//! serializes generation publication, activation, and CAS collection within one output root.

use std::fs::{File, OpenOptions};
use std::path::Path;

use anyhow::Context as _;

/// keep the file alive for the whole operation; never unlink the lock.
/// the OS releases this advisory lock on process exit, including crashes.
pub fn lock(root: &Path) -> anyhow::Result<File> {
    std::fs::create_dir_all(root).context("create output root")?;
    let file = OpenOptions::new()
        .create(true)
        .read(true)
        .write(true)
        .open(root.join(".admin.lock"))
        .context("open output lock")?;
    file.lock().context("lock output root")?;
    Ok(file)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn lock_is_exclusive_and_released_on_drop() {
        let root = tempfile::tempdir().unwrap();
        let first = lock(root.path()).unwrap();
        let second = OpenOptions::new()
            .read(true)
            .write(true)
            .open(root.path().join(".admin.lock"))
            .unwrap();
        assert!(second.try_lock().is_err());
        drop(first);
        second.try_lock().unwrap();
    }
}