build.rs

/*
 * Copyright 2020 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

use std::io::{Error, ErrorKind, Result as IoResult};

/// Reads the contents of a git file, and registers the build to rerun if it changes
fn read_git_file(file_path: String) -> IoResult<String> {
    let string = std::fs::read_to_string(&file_path)?;
    println!("cargo:rerun-if-changed={file_path}");
    Ok(string)
}

fn git_path(path: &str) -> IoResult<String> {
    let output = std::process::Command::new("git")
        .args(["rev-parse", "--git-path", path])
        .output()
        .map_err(|e| Error::new(e.kind(), "failed to run `git`"))?;

    if !output.status.success() {
        return Err(Error::new(
            ErrorKind::Other,
            format!(
                "`git` failed with status {}: {}",
                output.status,
                std::str::from_utf8(&output.stderr).unwrap_or("no output")
            ),
        ));
    }

    // Trim whitespace cruft
    let output = std::str::from_utf8(&output.stdout)
        .map_err(|_e| Error::new(ErrorKind::InvalidData, "`git` output was not utf-8"))?
        .trim();

    if output.is_empty() {
        Err(Error::new(ErrorKind::InvalidData, "`git` output was empty"))
    } else {
        Ok(output.to_owned())
    }
}

/// Embed the git commit hash into the binary
fn embed_commit_hash() -> Result<(), (Error, &'static str)> {
    // 1. Read HEAD (this should always be .git/HEAD, but better safe than sorry)
    let head_path = git_path("HEAD").map_err(|e| (e, "failed to get HEAD path"))?;
    let head_contents = read_git_file(head_path).map_err(|e| (e, "failed to read HEAD"))?;

    // 2. HEAD usually points to symbolic ref, so peel that to the actual SHA1
    let commit = if let Some(ref_path) = head_contents.strip_prefix("ref: ") {
        let ref_path = git_path(ref_path).map_err(|e| (e, "failed to get ref path"))?;
        read_git_file(ref_path).map_err(|e| (e, "failed to read ref"))?
    } else {
        head_contents
    };

    // 3. Profit
    println!("cargo:rustc-env=GIT_COMMIT_HASH={commit}");

    Ok(())
}

// This build script is used to generate the rust source files that
// we need for XDS GRPC communication.
fn main() -> Result<(), Box<dyn std::error::Error>> {
    // We could use an env var etc to make this fatal if needed
    if let Err((err, details)) = embed_commit_hash() {
        println!("cargo:warning={details}: {err}");
    }

    Ok(())
}