`msg.value` is not the same as the actual amount that gets sent to a smart contract

[sbinabdullah]

Describe the bug

When sending ether to a smart contract on the subtensor's evm, the msg.value is not the same as the actual amount that is deposited - it appears that msg.value does not consider the 0.00005 TAO fee. This is very different to how i.e. the Ethereum Mainnet evm behaves.

To Reproduce

• Deploy a smart contract with a payable modifier in solidity
• Call the function whilst sending it ether (TAO).
• Notice that the amount that gets deposited is 0.00005 TAO less than what you intended to deposit.

Expected behavior

msg.value should reflect the actual amount that gets deposited.

[open-junius]

I tested the scenario, and the less part is EXISTENTIAL_DEPOSIT for an account in subtensor. After a contracted is deployed, its balance is 0. When someone transfer TAO to its account, the EXISTENTIAL_DEPOSIT is deducted from the account. that's why we have the difference. If you transfer again, the contract will get the same amount as in msg.value.

[open-junius]

found out the root cause. the account in subtensor get the exact amount of token set in msg.value.
but for get balance implementation in frontier, it return the reducible_balance.
that's why balance from evm interface is different with substrate, and the difference is EXISTENTIAL_DEPOSIT

[open-junius]

We can fix it by returning all free part for the account. but for smart contract, it need to keep the account in subtensor. so reducible_balance make more sense, and the user know exactly how much the user can withdraw from contract and keep the account alive.