Arbitrum gateways

[aalavandhan]

AMPL Arbitrum Bridge gateways.

To test the UI:

1. Goto https://bridge.arbitrum.io/ and connect to the Rinkeby testnet.
2. Add the AMPL testnet deployment address "0x244E12361f488adFa757E706466023673a96Fa3d" to the token list.
3. This should pull up the counterpart L2 address and allow you to send AMPL from rinkeby to arb testnet.

[aalavandhan]

Initially assumed we could abstract all L2s to implement the same gateway interface, however there are operational differences between L2s which prevent this.

eg) Matic doesn't charge a ETH fee to send data from ethereum to matic but Arbitrum does. Arbitrum expects the caller to "pre-pay" the storage and execution cost of the cross chain transaction and thus has some additional parameters.

Leaning toward defining separate gateway interfaces for each bridge.

[brandoniles]

Yeah. Unfortunately I think it's unlikely these bridges will be similar enough to abstract away completely.

[aalavandhan]

We do this to keep the audit trail on the vault. Explicitly calling lock over just transferring directly into the vault logs the GatewayLocked event.

https://github.com/ampleforth/cross-chain-ample/blob/master/contracts/base-chain/TokenVault.sol#L31

However I wonder if the GatewayLocked and GatewayUnlocked events are redundant as one could always reconstruct the same information using merely the token transfer logs.

If so we can safeTransferFrom the user's wallet directly into the vault which is cleaner and more gas efficient.