add allowSgtValue/disallowSgtValue to SGT contract

[blockchaindevsh]

This PR prepares SoulGasToken contract so that if:

1. IS_BACKED_BY_NATIVE is true
2. a whitelisted contract is used as tx.to

then SoulGasToken balance can be used as msg.value.

[qzhodl]

Is there an issue to track the motivation and background for this?

Additionally, in many cases, the EthStorage contract might be called by a dApp contract to store data, rather than being the direct transaction target. Can we also support this scenario?

[blockchaindevsh]

Is there an issue to track the motivation and background for this?

I updated the description above.

Additionally, in many cases, the EthStorage contract might be called by a dApp contract to store data, rather than being the direct transaction target. Can we also support this scenario?

@qizhou What's your opinion? The implementation will be much trickier if we want to support indirect caller.

I'm not sure if we should give SoulGasToken so much flexibility.

[blockchaindevsh]

I pushed the tests in this PR, since it depends on the contract change.

[qizhou]

Is there an issue to track the motivation and background for this?

I updated the description above.

Additionally, in many cases, the EthStorage contract might be called by a dApp contract to store data, rather than being the direct transaction target. Can we also support this scenario?

@qizhou What's your opinion? The implementation will be much trickier if we want to support indirect caller.

I'm not sure if we should give SoulGasToken so much flexibility.

The indirect caller is tricker, but Qiang is right: the EthStorage contract is mostly called by the contracts.

One solution that comes to my mind is:

• EthStorage contract can transfer SGT balance: call chargeFrom(addr, amount) returns (amountCharged). In the call, it will charge maximum amountCharged <= SGT balance from tx.origin (msg.sender does not have the SGT), then burn the SGT, and send the value to the contract)
• A whitelist to allow which contracts can call chargeFrom()

[blockchaindevsh]

Is there an issue to track the motivation and background for this?

I updated the description above.

Additionally, in many cases, the EthStorage contract might be called by a dApp contract to store data, rather than being the direct transaction target. Can we also support this scenario?

@qizhou What's your opinion? The implementation will be much trickier if we want to support indirect caller.
I'm not sure if we should give SoulGasToken so much flexibility.

The indirect caller is tricker, but Qiang is right: the EthStorage contract is mostly called by the contracts.

One solution that comes to my mind is:

• EthStorage contract can transfer SGT balance: call chargeFrom(addr, amount) returns (amountCharged). In the call, it will charge maximum amountCharged <= SGT balance from tx.origin (msg.sender does not have the SGT), then burn the SGT, and send the value to the contract)
• A whitelist to allow which contracts can call chargeFrom()

I see how it works, but the trust model is a bit different from ethereum's convention in that evil contracts can upgrade and spend user's SGT balance without permission. But since the permission is whitelisted, this concern should be controllable since we can revoke the whitelist if bad things happen.

[blockchaindevsh]

@qizhou @qzhodl Added a function: chargeFromOrigin(uint256 amount) returns (uint256 amountCharged) in b1d4c1e.

[qizhou]

LGTM. This seems to be simplest change to allow storage contract to charge SGT without changing op-geth.

[qzhodl]

It seems a bit odd that if the amount to be charged exceeds the balance, we still attempt to charge it instead of returning 0.

How will the EthStorage contract call this method? Based on my understanding, the EthStorage contract will first check if msg.value is sufficient. If not, it will call chargeFromOrigin(amountNeeded - msg.value). If the balance is still insufficient, it will revert.

[blockchaindevsh]

chargeFromOrigin may charge less than requested, and the remaining amount is expected to be charged from native balance by the caller contract. The caller contract can revert if it can't charge from native balance.

[qizhou]

Yes, I prefer charging SGT first in EthStorage and then charging native balance so that it may provider better UX.

[qzhodl]

A new issue has been opened to track the corresponding changes: ethstorage/storage-contracts-v1#122