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Developing.md

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Developing

If you have recently created a contract with this template, you probably could use some help on how to build and test the contract, as well as prepare it for production. This file attempts to provide a brief overview, assuming you have installed a recent version of Rust already (eg. 1.44.1+).

Prerequisites

Before starting, make sure you have rustup along with a recent rustc and cargo version installed. Currently, we are testing on 1.44.1+.

And you need to have the wasm32-unknown-unknown target installed as well.

You can check that via:

rustc --version
cargo --version
rustup target list --installed
# if wasm32 is not listed above, run this
rustup target add wasm32-unknown-unknown

Compiling and running tests

Now that you created your custom contract, make sure you can compile and run it before making any changes. Go into the repository and do:

# this will produce a wasm build in ./target/wasm32-unknown-unknown/release/YOUR_NAME_HERE.wasm
cargo wasm

# this runs unit tests with helpful backtraces
RUST_BACKTRACE=1 cargo unit-test

# auto-generate json schema
cargo schema

Understanding the tests

The main code is in src/contract.rs and the unit tests there run in pure rust, which makes them very quick to execute and give nice output on failures, especially if you do RUST_BACKTRACE=1 cargo unit-test.

We consider testing critical for anything on a blockchain, and recommend to always keep the tests up to date.

Generating JSON Schema

While the Wasm calls (init, handle, query) accept JSON, this is not enough information to use it. We need to expose the schema for the expected messages to the clients. You can generate this schema by calling cargo schema, which will output 4 files in ./schema, corresponding to the 3 message types the contract accepts, as well as the internal State.

These files are in standard json-schema format, which should be usable by various client side tools, either to auto-generate codecs, or just to validate incoming json wrt. the defined schema.

Preparing the Wasm bytecode for production

Before we upload it to a chain, we need to ensure the smallest output size possible, as this will be included in the body of a transaction. We also want to have a reproducible build process, so third parties can verify that the uploaded Wasm code did indeed come from the claimed rust code.

To solve both these issues, we have produced rust-optimizer, a docker image to produce an extremely small build output in a consistent manner. The suggest way to run it is this:

docker run --rm -v "$(pwd)":/code \
  --mount type=volume,source="$(basename "$(pwd)")_cache",target=/code/target \
  --mount type=volume,source=registry_cache,target=/usr/local/cargo/registry \
  cosmwasm/rust-optimizer:0.10.3

We must mount the contract code to /code. You can use a absolute path instead of $(pwd) if you don't want to cd to the directory first. The other two volumes are nice for speedup. Mounting /code/target in particular is useful to avoid docker overwriting your local dev files with root permissions. Note the /code/target cache is unique for each contract being compiled to limit interference, while the registry cache is global.

This is rather slow compared to local compilations, especially the first compile of a given contract. The use of the two volume caches is very useful to speed up following compiles of the same contract.

This produces a contract.wasm file in the current directory (which must be the root directory of your rust project, the one with Cargo.toml inside). As well as hash.txt containing the Sha256 hash of contract.wasm, and it will rebuild your schema files as well.

Testing production build

Once we have this compressed contract.wasm, we may want to ensure it is actually doing everything it is supposed to (as it is about 4% of the original size). If you update the "WASM" line in tests/integration.rs, it will run the integration steps on the optimized build, not just the normal build. I have never seen a different behavior, but it is nice to verify sometimes.

static WASM: &[u8] = include_bytes!("../contract.wasm");

Note that this is the same (deterministic) code you will be uploading to a blockchain to test it out, as we need to shrink the size and produce a clear mapping from wasm hash back to the source code.