# Contracts Module

The Contracts module provides functionality for the runtime to deploy and execute WebAssembly smart-contracts.

- [`Call`](https://paritytech.github.io/substrate/master/pallet_contracts/pallet/enum.Call.html)
- [`Config`](https://paritytech.github.io/substrate/master/pallet_contracts/pallet/trait.Config.html)
- [`Error`](https://paritytech.github.io/substrate/master/pallet_contracts/pallet/enum.Error.html)
- [`Event`](https://paritytech.github.io/substrate/master/pallet_contracts/pallet/enum.Event.html)

## Overview

This module extends accounts based on the [`frame_support::traits::fungible`] traits to have smart-contract
functionality. It can be used with other modules that implement accounts based on [`frame_support::traits::fungible`].
These "smart-contract accounts" have the ability to instantiate smart-contracts and make calls to other contract and
non-contract accounts.

The smart-contract code is stored once, and later retrievable via its `code_hash`. This means that multiple
smart-contracts can be instantiated from the same `code`, without replicating the code each time.

When a smart-contract is called, its associated code is retrieved via the code hash and gets executed. This call can
alter the storage entries of the smart-contract account, instantiate new smart-contracts, or call other smart-contracts.

Finally, when an account is reaped, its associated code and storage of the smart-contract account will also be deleted.

### Weight

Senders must specify a [`Weight`](https://paritytech.github.io/substrate/master/sp_weights/struct.Weight.html) limit
with every call, as all instructions invoked by the smart-contract require weight. Unused weight is refunded after the
call, regardless of the execution outcome.

If the weight limit is reached, then all calls and state changes (including balance transfers) are only reverted at the
current call's contract level. For example, if contract A calls B and B runs out of weight mid-call, then all of B's
calls are reverted. Assuming correct error handling by contract A, A's other calls and state changes still persist.

One `ref_time` `Weight` is defined as one picosecond of execution time on the runtime's reference machine.

### Revert Behaviour

Contract call failures are not cascading. When failures occur in a sub-call, they do not "bubble up", and the call will
only revert at the specific contract level. For example, if contract A calls contract B, and B fails, A can decide how
to handle that failure, either proceeding or reverting A's changes.

### Off-chain Execution

In general, a contract execution needs to be deterministic so that all nodes come to the same conclusion when executing
it. To that end we disallow any instructions that could cause indeterminism. Most notable are any floating point
arithmetic. That said, sometimes contracts are executed off-chain and hence are not subject to consensus. If code is
only executed by a single node and implicitly trusted by other actors is such a case. Trusted execution environments
come to mind. To that end we allow the execution of indeterminstic code for off-chain usages with the following
constraints:

1. No contract can ever be instantiated from an indeterministic code. The only way to execute the code is to use a
delegate call from a deterministic contract.
2. The code that wants to use this feature needs to depend on `pallet-contracts` and use
[`bare_call()`](https://paritytech.github.io/substrate/master/pallet_contracts/pallet/struct.Pallet.html#method.bare_call)
directly. This makes sure that by default `pallet-contracts` does not expose any indeterminism.

#### How to use

An indeterministic code can be deployed on-chain by passing `Determinism::Relaxed` to
[`upload_code()`](https://paritytech.github.io/substrate/master/pallet_contracts/pallet/struct.Pallet.html#method.upload_code).
A deterministic contract can then delegate call into it if and only if it is ran by using
[`bare_call()`](https://paritytech.github.io/substrate/master/pallet_contracts/pallet/struct.Pallet.html#method.bare_call)
and passing
[`Determinism::Relaxed`](https://paritytech.github.io/substrate/master/pallet_contracts/enum.Determinism.html#variant.Relaxed)
to it. **Never use this argument when the contract is called from an on-chain transaction.**

## Interface

### Dispatchable functions

Those are documented in the [reference
documentation](https://paritytech.github.io/substrate/master/pallet_contracts/index.html#dispatchable-functions).

### Interface exposed to contracts

Each contract is one WebAssembly module that looks like this:

```wat
(module
    ;; Invoked by pallet-contracts when a contract is instantiated.
    ;; No arguments and empty return type.
    (func (export "deploy"))

    ;; Invoked by pallet-contracts when a contract is called.
    ;; No arguments and empty return type.
    (func (export "call"))

    ;; If a contract uses memory it must be imported. Memory is optional.
    ;; The maximum allowed memory size depends on the pallet-contracts configuration.
    (import "env" "memory" (memory 1 1))

    ;; This is one of many functions that can be imported and is implemented by pallet-contracts.
    ;; This function is used to copy the result buffer and flags back to the caller.
    (import "seal0" "seal_return" (func $seal_return (param i32 i32 i32)))
)
```

The documentation of all importable functions can be found
[here](https://paritytech.github.io/substrate/master/pallet_contracts/api_doc/trait.Current.html).

## Usage

This module executes WebAssembly smart contracts. These can potentially be written in any language that compiles to
Wasm. However, using a language that specifically targets this module will make things a lot easier. One such language
is [`ink!`](https://use.ink). It enables writing WebAssembly-based smart-contracts in the Rust programming language.

## Debugging

Contracts can emit messages to the client when called as RPC through the
[`debug_message`](https://paritytech.github.io/substrate/master/pallet_contracts/api_doc/trait.Current.html#tymethod.debug_message)
API. This is exposed in [ink!](https://use.ink) via
[`ink_env::debug_message()`](https://paritytech.github.io/ink/ink_env/fn.debug_message.html).

Those messages are gathered into an internal buffer and sent to the RPC client. It is up the the individual client if
and how those messages are presented to the user.

This buffer is also printed as a debug message. In order to see these messages on the node console the log level for the
`runtime::contracts` target needs to be raised to at least the `debug` level. However, those messages are easy to
overlook because of the noise generated by block production. A good starting point for observing them on the console is
using this command line in the root directory of the Substrate repository:

```bash
cargo run --release -- --dev -lerror,runtime::contracts=debug
```

This raises the log level of `runtime::contracts` to `debug` and all other targets to `error` in order to prevent them
from spamming the console.

`--dev`: Use a dev chain spec `--tmp`: Use temporary storage for chain data (the chain state is deleted on exit)

## Host function tracing

For contract authors, it can be a helpful debugging tool to see which host functions are called, with which arguments,
and what the result was.

In order to see these messages on the node console, the log level for the `runtime::contracts::strace` target needs to
be raised to the `trace` level.

Example:

```bash
cargo run --release -- --dev -lerror,runtime::contracts::strace=trace,runtime::contracts=debug
```

## Unstable Interfaces

Driven by the desire to have an iterative approach in developing new contract interfaces this pallet contains the
concept of an unstable interface. Akin to the rust nightly compiler it allows us to add new interfaces but mark them as
unstable so that contract languages can experiment with them and give feedback before we stabilize those.

In order to access interfaces marked as `#[unstable]` in [`runtime.rs`](src/wasm/runtime.rs) one need to set
`pallet_contracts::Config::UnsafeUnstableInterface` to `ConstU32<true>`. **It should be obvious that any production
runtime should never be compiled with this feature: In addition to be subject to change or removal those interfaces
might not have proper weights associated with them and are therefore considered unsafe**.

New interfaces are generally added as unstable and might go through several iterations before they are promoted to a
stable interface.

License: Apache-2.0