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 easy to use with Prost or Tonic.

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 time (approximately 2 minutes on my 2020 4 core Intel Linux desktop), and has more dependencies. Notably: protobuf-src requires cmake and a C++ compiler.

Quick Start

In your 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(())
}

Trust/Security

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

• The author of this crate (me! Evan Jones)
  • I manually add SHA256 hashes for protoc releases, to allow them to be downloaded.
  • I wrote this crate. Let's hope it is mostly free from bugs
• Crates.io: To give you a non-malicious version of this crate.
• Github: To provide the protoc binaries that Google uploaded.
• Google Protobuf maintainers: To upload 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 lib.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.