//! Atomic file writes: write to a sibling temp file, then `rename` over the
//! target. Matters here specifically for the instance JSON file, which the
//! supervisor rewrites on every state change while it may be alive for
//! months; a reader (`status`/`list`) must never observe a half-written
//! file, and a crash mid-write must never corrupt the last-known-good state.

use std::io::Write;
use std::path::Path;

pub fn write(path: &Path, contents: &[u8]) -> std::io::Result<()>
{
    let tmp = path.with_extension(format!(
        "{}.tmp.{}",
        path.extension().and_then(|e| e.to_str()).unwrap_or(""),
        std::process::id()
    ));

    {
        let mut f = std::fs::File::create(&tmp)?;
        f.write_all(contents)?;
        f.sync_all()?;
    }

    std::fs::rename(&tmp, path)
}