velra 0.1.0

Lossless compaction for Claude Code: task continuity across context compactions.
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//! Atomic file replacement: temp file in the same directory, fsync, rename,
//! then fsync the directory on POSIX (ยง6.2 step 8).

use std::io::Write;
use std::path::{Path, PathBuf};

fn temp_path(path: &Path) -> PathBuf {
    let dir = path.parent().unwrap_or_else(|| Path::new("."));
    let name = path
        .file_name()
        .map(|n| n.to_string_lossy().into_owned())
        .unwrap_or_else(|| "velra".into());
    dir.join(format!(".{name}.velra-tmp-{}", std::process::id()))
}

/// Writes `bytes` to `path` atomically, preserving the existing file's
/// permissions when there is one.
pub fn write(path: &Path, bytes: &[u8]) -> std::io::Result<()> {
    if let Some(dir) = path.parent() {
        if !dir.as_os_str().is_empty() && !dir.is_dir() {
            crate::home::ensure_dir(dir)?;
        }
    }
    let tmp = temp_path(path);
    {
        let mut opts = std::fs::OpenOptions::new();
        opts.write(true).create(true).truncate(true);
        #[cfg(unix)]
        {
            use std::os::unix::fs::OpenOptionsExt;
            opts.mode(0o600);
        }
        let mut f = opts.open(&tmp)?;
        f.write_all(bytes)?;
        f.flush()?;
        f.sync_all()?;
    }
    #[cfg(unix)]
    if let Ok(meta) = std::fs::metadata(path) {
        let _ = std::fs::set_permissions(&tmp, meta.permissions());
    }
    match std::fs::rename(&tmp, path) {
        Ok(()) => {}
        Err(e) => {
            let _ = std::fs::remove_file(&tmp);
            return Err(e);
        }
    }
    #[cfg(unix)]
    if let Some(dir) = path.parent() {
        if let Ok(d) = std::fs::File::open(dir) {
            let _ = d.sync_all();
        }
    }
    Ok(())
}

/// Writes a new file and fsyncs it (used for backups).
pub fn write_synced(path: &Path, bytes: &[u8]) -> std::io::Result<()> {
    if let Some(dir) = path.parent() {
        if !dir.is_dir() {
            crate::home::ensure_dir(dir)?;
        }
    }
    let mut opts = std::fs::OpenOptions::new();
    opts.write(true).create(true).truncate(true);
    #[cfg(unix)]
    {
        use std::os::unix::fs::OpenOptionsExt;
        opts.mode(0o600);
    }
    let mut f = opts.open(path)?;
    f.write_all(bytes)?;
    f.sync_all()?;
    Ok(())
}

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

    #[test]
    fn replaces_in_place() {
        let dir = tempfile::tempdir().unwrap();
        let p = dir.path().join("settings.json");
        write(&p, b"{}\n").unwrap();
        assert_eq!(std::fs::read(&p).unwrap(), b"{}\n");
        write(&p, b"{\"a\":1}").unwrap();
        assert_eq!(std::fs::read_to_string(&p).unwrap(), "{\"a\":1}");
        // No temp files left behind.
        let leftovers: Vec<_> = std::fs::read_dir(dir.path())
            .unwrap()
            .filter_map(|e| e.ok())
            .filter(|e| e.file_name().to_string_lossy().contains("velra-tmp"))
            .collect();
        assert!(leftovers.is_empty());
    }
}