; persist/concurrency — optimistic concurrency for the log facet. The caller
; passes the seq it believes is current (the last-seq it last observed). If the
; stream has advanced since, the append is refused and a conflict VALUE is
; returned — never a crash, never a silent overwrite. The caller re-reads the
; tail and retries. This is the substrate-level answer to "two writers, one
; stream": the loser gets a result it can act on.
; Requires: lib/persist/log.sx.

(define
  persist/append-expect
  (fn
    (b stream expected type at data)
    (let
      ((actual (persist/last-seq b stream)))
      (if
        (= actual expected)
        (persist/append b stream type at data)
        {:actual actual :expected expected :conflict true}))))

(define
  persist/conflict?
  (fn (r) (if (has-key? r :conflict) (get r :conflict) false)))
(define persist/conflict-expected (fn (r) (get r :expected)))
(define persist/conflict-actual (fn (r) (get r :actual)))