[KIP-247] Gasless Transaction

KIPs/kip-247.md

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+---
+kip: 247
+title: Gasless Transaction
+author: Ian (@ian0371) and Ollie (@blukat29)
+discussions-to: TBU
+status: Draft
+type: Core
+created: 2025-02-18
+---
+
+## Abstract
+
+This KIP introduces Gasless Transaction (GaslessTx) where users can swap whitelisted fungible tokens into KAIA without possessing any KAIA.
+
+## Motivation
+
+For users who have onboarded onto KAIA from another blockchain via a bridge, it is possible that they do not possess any KAIA tokens and hold only fungible tokens from the bridged assets.
+These users will be unable to perform any on-chain actions unless they acquire KAIA through alternative channels.
+This limitation effectively restricts their ability to engage with decentralized applications, execute transactions, or participate in any protocol activities within the KAIA ecosystem.
+
+To enhance the user experience for those, this KIP introduces a concept of gasless transaction (GaslessTx) where transactions with a specific pattern is recognized as the GaslessTx.
+Upon detection of GaslessTx, the block proposer may lend the user KAIA to facilitate the GaslessTx execution.
+A gasless swap from Token to KAIA shall pay back the lent amount.
+
+## Specification
+
+### Definition of GaslessTx
+
+There are two types of GaslessTx: GaslessApproveTx, and GaslessSwapTx.
+
+#### GaslessApproveTx
+
+A GaslessApproveTx must meet all the following conditions to be inserted to `txpool.queue`:
+
+- A1: `GaslessApproveTx.to` is a whitelisted ERC-20 token.
+- A2: `GaslessApproveTx.data` is `approve(spender, amount)`.
+- A3: `spender` is a whitelisted `GaslessSwapRouter`.
+- A4: `amount` is a nonzero.
+
+(Note that above statements can be verified solely through a static validation of a transaction.)
+
+A GaslessApproveTx must meet all the following conditions to be promoted to `txpool.pending`:
+
+- AP1: GaslessApproveTx is followed by a GaslessSwapTx of the same sender and the `GaslessSwapTx` can be promoted to `txpool.pending`.
+
+#### GaslessSwapTx
+
+A `GaslessSwapTx` must meet all the following conditions to be inserted to `txpool.queue`:
+
+- S1: `GaslessSwapTx.to` is a whitelisted `GaslessSwapRouter`.
+- S2: `GaslessSwapTx.data` is `swapForGas(token, amountIn, amountOut, amountRepay)`.
+- S3. `token` is a whitelisted ERC20 token.
+
+(Note that above statements can be verified solely through a static validation of a transaction.)
+
+A `GaslessSwapTx` must meet all the following conditions to be promoted to `txpool.pending`:
+
+- SP1: (If GaslessApproveTx exists in `txpool.queue`) `GaslessApproveTx.to=token`.
+- SP2: (If GaslessApproveTx exists in `txpool.queue`) `GaslessApproveTx.data.amount>=amountIn`.
+- SP3: `amountRepay` is the correct value.
+  - (If GaslessApproveTx exists in `txpool.queue`) `amountRepay = CalcRepayAmount(GaslessApproveTx, GaslessSwapTx)`.
+  - (otherwise) `amountRepay = amountRepay(GaslessSwapTx)`.
+  - `amountRepay(GaslessApproveTx, GaslessSwapTx) = R1 + R2 + R3` and `amountRepay(GaslessSwapTx) = R1 + R3` where
+    - `R1 = LendTx.Fee() = SwapTx.GasPrice() * 21000`
+    - `R2 = ApproveTx.Fee()` (if exists)
+    - `R3 = SwapTx.Fee()`
+
+While GaslessApproveTx must be followed by with GaslessSwapTx in order to be promoted, GaslessSwapTx can exist by itself.
+
+### Definition of LendTx
+
+Upon detection of a GaslessTx (either GaslessApproveTx or GaslessSwapTx), the block proposer shall inject a transaction, denoted by LendTx, which transfers KAIA to the GaslessTx sender. LendTx must meet all the following requirements:
+
+- L1: `LendTx.type` is 0x7802, a Ethereum dynamic fee transaction type.
+- L2: `LendTx.from` is the proposer itself.
+- L3: `LendTx.to` is `GaslessSwapTx.from`
+- L4: `LendTx.value` is `amountLend`
+  - `amountLend` is `R2 + R3` (from aforementioned `amountRepay` definition).
+
+### GaslessSwapRouter
+
+GaslessSwapRouter is a smart contract responsible for token swap and repayment. It must support the following interface:
+
+```
+interface IKIP247 {
+    function swapForGas(address token, uint256 amountIn, uint256 minAmountOut, uint256 amountRepay) external;
+    function addToken(address token, address dex) external;
+    function removeToken(address token) external;
+
+    function claimCommission() external;
+    function updateCommissionRate(uint256 _commissionRate) external;
+
+    // view functions
+    function dexAddress(address token) external view returns (address dex);
+    function getAmountIn(address token, uint amountOut) external view returns (uint amountIn);
+}
+```
+
+`swapForGas()` must ensure that all the following conditions are satisfied:
+
+- R1: Sender has enough tokens. That is, `token.balanceOf(msg.sender) >= amountIn`.
+- R2: Token is whitelisted and has a corresponding DEX address other than zero.
+- R3: `amountReceived >= minAmountOut >= amountRepay` where `amountReceived` is the swap output.
+
+`swapForGas()` must distribute the swap output (`amountReceived`) as follows:
+
+- The block proposer (`block.coinbase`) receives `amountRepay`.
+- User receives `(amountReceived - amountRepay) * (1 - commissionRate)`.
+- The commission `(amountReceived - amountRepay) * commissionRate` is stored in the contract for later claim.
+
+### TxPool Policy Updates
+
+#### GaslessTx Validation (validateTx)
+
+To prevent GaslessTx to be discarded, the validation logic at `txpool.queue` insertion needs to detect GaslessTx and skip the account balance check.
+
+```
+// txpool.validateTx()
+if ...
+else: // if tx is NOT feeDelegatedTx
+    if senderBalance.Cmp(tx.Cost()) < 0:
+        if tx is GaslessTx:
+            validation ok
+        else:
+            insufficientFund error
+```
+
+#### Promoting GaslessTx (promoteExecutables)
+
+GaslessApproveTx cannot be executed unless the corresponding GaslessSwapTx from the same sender is already in the transaction pool.
+Conversely, if the token has not been approved, GaslessSwapTx cannot be executed before GaslessApproveTx.
+In such cases, both transactions need to be promoted simultaneously.
+
+```
+// txpool.promoteExecutables()
+if tx is GaslessApproveTx:
+    if GaslessSwapTx is in queue:
+        promote GaslessApproveTx and GaslessSwapTx to pending
+    else:
+        push GaslessApproveTx to queue
+
+if tx is GaslessSwapTx:
+    if tx is GaslessApproveTx is in queue:
+        promote GaslessApproveTx and GaslessSwapTx to pending
+    else if GaslessSwapTx.nonce == getNonce(user):
+        push GaslessApproveTx to pending
+    else if GaslessSwapTx.nonce == getNonce(user) + 1:
+        promote GaslessSwapTx to queue
+```
+
+#### Transaction Bundling
+
+This KIP implements `TxBundlingModule` specified in [KIP-245](https://kips.kaia.io/KIPs/kip-245). These are the possible bundles:
+
+- LendTx - GaslessApproveTx - GaslessSwapTx
+- LendTx - GaslessSwapTx
+
+```
+ExtractTxBundles(txs, prevBundles):
+    if GaslessTx is disabled:
+        return nothing
+
+    filter ApproveTx, SwapTx from txs
+    move ApproveTx (if exists) before SwapTx
+    inject LendTx before ApproveTx if exists, otherwise before SwapTx
+```
+
+## Rationale
+
+### Disabling GaslessTx
+
+If many consensus nodes disabled GaslessTx feature, users may experience increased transaction latency.
+However, as long as there are still nodes supporting GaslessTx, the transactions will eventually be executed.
+Also, because this feature is enabled by default, few consensus nodes are expected to disable it.
+
+## Backward Compatibility
+
+This does not affect the backward compatibility because this does not involve hardfork.
+
+## References
+
+- [KIP-245](https://kips.kaia.io/KIPs/kip-245)
+
+## Copyright
+
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).