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experimental: allow the batch inbox to be a contract #284

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experimental: allow the batch inbox to be a contract #284

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add inbox-contract.md
blockchaindevsh Jul 23, 2024
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update inbox-contract.md
blockchaindevsh Aug 3, 2024
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modify L1Block Deployment from address to the existing 0x421000000000…
blockchaindevsh Aug 9, 2024
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modification towards the idea of "Separate Dynamic and Static L1 Attr…
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264 changes: 264 additions & 0 deletions specs/protocol/holocene/inbox-contract.md
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<!-- omit in toc -->
# Inbox Contract

<!-- START doctoc generated TOC please keep comment here to allow auto update -->
<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
**Table of Contents**
- [Motivation](#motivation)
- [How It Works](#how-it-works)
- [Supporting Dynamic Updates to Inbox Address](#supporting-dynamic-updates-to-inbox-address)
- [SystemConfigHolocene](#systemconfigholocene)
- [setBatchInbox](#setbatchinbox)
- [L1BlockHolocene](#l1blockholocene)
- [batchInbox](#batchinbox)
- [ConfigType](#configtype)
- [setConfig](#setconfig)
- [StaticConfig](#staticconfig)
- [encodeSetBatchInbox](#encodesetbatchinbox)
- [decodeSetBatchInbox](#decodesetbatchinbox)
- [L2Client.SystemConfigByL2Hash](#l2clientsystemconfigbyl2hash)
- [L1Traversal.AdvanceL1Block](#l1traversaladvancel1block)
- [How `op-node` knows the canonical batch inbox](#how-op-node-knows-the-canonical-batch-inbox)
- [How `op-batcher` knows canonical batch inbox](#how-op-batcher-knows-canonical-batch-inbox)
- [Upgrade](#upgrade)
- [L1Block Deployment](#l1block-deployment)
- [L1Block Proxy Update](#l1block-proxy-update)
- [SystemConfigHolocene Upgrade](#systemconfigholocene-upgrade)
- [Security Considerations](#security-considerations)
- [Inbox Sender](#inbox-sender)
- [Reference Implementation](#reference-implementation)

## Motivation

The batch inbox is currently an Externally Owned Account (EOA), which has both advantages and disadvantages:

Advantages:
- Low submission gas cost due to the absence of onchain execution.
- Verification logic is moved offchain to the derivation part, protected by a fault dispute game with a correct absolute prestate.

Disadvantage:
- Onchain verification is not possible.


This specification aims to allow the batch inbox to be a contract, enabling customized batch submission conditions such as:
- Requiring the batch transaction to be signed by a quorum of sequencers in a decentralized sequencing network; or
- Mandating that the batch transaction call a BLOB storage contract (e.g., EthStorage) with a long-term storage fee, which is then distributed to data nodes that prove BLOB storage over time.


## How It Works

The integration process consists of three primary components:
1. The [`BatchInboxAddress`](https:/ethereum-optimism/optimism/blob/db107794c0b755bc38a8c62f11c49320c95c73db/op-chain-ops/genesis/config.go#L77) can now be set to either an Externally Owned Account (EOA) or a smart contract. When a contract is used, it assumes responsibility for verifying and enforcing batch submission conditions.
2. Modification of the `op-node` derivation process: The `op-node` will be updated to exclude failed batch transactions during the derivation process. This change ensures that only successfully executed batch transactions are processed and included in the derived state.
3. Modification of the op-batcher submission process: The op-batcher will be updated to [call `recordFailedTx`](https:/blockchaindevsh/optimism/blob/02e3b7248f1b590a2adf1f81488829760fa2ba03/op-batcher/batcher/driver.go#L537) for failed batch transactions. This modification ensures that the data contained in failed transactions will be resubmitted automatically.
1. Most failures will be detected during the [`EstimateGas`](https:/ethereum-optimism/optimism/blob/8f516faf42da416c02355f9981add3137a3db190/op-service/txmgr/txmgr.go#L266) call. However, under certain race conditions, failures may occur after the transaction has been included in a block.

These modifications aim to enhance the security and efficiency of the batch submission and processing pipeline, allowing for more flexible and customizable conditions while maintaining the integrity of the derived state.


## Supporting Dynamic Updates to Inbox Address

### SystemConfigHolocene

The `SystemConfigHolocene` is the source of truth for the address of inbox. It stores information about the inbox address and passes the information to L2 as well.


#### setBatchInbox

A new function `setBatchInbox` is introduced to the `SystemConfigHolocene` contract, enabling dynamic updates to the inbox:

```solidity
/// @notice Updates the batch inbox address. Can only be called by the owner.
/// @param _batchInbox New batch inbox address.
function setBatchInbox(address _batchInbox) external onlyOwner {
if (_batchInbox != batchInbox()) {
Storage.setAddress(BATCH_INBOX_SLOT, _batchInbox);
OptimismPortal(payable(optimismPortal())).setConfig(
ConfigType.SET_BATCH_INBOX,
StaticConfig.encodeSetBatchInbox({
_batchInbox: _batchInbox,
})
);
}
}
```



### L1BlockHolocene

The `L1BlockHolocene` stores Layer 1 (L1) related information on Layer 2 (L2). It is extended to store the dynamic inbox address.

#### batchInbox

A new field `batchInbox` is added to the `L1BlockHolocene`:

```solidity
/// @notice The canonical batch inbox.
bytes32 public batchInbox;
```

#### ConfigType

A new enum value `SET_BATCH_INBOX` is added to the `ConfigType` enumeration.

```solidity
enum ConfigType {
SET_GAS_PAYING_TOKEN,
ADD_DEPENDENCY,
REMOVE_DEPENDENCY,
SET_BATCH_INBOX
}
```


#### setConfig

This function stores Layer 1 (L1) system configs for Layer 2 (L2). It is extended to support the newly introduced `SET_BATCH_INBOX` ConfigType.

```solidity
function setConfig(ConfigType _type, bytes calldata _value) external {
...
else if (_type == ConfigType.SET_BATCH_INBOX) {
_setBatchInbox(_value);
}
}

/// @notice Internal method to set the batch inbox address.
/// @param _value The encoded value with which to set the batch inbox address.
function _setBatchInbox(bytes calldata _value) internal {
batchInbox = StaticConfig.decodeSetBatchInbox(_value);
}
```

### StaticConfig

The `StaticConfig` library is used for encoding and decoding static configuration data.

#### encodeSetBatchInbox

This function is used on L1 to encode the static configuration data for setting a batch inbox.

```solidity
/// @notice Encodes the static configuration data for setting a batch inbox.
/// @param _batchInbox Address of the batch inbox.
/// @return Encoded static configuration data.
function encodeSetBatchInbox(address _batchInbox) internal pure returns (bytes memory) {
return abi.encode(_batchInbox);
}
```

#### decodeSetBatchInbox

This function is used on L2 to decode the static configuration data for setting a batch inbox.

```solidity
/// @notice Decodes the static configuration data for setting a batch inbox.
/// @param _data Encoded static configuration data.
/// @return Decoded Address of the batch inbox.
function decodeSetBatchInbox(bytes memory _data) internal pure returns (address) {
return abi.decode(_data, (address));
}
```

### L2Client.SystemConfigByL2Hash

This function is used to retrieve the system configuration for a specific L2 block hash. Prior to the fork, this information was obtained from the deposit transaction. However, post-fork, it should be read from the L2 state instead, in accordance with the concepts outlined in [this issue](https:/ethereum-optimism/specs/issues/122).

### L1Traversal.AdvanceL1Block

This function is used to fetch the next L1 block. It also caches the up-to-date `SystemConfig` by iteratively applying the `SystemConfig` updates in each L1 block. The way it retrieves the `SystemConfig` updates differs pre- and post-fork:

Pre-fork:
- It retrieves the `SystemConfig` updates by parsing the [`ConfigUpdate`](https:/ethereum-optimism/optimism/blob/f20b92d3eb379355c876502c4f28e72a91ab902f/packages/contracts-bedrock/src/L1/SystemConfig.sol#L124) event.

Post-fork:
- It retrieves the `SystemConfig` updates by parsing the `TransactionDeposited` event emitted by [`OptimismPortalInterop.setConfig`](https:/ethereum-optimism/optimism/blob/f20b92d3eb379355c876502c4f28e72a91ab902f/packages/contracts-bedrock/src/L1/OptimismPortalInterop.sol#L41) function.
- It needs to parse the `SystemConfig` values according to [`ConfigType`](https:/ethereum-optimism/optimism/blob/f20b92d3eb379355c876502c4f28e72a91ab902f/packages/contracts-bedrock/src/L2/L1BlockInterop.sol#L14), similar to [`L1BlockInterop.setConfig`](https:/ethereum-optimism/optimism/blob/f20b92d3eb379355c876502c4f28e72a91ab902f/packages/contracts-bedrock/src/L2/L1BlockInterop.sol#L59).



### How `op-node` knows the canonical batch inbox

We define that the canonical batch inbox at a specific L2 block is the batch inbox of `SystemConfig` for the origin of the L2 block.

Under normal conditions, `op-node` knows the canonical batch inbox through the derivation pipeline:
1. The `L1Traversal` component first identifies the L1 `SystemConfig` updates and caches the up-to-date `SystemConfig` while traversing the L1 block, via [`UpdateSystemConfigWithL1Receipts`](https:/ethereum-optimism/optimism/blob/71928829ca7ece48152159daa1d231eac2df03b3/op-node/rollup/derive/l1_traversal.go#L78).
2. The `L1Retrieval` component then fetches the canonical batch inbox from the `L1Traversal` componenet and [pass](https:/ethereum-optimism/optimism/blob/71928829ca7ece48152159daa1d231eac2df03b3/op-node/rollup/derive/l1_retrieval.go#L57) it to the `DataSourceFactory` component, via `OpenData` function, similar to how [`SystemConfig.BatcherAddr`](https:/ethereum-optimism/optimism/blob/71928829ca7ece48152159daa1d231eac2df03b3/op-service/eth/types.go#L382) is handled.
3. The `FetchingAttributesBuilder` component will convert `SystemConfig` updates into deposit transactions via [`DeriveDeposits`](https:/ethereum-optimism/optimism/blob/f20b92d3eb379355c876502c4f28e72a91ab902f/op-node/rollup/derive/attributes.go#L70). After the deposit transactions are executed, the `SystemConfig` will be persisted in L2 state.

During L2 reorganization, `op-node` knows the canonical batch inbox using the `SystemConfig` parameter of [`ResettableStage.Reset(context.Context, eth.L1BlockRef, eth.SystemConfig)`](https:/ethereum-optimism/optimism/blob/71928829ca7ece48152159daa1d231eac2df03b3/op-node/rollup/derive/pipeline.go#L38) function, where `SystemConfig` is derived from the `DepositTx` of the corresponding L2 block.

### How `op-batcher` knows canonical batch inbox

Immediately before submitting a new batch, `op-batcher` fetches the current inbox address from L1 and submits to that address. After the transaction is successfully included in L1 at block `N`, `op-batcher` verifies that the inbox address hasn't changed at block `N`. If the address has changed, it resubmits the batch to the new address.

## Upgrade

The inbox contract upgrade is not yet defined to be part of a particular network upgrade, but it will be scheduled as part of a future hardfork. On the network upgrade block, a set of deposit transaction based upgrade transactions are deterministically generated by the derivation pipeline in the following order:

- L1 Attributes Transaction calling `setL1BlockValuesEcotone`
- User deposits from L1
- Network Upgrade Transactions
- L1Block deployment
- Update L1Block Proxy ERC-1967 Implementation Slot

The deployment transactions MUST have a `from` value that has no code and has no known
private key. This is to guarantee it cannot be frontran and have its nonce modified.
If this was possible, then an attacker would be able to modify the address that the
implementation is deployed to because it is based on `CREATE` and not `CREATE2`.
This would then cause the proxy implementation set transactions to set an incorrect
implementation address, resulting in a bricked contract. The calldata is not generated
dynamically to enable deterministic upgrade transactions across all networks.

The proxy upgrade transactions are from `address(0)` because the `Proxy` implementation
considers `address(0)` to be an admin. Going straight to the `Proxy` guarantees that
the upgrade will work because there is no guarantee that the `Proxy` is owned by the
`ProxyAdmin` and going through the `ProxyAdmin` would require stealing the identity
of its owner, which may be different on every chain. That would require adding L2
RPC access to the derivation pipeline and make the upgrade transactions non deterministic.

### L1Block Deployment

- `from`: `0x4210000000000000000000000000000000000000`
- `to`: `null`
- `mint`: `0`
- `value`: `0`
- `gasLimit`: TODO
- `data`: TODO
- `sourceHash`: TODO

### L1Block Proxy Update

- `from`: `0x0000000000000000000000000000000000000000`
- `to`: `0x4200000000000000000000000000000000000015`
- `mint`: `0`
- `value`: `0`
- `gasLimit`: `50,000`
- `data`: TODO
- `sourceHash`: TODO

### SystemConfigHolocene Upgrade

Finally, to dynamically change the inbox address, `SystemConfigHolocene` on L1 will be upgraded to accept a new inbox address.

Note that according to the [Optimism Style Guide](https:/ethereum-optimism/optimism/blob/9d31040ecf8590423adf267ad24b03bc1bf7273b/packages/contracts-bedrock/STYLE_GUIDE.md), The process for upgrading the implementation is as follows:
1. Upgrade the implementation to the `StorageSetter` contract.
2. Use that to set the initialized slot (typically slot 0) to zero.
3. Upgrade the implementation to the desired new implementation and initialize it.

For a practical example of this process, refer to [this](https:/ethereum-optimism/superchain-ops/blob/55e9520e27c7e916d8992ce351c6d5cfa8a511d8/tasks/eth/009-fp-upgrade/EXEC.md) execution document in the Optimism Superchain Operations repository.

## Security Considerations

### Inbox Sender

The inbox contract is a special contract that, if invoked by the batcher, all of its calldata / blob will be used as L2 derivation data, unless the transaction fails. In order to invoke and modify the state of the inbox contract, it is recommended to use a non-batcher sender.


## Reference Implementation

1. [example inbox contract for EthStorage](https:/blockchaindevsh/es-op-batchinbox/blob/main/src/BatchInbox.sol)
2. [op-node & op-batcher changes](https:/blockchaindevsh/optimism/compare/5137f3b74c6ebcac4f0f5a118b0f4909df03aec6...02e3b7248f1b590a2adf1f81488829760fa2ba03)

TODO: implement `Supporting Dynamic Updates to Inbox Address` mentioned above.