Function atomic_write_fsync

nodedb-wal/src/segment/atomic_io.rs:51–68

Atomically write `bytes` to `dst` via a `tmp` file with full durability. Order of operations (must not change): 1. Create / truncate `tmp` and write `bytes`. 2. `sync_data()` on `tmp` — forces file data pages to stable storage. 3. `rename(tmp, dst)` — atomic on POSIX filesystems. 4. `fsync_directory(parent)` — forces the directory entry durable so the new name survives power loss. `tmp` and `dst

(tmp: &Path, dst: &Path, bytes: &[u8])

/// `tmp` and `dst` MUST be in the same directory; otherwise rename is not
/// atomic and the parent fsync won't cover both entries.
pub fn atomic_write_fsync(tmp: &Path, dst: &Path, bytes: &[u8]) -> Result<()> {
    let parent = dst.parent().ok_or_else(|| {
        WalError::Io(std::io::Error::new(
            std::io::ErrorKind::InvalidInput,
            "atomic_write_fsync: dst has no parent directory",
        ))
    })?;

    {
        let mut f = fs::File::create(tmp).map_err(WalError::Io)?;
        f.write_all(bytes).map_err(WalError::Io)?;
        f.sync_data().map_err(WalError::Io)?;
    }

    fs::rename(tmp, dst).map_err(WalError::Io)?;
    fsync_directory(parent)?;
    Ok(())
}

/// Atomically swap a directory: `rename(live, backup); rename(staged, live)`,
/// fsyncing the parent directory once both renames have completed.