Saving Safely -- Never Half a File

The simple way to save -- wipe the file, then write it out again -- has a nasty problem: if the computer crashes halfway through, you're left with half a file, and your data is gone.

So saving works like a careful student copying out their homework:

1. Write the new version on a fresh sheet -- a temporary file right next to the real one.
2. Make sure the ink is dry -- fsync pushes the bytes out of the computer's memory and onto the disk.
3. Swap the fresh sheet in for the old one in a single instant step.

At every moment there's a complete file: the old one or the new one, never half of each. Real databases like SQLite and PostgreSQL use the same trick. The code lives in atomic.py.