOCKED. This is still the case with this PR.
   

   polymath-core/contracts/tokens/SecurityToken.sol

   Lines 538 to 544 in 6ff637c

   	function _returnPartition(uint256 _beforeBalance, uint256 _afterBalance, uint256 _value) internal pure returns(bytes32 toPartition) {
   	// return LOCKED only when the transaction `_value` should be equal to the change in the LOCKED partition
   	// balance otherwise return UNLOCKED
   	if (_afterBalance.sub(_beforeBalance) == _value)
   	toPartition = LOCKED;
   	// Returning the same partition UNLOCKED
   	toPartition = UNLOCKED;

From the issue reported in the initial report:

It's commented in the function that it should return LOCKED under some circumstances.
Function _returnPartition always returns UNLOCKED partition.
Additionally, the comment says that LOCKED should be returned when a change in the locked partition equals value but it actually checks the change in _partition which is usually UNLOCKED

2. as mentioned inline please elaborate a bit on the changes.

thanks!
tin/diligence

[coveralls]

Coverage decreased (-0.003%) to 95.181% when pulling d4465e3 on task_3.11 into db6ee34 on dev-3.0.0.

[tintinweb]

👍 tin/diligence