Get GC working with Nix shells

[allenap]

Nix shells and GC equals frustration. Below are my notes so far. I've managed to find a 95% solution or something like that; see DO THIS ONE.

    // 3c. Capture the environment's closure to prevent GC.
    // nix-build --no-out-link stuff.nix | xargs -r nix-store -qR | \
    //   xargs -r nix-store --realise --add-root _build/firstaide/gc/$random --indirect
    // log::info!("Capture closure of inside environment to prevent GC.");
    // let gcroot_dir = &config.cache_dir.join("gc");
    // fs::create_dir(gcroot_dir)?;
    // let gcroot_output = process::Command::new("nix-build")
    //     .current_dir(&config.build_dir)
    //     .arg("--no-out-link")
    //     .arg("stuff.nix")
    //     .stdin(process::Stdio::null())
    //     .stdout(process::Stdio::piped())
    //     .stderr(process::Stdio::inherit())
    //     .output()?;
    // let gcroot_inputs: Vec<&[u8]> = gcroot_output.stdout.fields().collect();

    // Another approach?
    //   nix-instantiate shell.nix --add-root gc/shell --indirect
    //   nix-store -r gc/shell --add-root gc/root --indirect


    // An approach that seems to work:  *** DO THIS ONE ***
    //   nix-instantiate shell.nix --add-root _build/firstaide/shell --indirect
    //   nix-shell _build/firstaide/shell --run 'firstaide env ...'
    //   ALSO: Check that user has `keep-outputs` enabled. <-- IMPORTANT

    // 3c. Add a GC root for this environment. This alone is not enough to
    // ensure that the environment and its constituents are not garbage
    // collected – Nix does *not* make this easy – but if we also check if users
    // have `keep-outputs` enabled in their configuration then we're fairly
    // close: Nix will only think it needs to GC a couple of dozen packages.

    log::info!("Capture closure of inside environment to prevent GC.");
    let gcroot_dir = &config.cache_dir.join("gc");
    fs::create_dir(gcroot_dir)?;
    let gcroot_output = process::Command::new("nix-build")
        .current_dir(&config.build_dir)
        .arg("--no-out-link")
        .arg("stuff.nix")
        .stdin(process::Stdio::null())
        .stdout(process::Stdio::piped())
        .stderr(process::Stdio::inherit())
        .output()?;
    let gcroot_inputs: Vec<&[u8]> = gcroot_output.stdout.fields().collect();

    
    // let gcroot_dir = tempfile::TempDir::new_in(
    // let gcroot_path = temp_path.join("inside");
    // let mut dump_cmd = config.command_to_dump_env_inside(&dump_path, &env_outside);
    // log::debug!("{:?}", dump_cmd);
    // let mut dump_proc = dump_cmd.spawn()?;
    // if !dump_proc.wait()?.success() {
    //     return Err(Error::EnvInsideCapture);
    // }
    // match bincode::deserialize(&fs::read(dump_path)?) {
    //     Ok(env) => Ok(env),
    //     Err(err) => Err(Error::EnvInsideDecode(err)),
    // }