dlprotoc: Download protoc for Cargo build scripts

This crate downloads the official binary releases of protoc from Google's protobuf Github repo, verifies a SHA256 hash, then extracts it. It is intended to be used in Cargo build scripts (build.rs) with Prost or Tonic, so you don't need to have protoc installed to build Rust projects that use Protocol Buffers.

This fixes Cargo errors like the following:

error: failed to run custom build command for `example v0.1.0 (/home/user/example)`

Caused by:
  process didn't exit successfully: `target/debug/build/example-3f5090329e3c4e4b/build-script-build` (exit status: 1)
  --- stderr
  Error: Custom { kind: NotFound, error: "Could not find `protoc`. If `protoc` is installed, try setting the `PROTOC` environment variable to the path of the `protoc` binary. To install it on Debian, run `apt-get install protobuf-compiler`. It is also available at https://github.com/protocolbuffers/protobuf/releases  For more information: https://docs.rs/prost-build/#sourcing-protoc" }

An alternative is the protobuf-src crate, which compiles protoc from source. Unfortunately, compiling protoc is quite slow (approximately 2 minutes on my 4 core Intel desktop from 2020), and requires cmake and a C++ compiler. This crate only requires Rust.

Quick Start

Add dlprotoc to In build.rs, call dlprotoc::download_protoc() before calling compile_protos(...):

fn main() -> Result<(), Box<dyn std::error::Error>> {
    dlprotoc::download_protoc()?;
    prost_build::compile_protos(&["src/example.proto"], &["src/"])?;
    Ok(())
}

For a complete example, see protoc-cargo-example-rs.

Trust/Security

This downloads pre-compiled executables on Github, which is somewhat dangerous. You need to trust:

• The SHA256 hashes embedded in the source code are for protoc binaries that are non-malicious. The hashes are from Google's Github releases.
• The crate does not have bugs, and will not run protoc binaries that have not been seen before.
• Crates.io: Must give you a non-malicious version of this crate.
• Google Protobuf maintainers: Uploads non-malicious protoc binaries to Github.

Updating to new protoc releases (for maintainers)

1. Run: cargo run -- (version e.g 27.0)
2. Append the printed struct definitions into the KNOWN_VERSIONS array in versions.rs.
3. Run make to execute all checks.
4. Update the version in Cargo.toml to include the version of protoc. E.g. "0.1.0+27.0".
5. Send a pull request.

Releasing the crate (for maintainers)

1. Test it: make && cargo publish --dry-run
2. Publish to crates.io: cargo publish
3. Tag the release: (VERSION=$(cargo pkgid | sed 's/.*@//'); git tag -a "v$VERSION" -m "release version $VERSION")
4. Push the tag: git push --tags
5. Create a release from the existing tag on Github: Go to Tags, click the "..." menu, choose Create Release.