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Contract Pallet Implementation

On pallet contracts layer#

During the contract debugging process, Europa believes that developers need:

  1. Rich error information: Wasm records the error information during the entire execution process, including Wasm executor errors and host_function errors.
  2. Execution in the debugging process: The main modification information of pallet contracts, the "contract stack" is used to record the process of contract calling contract, and any information that can assist debugging during the execution of this layer of contract, such as the situation of calling the host_function, selector, and calling contract parameters, etc.

Europa made the following modifications:

Error on the wasm executor layer:#

Europa designed our own ep-sandbox to replace the original sp-sandbox used by pallet contracts, and modified ep_sandbox::Error

pub enum Error {   Module,   OutOfBounds,   Execution,   /// Wasm inner trap   Trap(imp::Trap),}

Trap(imp::Trap) carries backtrace information of the Wasm layer, it can help developers to get the detailed information for execution stack.

Europa's own ep-sandbox only has the std version (because Europa has removed all Wasm parts, there is no need for ep-sandbox to support no-std), so in the future, ep-sandbox can be replaced with different wasm executors to support running tests of different wasm executors, and replaced with wasm executors that support debugging and other features.

Currently ep-sandbox uses a forked version of wasmi as the executor, so the error it throws is WasmiError. See the next chapter for errors inwasmi.

Error of host_functions:#

The host function execution error will cause Trap, and will record TrapReason. No modification to the data structure, just record.

Execution during debugging#

The Europa forked version of pallet-contracts has designed an object to record any information that can help debugging during contract execution:

/// Record the contract execution struct NestedRuntime {    /// Current depth    depth: usize,    /// The current contract execute result    ext_result: ExecResult,    /// The value in sandbox successful result    sandbox_result_ok: Option<ReturnValue>,    /// Who call the current contract    caller: AccountId32,    /// The account of the current contract    self_account: Option<AccountId32>,    /// The input selector    selector: Option<HexVec>,    /// The input arguments    args: Option<HexVec>,    /// The value in call or the endowment in instantiate    value: u128,    /// The gas limit when this contract is called    gas_limit: Gas,    /// The gas left when this contract return    gas_left: Gas,    /// The host function call stack    env_trace: EnvTraceList,    /// The error in wasm    wasm_error: Option<WasmErrorWrapper>,    /// The trap in host function execution    trap_reason: Option<TrapReason>,    /// Nested contract execution context    nest: Vec<NestedRuntime>,}

In the model of pallet contracts, a contract calling another contract is in the "contract stack" model, so NestedRuntime will track the execution process of the entire contract stack, and use the property of nest to store a list of NestedRuntime to represent other contracts the the contract called.

In the process of executing a contract by pallet contracts, Europa records the relevant information in the execution process in the structure of NestedRuntime in the form of a bypass, and will print the NestedRuntime to the log (show the case later) in a certain format after the contract call ends. Contract developers can analyze the information printed by NestedRuntime to obtain various detailed information during the execution of the contract, which can be used in various situations:

  1. help to locate where the error occurs, including the following situations:
    1. pallet contracts layer
    2. ink! layer
    3. The specific position in the contract layer
    4. Locate which level of the contract is when a contract calling another contract
  2. Analyze the information during the execution of the contract at this timing:
    1. Analyze the consumption of gas execution
    2. Analyze the call of get_storage and set_storage, help reconstruct the contract code and analyze the demand of rent
    3. According to selector, args and value, analyze and locate whether the transaction parameters of the third-party SDK are legal.
    4. Analyze the execution path of the contract and adjust the contract based on the nest information and combined with the seal_call information.
    5. etc.

The process of recording pallet contracts executing contract to NestEdRuntime is relatively fine-grained. The process of logging the information of the execution contract of pallet contracts to NestEdRuntime is relatively fine-grained. Take seal_call in define_env! as an example:

pub struct SealCall {    callee: Option<HexVec>,    gas: u64,    value: Option<u128>,    input: Option<HexVec>,    output: Option<HexVec>,}

The attributes are basically Option<>. For example, before calling the contract, the input will be set to Some, and the return value will be set after the calling contract is normal. If there is an error in the calling contract, then output will remain None. Therefore, if input is Some and output is None, it means that there is a problem with the called contract during the process of calling the contract.

The current information of NestedRuntime is only printed in the log. In the future, NestedRuntime will be stored locally and provide corresponding RPC for external access. Therefore, in the future, third-party applications can obtain NestedRuntime for further processing. For example, in our Redspot, a plug-in can be designed to generate a contract call another contract topology based on the information of NestedRuntime, and a visual contract call path can be generated on the web wallet interface, etc.

On wasmtime Layer#

Currently, europa use wasmtime for execution. And wasmtime support to record the backtrace. europa collects them and record in local.

Contract Log functions#

In the process of contract debugging, you need to know the internal execution of the contract and the intermediate data. Currently, due to lack of debugging conditions (such as using gdb for debugging), log printing is the most convenient way. As mentioned in the Europa v0.2 proposal, the current pallet contracts and ink! already support format!+seal_println to format and print strings, but this mode has two defects :

  1. All the logs of seal_println printed on the node side are target: runtime and level DEBUG, but when developing complex contracts, a lot of logs will be printed. If you cannot filter by target and log level, then the development process will be full of interference from irrelevant information.
  2. The contract developer wrote seal_println when needed during the development process, but all seal_println must be deleted when the contract is released. Although the contract developer can encapsulate a conditionally compiled function to control it, it is more convenient if a tool library already provides such a function.

Therefore, Europa provides a log library patractlabs/ink-log that mimics Rust's log crete to solve the above problems. Its usage is the same as that of Rust. log is completely consistent, which reduces the learning cost of developers.

The ink-log is generally implemented by the ChainExtension of pallet contracts, the agreed function_id is 0xfeffff00, and the message is transmitted in the wasm memory through the structure LoggerExt. Therefore this library is divided into the following two parts:


In the ink-log/contracts directory, provide info!, debug!, warn!, error!, trace!, same as Rust's log library in the same macro, and the call method of the macro is also the same. These macros are packaged implementations of seal_chain_extensions on the ink! side, and are tool library for contract developers. For example, after this library is introduced in the contract Cargo.toml, the log can be printed as follows:

In Cargo.toml:

[dependencies]ink_log = { version = "0.1", git = "", default-features = false, features = ["ink-log-chain-extensions"] }
[features]default = ["std"]std = [ # ... "ink_log/std"]

In the contract, you can use the following methods to print logs in the node:

ink_log::info!(target: "flipper-contract", "latest value is: {}", self.value);


In the ink-log/runtime directory, this library is based on the contents of the function_id and LoggerExt structures passed from ChainExtensions to call the corresponding logs under debug in frame_support to print. It is an implementation library of ink_log prepared for developers of the chain. **For example, chain developers can use it in their own ChainExtensions:

In Cargo.toml

[dependencies]runtime_log = { version = "0.1", git = "", default-features = false }
[features]default = ["std"]std = [ # ... "runtime_log/std"]

In ChainExtensions's implementation:

pub struct CustomExt;impl ChainExtension for CustomExt { fn call<E: Ext>(func_id: u32, env: Environment<E, InitState>) -> Result<RetVal, DispatchError> where     <E::T as SysConfig>::AccountId: UncheckedFrom<<E::T as SysConfig>::Hash> + AsRef<[u8]>, {        match func_id {            ... => {/* other ChainExtension */ }            0xfeffff00 => {                // TODO add other libs             runtime_log::logger_ext!(func_id, env);             // or use                // LoggerExt::call::<E>(func_id, env)                Ok(RetVal::Converging(0))            }`europa_forwardToHeight`        }    }}

ink_log corresponds to runtime_log, so if contract developers need to use ink_log, they need to pay attention to the chain corresponding to the debugging contract that needs to implement runtime_log.

On the other hand, after contract developers introduce ink_log, they need to pay attention to features = ["ink-log-chain-extensions"], ink_log will call seal_chain_extensions to interact with the chain only when this feature is enabled. Without this feature, noop will be used to skip the process of contract printing.

Therefore, contract developers can control the contract to print logs in the debugging environment and the production environment through features. The contract compiled in the debugging environment opens the "ink-log-chain-extensions" feature, and the contract compiled in the production environment removes this feature.

For detailed usage examples, please check Custom ChainExtensions