Add POSDAO post-merge specification

execution.md

@@ -4,7 +4,7 @@
 
 - [ ] AuRa
 - [ ] POSDAO
-- [ ] POSDAO post-merge
+- [POSDAO post-merge](./execution/posdao-post-merge.md)
 - [Withdrawals](./execution/withdrawals.md)
 - [Shutter](./shutter/high-level.md)
 

execution/posdao-post-merge.md

@@ -0,0 +1,65 @@
+# POSDAO post-merge
+
+[POSDAO](https://github.com/poanetwork/posdao-contracts) (Proof of Stake Decentralized Autonomous Organization consensus) is a DPOS consensus implemented in Solidity and running within EVM that drives the consensus of Gnosis chain up until the merge.
+
+All POSDAO components are de-activated after the merge transition block except the capability to mint native tokens in response to events from the `erc-to-native` bridge.
+
+
+## Specification
+
+| Name | Value |
+| - | - |
+| `BLOCK_REWARD_CONTRACT` | `0x481c034c6d9441db23Ea48De68BCAe812C5d39bA` |
+| `SYSTEM_SENDER`         | `0xfffffffffffffffffffffffffffffffffffffffe` |
+
+Beginning with the merge transition block, execution clients **MUST** include the following payload validation and processing:
+
+- System-level operation: block rewards call
+
+**State transition**
+
+The `reward` function of the `BlockRewardAuRa` contract MUST be called by with a [system call](#system-call) when producing and closing a block **after** any user-level transactions are applied. 
+
+The implementation must construct and execute one system EVM transaction as follows:
+
+```
+sender: SYSTEM_SENDER
+max_priority_fee_per_gas: 0
+max_fee_per_gas: 0
+gas_limit: 18446744073709551615
+destination: BLOCK_REWARD_CONTRACT
+amount: 0
+payload: PAYLOAD
+```
+
+System transactions rules are:
+
+- Gas limit checks are disabled compared to `block.gas_limit - block.gas_used` ([ref](https://github.com/NethermindEth/nethermind/blob/master/src/Nethermind/Nethermind.Evm/TransactionProcessing/TransactionProcessor.cs#L204-L220)).
+- Caller balance and nonce checks are disabled. Nonce for system address is not incremented ([ref_1](https://github.com/NethermindEth/nethermind/blob/master/src/Nethermind/Nethermind.Evm/TransactionProcessing/TransactionProcessor.cs#L256-L287), [ref_2](https://github.com/NethermindEth/nethermind/blob/master/src/Nethermind/Nethermind.Evm/TransactionProcessing/TransactionProcessor.cs#L468-L471)).
+- Fees are not added ([ref](https://github.com/NethermindEth/nethermind/blob/master/src/Nethermind/Nethermind.Evm/TransactionProcessing/TransactionProcessor.cs#L421-L448))
+- `block.gas_used` is not incremented ([ref](https://github.com/NethermindEth/nethermind/blob/master/src/Nethermind/Nethermind.Evm/TransactionProcessing/TransactionProcessor.cs#L482-L485))
+- If the transaction reverts, or runs out of the gas, the entire block **MUST** be considered invalid.
+
+
+`PAYLOAD` is the ABI encoded arguments for a solidity function with ABI:
+
+```solidity
+function reward(
+    address[] benefactors,
+    uint16[] kind
+) returns(
+    address[] receiversNative,
+    uint256[] memory rewardsNative
+)
+```
+
+_[contracts/base/BlockRewardAuRaBase.sol](https://github.com/poanetwork/posdao-contracts/blob/0315e8ee854cb02d03f4c18965584a74f30796f7/contracts/base/BlockRewardAuRaBase.sol#L234)_
+
+Where arguments MUST equal to a 1 element list with a single `benefactor` equal to `header.Coinbase` and a single kind of type 0 `RewardAuthor`.
+
+```
+rewards_call_data = abi_encode("reward", [header.coinbase], [0])
+```
+
+The return values of the system call are two lists of equal length. For each tuple item `(receiverNative, rewardNative)` the balance of `receiverNative` must be increased by `rewardNative`. The returned list may be empty. In case of overflow the block MUST be considered invalid.
+