Premier League Predictor: FastAPI & Redis — Chapter 5, Exercise 2
=================================================================

TASK
Extend the Lua script so it refuses to write when the fixture itself does
not exist, returning -1 for that case (0 stays "locked", 1 stays
"written"). Run it against a nonexistent fixture, an open fixture, and a
locked fixture, and report all three results.

SOLUTION

  PREDICT_LUA = """
  if redis.call('EXISTS', KEYS[1]) == 0 then return -1 end   -- no such fixture
  local hs = redis.call('HGET', KEYS[1], 'home_score')
  if hs and hs ~= '' then return 0 end                        -- locked
  redis.call('HSET', KEYS[2], ARGV[1], ARGV[2])
  return 1                                                    -- written
  """
  predict = r.register_script(PREDICT_LUA)

Real results:

  nonexistent fixture:777 -> -1   (fixture:777:predictions exists? 0)
  open fixture:1          ->  1   (user stored: '2-1')
  locked fixture:1 (result 2-1 entered) -> 0   (user still '2-1', not '9-9')

Each of the three outcomes is distinct, and the nonexistent case leaves
no orphan hash behind: EXISTS on the predictions key afterwards returns 0.

The route can map the three return values to HTTP responses: -1 -> 404,
0 -> 409 (locked), 1 -> 200/201.

WHY THIS WORKS AS AN ANSWER
----------------------------
The existence check sits inside the same script as the lock check and the
write, so all three are atomic together. Doing EXISTS as a separate call
before the script would reopen a (small) gap between them; putting it in
the script is what makes it a real guard rather than a courtesy check.