; persist/idempotency — exactly-once append under retries. A command retried
; after a network blip must not append its event twice. The caller supplies an
; idempotency key; the first append for that (stream, key) stores the event and
; remembers the key in the kv facet; a repeat returns the SAME event without
; appending. Because the marker lives in kv, idempotency holds across a restart
; too. Keyed per stream. Requires: lib/persist/log.sx, lib/persist/kv.sx.

(define persist/idem-key (fn (stream key) (str "idem/" stream "/" key)))

; true if an append-once has already been recorded for (stream, key)
(define
  persist/seen?
  (fn (b stream key) (persist/kv-has? b (persist/idem-key stream key))))

; append at most once per (stream, key). Returns the stored event either way —
; freshly appended on first use, the remembered one on a repeat.
(define
  persist/append-once
  (fn
    (b stream key type at data)
    (let
      ((k (persist/idem-key stream key)))
      (if
        (persist/kv-has? b k)
        (persist/kv-get b k)
        (let
          ((ev (persist/append b stream type at data)))
          (begin (persist/kv-put b k ev) ev))))))