Using protoc-bin-vendored doesn't work when building on NixOS

[aeblyve]

NixOS is a Linux distribution which does not obey the FHS, and as such, the protoc supplied by protoc-bin-vendored-linux will fail to run:

error: failed to run custom build command for `meshtastic v0.1.6`
note: To improve backtraces for build dependencies, set the CARGO_PROFILE_DEV_BUILD_OVERRIDE_DEBUG=true environment variable to enable debug information generation.

Caused by:
  process didn't exit successfully: `/home/ablyve/my/83e_meshtastic-bot/target/debug/build/meshtastic-bf53c0c18fbe98d1/build-script-build` (exit status: 101)
  --- stdout
  cargo:rerun-if-changed=src/protobufs/

  --- stderr
  thread 'main' panicked at /home/ablyve/.cargo/registry/src/index.crates.io-6f17d22bba15001f/meshtastic-0.1.6/build.rs:49:54:
  called `Result::unwrap()` on an `Err` value: Custom { kind: NotFound, error: "failed to invoke protoc (hint: https://docs.rs/prost-build/#sourcing-protoc): (path: \"/home/ablyve/.cargo/registry/src/index.crates.io-6f17d22bba15001f/protoc-bin-vendored-linux-x86_64-3.0.0/bin/protoc\"): No such file or directory (os error 2)" }

If we run the binary manually:

/home/ablyve/.cargo/registry/src/index.crates.io-6f17d22bba15001f/protoc-bin-vendored-linux-x86_64-3.0.0/bin/protoc
bash: /home/ablyve/.cargo/registry/src/index.crates.io-6f17d22bba15001f/protoc-bin-vendored-linux-x86_64-3.0.0/bin/protoc: cannot execute: required file not found

As a workaround, doing this patchelf manually on NixOS lets the build finish:

patchelf --set-interpreter $(cat $NIX_CC/nix-support/dynamic-linker) /home/ablyve/.cargo/registry/src/index.crates.io-6f17d22bba15001f/protoc-bin-vendored-linux-x86_64-3.0.0/bin/protoc

[krant]

Same thing on Alpine Linux - provided protoc binary expects /lib64 to be present. It is unfortunate that tokio-rs/prost#771 got stalled - downloading precompiled binary is a terrible solution and Rust ecosystem deserves better.

[Princemachiavelli]

I think the issue is the installation of the vendored protoc is successful so even if PROTOC is already set to use the Nix provided version, the build.rs will always replace the environment variable even if already set.

[krant]

Fixed by #16