//! Writing files so a crash leaves the old content or the new, never half:
//! a temporary file in the same directory, flushed, then renamed over.

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

fn write(path: &Path, bytes: &[u8], private: bool) -> std::io::Result<()> {
    let dir = path.parent().filter(|d| !d.as_os_str().is_empty()).unwrap_or(Path::new("."));
    std::fs::create_dir_all(dir)?;
    let name = path.file_name().and_then(|n| n.to_str()).unwrap_or("file");
    let tmp = dir.join(format!(".{name}.{}.tmp", std::process::id()));
    let mut opts = std::fs::OpenOptions::new();
    opts.write(true).create(true).truncate(true);
    #[cfg(unix)]
    {
        use std::os::unix::fs::OpenOptionsExt;
        opts.mode(if private { 0o600 } else { 0o644 });
    }
    #[cfg(not(unix))]
    let _ = private;
    let mut f = opts.open(&tmp)?;
    f.write_all(bytes)?;
    f.sync_all()?;
    drop(f);
    std::fs::rename(&tmp, path)?;
    if let Ok(d) = std::fs::File::open(dir) {
        let _ = d.sync_all();
    }
    Ok(())
}

pub fn write_atomic(path: &Path, bytes: &[u8]) -> std::io::Result<()> {
    write(path, bytes, false)
}

/// As [`write_atomic`], readable by its owner only.
pub fn write_atomic_private(path: &Path, bytes: &[u8]) -> std::io::Result<()> {
    write(path, bytes, true)
}