Cache mounts for Rust packages break concurrent builds

[fence-sitter]

Is this a docs issue?

• My issue is about the documentation content or website

Type of issue

Information is incorrect

Description

Rust language-specific guide and Optimize cache usage in builds say to cache mount /usr/local/cargo/git/db and /usr/local/cargo/registry when building Rust projects. Eg.

RUN --mount=type=cache,target=/usr/local/cargo/git/db \
    --mount=type=cache,target=/usr/local/cargo/registry \
    cargo build

However, I believe this breaks multi-platform builds (Eg. docker buildx build --platform linux/amd64,linux/arm64 .) because while the package cache is shared between builds, the package cache lock is not. The lock can currently be found at $CARGO_HOME/.package-cache, which is /usr/local/cargo/.package-cache inside the container.

Location

https://docs.docker.com/guides/rust/

Suggestion

Potential solutions, worst-to-best in my opinion:

• Make the cache exclusive, eg. --mount=type=cache,sharing=locked,target=/usr/local/cargo/registry.
  • Why use Docker to do something that Cargo can do by itself?
• Share the package cache lock file, eg. --mount=type=cache,target=/usr/local/cargo/.package-cache.
  • The internal structure of the Cargo home is not stabilized and may be subject to change at any time. In fact, we've already missed the other package lock file: .package-cache-mutate.
• Cache the entire Cargo home directory, eg. --mount=type=cache,target=/usr/local/cargo,from=rust:1.92,source=/usr/local/cargo.
  • Doing this means we don't care about the internals of Cargo home, but we do have to specify from=rust:1.92 (assuming you are in a FROM rust:1.92 section) so that everything already in Cargo home isn't clobbered.

See https://hackmd.io/@kobzol/S17NS71bh for a good summary of Docker caching with cargo builds.