Three Presses, Three Remittances — Python Bug Hunt

Briefing

Modelled on the "Dalmellington bug" in the Post Office's Horizon system, one of the defects examined in the High Court's 2019 Horizon Issues judgment (Bates v Post Office). At the Dalmellington branch a cash remittance to an outreach branch was recorded several times: the terminal was slow to respond, the confirm button was pressed again, and each press was booked as another transfer. The duplicated entries appeared as a cash shortfall at the branch. Across the wider Horizon scandal, hundreds of subpostmasters were held liable — many prosecuted — for shortfalls the system produced.

remittance.py books a remittance. The counter terminal sends one call per press of the confirm button, each carrying the request_key generated when the remittance screen was opened.

Make record_remittance idempotent on that key.

Bug report

BUG-HNG-DAL · Priority: Critical (branch accounts) · Reported by: branch support

record_remittance(ledger, branch, request_key, amount_pence):

• amount_pence <= 0 raises ValueError (unchanged)
• the first call for a (branch, request_key) inserts one row and returns {"status": "recorded", "id": <row id>}
• a repeat call with the same branch, key and amount inserts nothing and returns {"status": "duplicate", "id": <the original row id>}
• a repeat with the same branch and key but a DIFFERENT amount raises ValueError and inserts nothing
• keys are scoped to a branch: the same key at another branch is a separate remittance

Observed: a remittance confirmed three times while the terminal hung is booked three times; the branch shows a shortfall of twice the amount.

Logs

[horizon] branch=DAL rem_out key=R-5510 amount=800000 row=41
[horizon] branch=DAL rem_out key=R-5510 amount=800000 row=42
[horizon] branch=DAL rem_out key=R-5510 amount=800000 row=43
[balancing] branch=DAL discrepancy=-1600000

The code as shipped

src/branch/remittance.py (editable)

# Books a cash remittance sent from a branch.


def record_remittance(ledger, branch, request_key, amount_pence):
    if amount_pence <= 0:
        raise ValueError("amount must be positive")
    row = ledger.insert(branch, request_key, amount_pence)
    return {"status": "recorded", "id": row["id"]}

Read-only context: src/branch/ledger.py.