The Backup That Always Said Yes — Python Bug Hunt

Briefing

Modelled on Pixar's Toy Story 2 deletion (1998): a recursive delete command run on the film's file server wiped out most of the production's files, and when the team turned to the backups they found they had been failing for some time without anyone noticing. The film was largely recovered from a copy a technical director happened to keep at home.

This project is a reconstruction. backup.py writes every file to a tape, but it swallows write errors and reports success based on what it tried to write — never checking what the tape actually holds.

Fix run_backup so failures are surfaced and the result is verified against the tape.

Bug report

BUG-TS2-1998 · Priority: Critical · Reported by: systems

run_backup(source, tape) copies source (name -> data) to an empty tape, in sorted name order, and returns {"ok", "files", "bytes"}:

• on the first TapeFull error, stop writing (attempt no further files)
• "files" and "bytes" describe what tape.catalogue() actually holds afterwards (count of entries, sum of sizes)
• "ok" is True only if no write failed AND every source file is in the catalogue with its exact size; otherwise False

Observed: the nightly job has reported ok for months while the tape held only part of the tree.

Logs

[backup] 1998-11-02 nightly: ok (files=18422)
[backup] 1998-11-03 nightly: ok (files=18422)
[restore] tape 0412: 4,117 of 18,422 expected files present

The code as shipped

src/backup/backup.py (editable)

tape_mod = bug_require("./tape.py")


def run_backup(source, tape):
    """Copy every file in source (name -> data) to the tape."""
    files = 0
    size = 0
    for name in sorted(source):
        data = source[name]
        try:
            tape.write(name, data)
        except tape_mod.TapeFull:
            continue
        files += 1
        size += len(data)
    return {"ok": True, "files": files, "bytes": size}

Read-only context: src/backup/tape.py.