mirror of
https://github.com/pezkuwichain/pezkuwi-subxt.git
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Alexander Gryaznov
137 lines
6.4 KiB
Markdown
137 lines
6.4 KiB
Markdown
# Contract Module
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The Contract module provides functionality for the runtime to deploy and execute WebAssembly smart-contracts.
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- [`Call`](https://paritytech.github.io/substrate/master/pallet_contracts/pallet/enum.Call.html)
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- [`Config`](https://paritytech.github.io/substrate/master/pallet_contracts/pallet/trait.Config.html)
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- [`Error`](https://paritytech.github.io/substrate/master/pallet_contracts/pallet/enum.Error.html)
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- [`Event`](https://paritytech.github.io/substrate/master/pallet_contracts/pallet/enum.Error.html)
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## Overview
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This module extends accounts based on the `Currency` trait to have smart-contract functionality. It can
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be used with other modules that implement accounts based on `Currency`. These "smart-contract accounts"
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have the ability to instantiate smart-contracts and make calls to other contract and non-contract accounts.
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The smart-contract code is stored once in a `code_cache`, and later retrievable via its `code_hash`.
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This means that multiple smart-contracts can be instantiated from the same `code_cache`, without replicating
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the code each time.
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When a smart-contract is called, its associated code is retrieved via the code hash and gets executed.
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This call can alter the storage entries of the smart-contract account, instantiate new smart-contracts,
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or call other smart-contracts.
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Finally, when an account is reaped, its associated code and storage of the smart-contract account
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will also be deleted.
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### Gas
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Senders must specify a gas limit with every call, as all instructions invoked by the smart-contract require gas.
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Unused gas is refunded after the call, regardless of the execution outcome.
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If the gas limit is reached, then all calls and state changes (including balance transfers) are only
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reverted at the current call's contract level. For example, if contract A calls B and B runs out of gas mid-call,
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then all of B's calls are reverted. Assuming correct error handling by contract A, A's other calls and state
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changes still persist.
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One gas is equivalent to one [weight](https://docs.substrate.io/v3/runtime/weights-and-fees)
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which is defined as one picosecond of execution time on the runtime's reference machine.
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### Notable Scenarios
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Contract call failures are not always cascading. When failures occur in a sub-call, they do not "bubble up",
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and the call will only revert at the specific contract level. For example, if contract A calls contract B, and B
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fails, A can decide how to handle that failure, either proceeding or reverting A's changes.
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## Interface
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### Dispatchable functions
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Those are documented in the [reference documentation](https://paritytech.github.io/substrate/master/pallet_contracts/index.html#dispatchable-functions).
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### Interface exposed to contracts
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Each contract is one WebAssembly module that looks like this:
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```wat
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(module
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;; Invoked by pallet-contracts when a contract is instantiated.
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;; No arguments and empty return type.
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(func (export "deploy"))
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;; Invoked by pallet-contracts when a contract is called.
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;; No arguments and empty return type.
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(func (export "call"))
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;; If a contract uses memory it must be imported. Memory is optional.
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;; The maximum allowed memory size depends on the pallet-contracts configuration.
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(import "env" "memory" (memory 1 1))
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;; This is one of many functions that can be imported and is implemented by pallet-contracts.
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;; This function is used to copy the result buffer and flags back to the caller.
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(import "seal0" "seal_return" (func $seal_return (param i32 i32 i32)))
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)
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```
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The documentation of all importable functions can be found
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[here](https://github.com/paritytech/substrate/blob/master/frame/contracts/src/wasm/runtime.rs).
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Look for the `define_env!` macro invocation.
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## Usage
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This module executes WebAssembly smart contracts. These can potentially be written in any language
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that compiles to web assembly. However, using a language that specifically targets this module
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will make things a lot easier. One such language is [`ink`](https://github.com/paritytech/ink)
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which is an [`eDSL`](https://wiki.haskell.org/Embedded_domain_specific_language) that enables
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writing WebAssembly based smart contracts in the Rust programming language.
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## Debugging
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Contracts can emit messages to the client when called as RPC through the `seal_debug_message`
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API. This is exposed in ink! via
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[`ink_env::debug_message()`](https://paritytech.github.io/ink/ink_env/fn.debug_message.html).
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Those messages are gathered into an internal buffer and send to the RPC client.
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It is up the the individual client if and how those messages are presented to the user.
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This buffer is also printed as a debug message. In order to see these messages on the node
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console the log level for the `runtime::contracts` target needs to be raised to at least
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the `debug` level. However, those messages are easy to overlook because of the noise generated
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by block production. A good starting point for observing them on the console is using this
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command line in the root directory of the substrate repository:
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```bash
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cargo run --release -- --dev -lerror,runtime::contracts=debug
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```
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This raises the log level of `runtime::contracts` to `debug` and all other targets
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to `error` in order to prevent them from spamming the console.
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`--dev`: Use a dev chain spec
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`--tmp`: Use temporary storage for chain data (the chain state is deleted on exit)
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## Unstable Interfaces
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Driven by the desire to have an iterative approach in developing new contract interfaces
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this pallet contains the concept of an unstable interface. Akin to the rust nightly compiler
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it allows us to add new interfaces but mark them as unstable so that contract languages can
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experiment with them and give feedback before we stabilize those.
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In order to access interfaces marked as `__unstable__` in `runtime.rs` one need to compile
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this crate with the `unstable-interface` feature enabled. It should be obvious that any
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live runtime should never be compiled with this feature: In addition to be subject to
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change or removal those interfaces do not have proper weights associated with them and
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are therefore considered unsafe.
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The substrate runtime exposes this feature as `contracts-unstable-interface`. Example
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commandline for running the substrate node with unstable contracts interfaces:
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```bash
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cargo run --release --features contracts-unstable-interface -- --dev
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```
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New interfaces are generally added as unstable and might go through several iterations
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before they are promoted to a stable interface.
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License: Apache-2.0
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