add W3ip-6

W3IPS/w3ip-6.md

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+---
+w3ip: 6
+title: Add external calls within consensus mechanism
+author: 0xhhh https://github.com/cyl19970726
+discussions-to: 
+status: Draft
+type: Standards
+Track category: Core
+created: 2022-06-25
+requires: 
+---
+
+
+## Simple Summary
+
+Support Web3Q to directly call Ethereum's on-chain data in transactions.
+
+## Abstract
+
+Any account can make a cross-chain data call by calling the precompiled contract in the initiated transaction. If the Web3Q Proposer receives a transaction with cross-chain call data, it will put the result of the cross-chain call in `receipt.externalCallResult`, and Participate in the generation of the merkle root of receipt trie, so as to include the result of the cross-chain call into the proposer block. 
+
+The validators of Web3Q will re-execute these transactions including the cross-chain call and participate in the generation of a new receipt root with their results.
+Then the validator can verify the correctness of the cross-chain call by verifying whether the receipt root is consistent with the receipt root in the proposer block.
+Therefore, Web3Q cross-chain call is combined with Web3Q's block generation consensus, and the security of cross-chain call is equivalent to that of Web3Q.
+
+## Motivation
+
+1. Web3Q is the sidechain of ethereum,  each Web3Q validator needs to run an Ethereum node, so the validators is born with the ability to verify the results of cross-chain calls.
+2. The security of Web3Q cross-chain calls is equivalent to the security of web3q, which can greatly improve the cross-chain security of the Web3Q bridge.
+
+## Specification
+
+### Perform cross-chain calls within a contract
+We implemented a precompiled contract at address `0x00000000000000000000000000000000033303` to perform cross-chain calls.
+At present, we provide a `getLogByTxHash` method in this precompiled contract to call the log data of ethereum.
+```
+getLogByTxHash(uint256 chainId , bytes32 txHash, uint256 logIdx ,uint256 maxDataLen, uint256 confirms)
+```
+#### params
+- chainId: The chainId of the target chain of the cross-chain call.
+- txHash: Transaction hash on the target chain.
+- logIdx: The index of the log to be obtained in the corresponding transaction.
+- maxDataLen: Maximum length of log.data.
+- confirms: The number of block confirmations required for this transaction
+#### returns
+##### no happen error
+- Encode `log.address` into an `address` type through `abi.encode`
+- Encode `log.topics` into an `bytes32[]` type through `abi.encode`
+- Encode `log.data` into an `bytes` type through `abi.encode`
+##### happen error
+- Encode `error msg` into an `string` type through `abi.encode`
+#### example: decode result
+```
+    bytes memory payload = abi.encodeWithSignature("getLogByTxHash(uint256,bytes32,uint256,uint256,uint256)", chainId,txHash,logIdx,maxDataLen,confirms);
+    (bool succeed,bytes memory log1) = crossChainCallContract.call(payload);
+    if (!succeed) {
+        string memory errMsg = abi.decode(log1, (string));
+        return;
+    }
+
+    (address c, bytes32[] memory _topics, bytes memory data) = abi.decode(log1, (address, bytes32[], bytes));
+```
+
+### view the result of the cross-chain via RPC
+Web3Q added an `externalCallResult` property to the results of `eth_getTransaction` and `eth_getTransactionReceipt`.
+
+### Parameters
+
+| Constant                  | Value            |
+| ------------------------- | ---------------- |
+| `ExternalCallByteGasCost` | 3               |
+| `ExternalCallGas`         | 200000  |
+
+For the gas cost of cross-chain calls, follow the formula below：
+```Gas Cost = ExternalCallGas + len(ExternalCallResult) * ExternalCallByteGasCost```.
+
+## Rationale
+
+Web3Q cross-chain call is combined with Web3Q's block generation consensus, and the security of cross-chain call is equivalent to that of Web3Q.
+
+## Copyright
+
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).