Premier League Predictor: FastAPI & Redis — Chapter 9, Exercise 2
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TASK
Call the rollover script directly on a 20-team season with only one
fixture entered, and report which clubs it relegates. Then call it a
second time and report the result and what happened to the season ID that
call took.

SOLUTION

Setup: season 1 has 20 teams at score -500000 and a single 3-0 result
entered for team 1 v team 2. New season ID from INCR: 2.

  rollover (first call)   -> 17
  relegated (ZRANGE season:1:table -3 -1) -> ['019', '020', '002']

The script rolled 17 clubs into season 2 and left team 2 (which had just
lost) and two clubs that had not played at all as the "bottom three".
Nothing in the script checks that the season is finished; the
season_complete check exists only in the route, so calling the script
directly bypasses it.

Second call, with a new ID from INCR (3):

  rollover (second call)  -> -1
  season IDs taken: 2 and 3
  EXISTS season:3         -> 0

The rolled-over flag (season:1:rolled) already existed, so the script
returned -1 before writing anything. Season ID 3 was taken by INCR before
the script ran and is never used - a harmless gap. The alternative,
claiming the flag first and taking the ID afterwards, would leave a
half-finished rollover if the process died between the two steps.

Fix for the first problem: keep the completeness check in the route AND
make sure nothing else calls the script; or pass a completeness result
into the script and refuse there.

WHY THIS WORKS AS AN ANSWER
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It shows that a guard living outside the atomic script can be bypassed,
with the actual wrong relegation as evidence, and explains the burned ID
from the order of operations rather than treating it as a bug.