writeonce/docs/examples/porch/middleware/idempotent.wo

-- porch/middleware/idempotent.wo — idempotency, actor-run.
-- Iteration 1 of the porch track, porch-store task 4.
--
-- Rebuilt, not patched: the old before/after shape ran the handler and
-- stored the response afterward, so two simultaneous duplicates both
-- missed and both executed — before() has nothing to find until after()
-- runs, which is too late for the request that is racing it. The fix is
-- that the ACTOR runs the handler: Idempotent wraps the route's own
-- Handler and hands both the request and that handler to the pool. A
-- duplicate for the same key then simply waits in the actor's mailbox
-- (one message at a time) and is dequeued once the owner's receive has
-- already committed the row — no in-flight heuristic needed, because
-- there is no window where a duplicate can see "nothing yet".
--
-- `call`'s reply is a copyable scalar only (WO-E226): keypool.wo's kind-2
-- arm answers with the SAME pool_pack(count, remaining_ms) encoding kind-1
-- uses, never the response itself. The response travels through the
-- @table instead — the actor stores it, this file reads the same row
-- back and builds the Resp from it, so owner and duplicate answer with
-- byte-identical bytes structurally, not by careful bookkeeping.
use http
use json

-- Idempotent wraps a route's Handler. Registration: Idempotent { key_header:
-- "Idempotency-Key", pool: p, inner: CreateOrder {} } in place of the bare
-- handler in app.post(...). Only the digest allowlists content-type for
-- replay (never Set-Cookie, never Date) — cookies arrive in porch 2.
pub class Idempotent {
  key_header:   Text        -- e.g., "Idempotency-Key"
  pool:         Pool
  inner:        Handler     -- the real route handler; the actor runs this
  include_body: Bool = true -- digest method+path+body, refuse a key reused with a different one
  ttl:          Int = 86_400_000_000  -- 24h in µs, lazy-expired on access

  fn handle(req: Req) -> Resp {
    let header_val = req.headers[self.key_header];
    if header_val == nil { return self.inner.handle(req); }

    let key = "idem:${self.key_header}:${header_val}";
    let digest = "";
    if self.include_body {
      let digest_input = "${req.method}|${req.path}|${req.body}";
      digest = base64_encode(bytes_slice(sha256(bytes_of_text(digest_input)), 0, 16));
    }

    let raw = try pool_begin(self.pool, key, digest, self.ttl, req, self.inner) catch (e) nil;
    if raw == nil {
      let r = Resp { status: 503, headers: {}, body: "{\"error\":\"idempotency store saturated\"}" };
      set_header(r, "content-type", "application/json");
      set_header(r, "retry-after", "1");
      return r;
    }

    let outcome = raw / 1_000_000_000;
    if outcome == 2 {
      -- Same key, a different request: refuse rather than serve the
      -- other request's response.
      let r = Resp { status: 422, headers: {}, body: "{\"error\":\"idempotency key reused with a different request\"}" };
      set_header(r, "content-type", "application/json");
      return r;
    }

    -- Outcome 1: the bare key names a durable (2xx/3xx) replay target --
    -- this file never deletes it; it is the whole point of a durable row.
    -- Outcome 3: this call's own 4xx/5xx answer lives under a row keyed
    -- by a nonce (the reply's low digits) that nobody else's message
    -- for this same bare key ever writes to -- rebuild that exact key
    -- rather than reading the bare one, so a race with a LATER message
    -- for this key (which never touches this row) can't hand back the
    -- wrong response.
    let lookup_key = key;
    if outcome == 3 { lookup_key = "${key}#eph:${raw % 1_000_000_000}"; }
    let hits = from k in IdempotencyKey where k.key == lookup_key take 1 select k;
    if len(hits) == 0 { return server_error(); }
    let row = hits[0];
    let stored = json.decode(row.response) as IdempotentStoredResp;
    if stored == nil { return server_error(); }
    -- `.. ""` forces a fresh, independently-owned Text for every key/value
    -- copied out of the decoded record: json.decode's Text values do not
    -- survive being handed onward as-is once the decoded record itself
    -- goes out of scope (a stale row read back corrupted mid-response
    -- otherwise) — concat is documented to always allocate new owned text.
    let hdrs: map<Text, Text> = {};
    for k, v in stored.headers { hdrs[k .. ""] = v .. ""; }
    let result = Resp { status: stored.status, headers: hdrs, body: stored.body .. "" };
    if outcome == 3 {
      -- Safe to delete here, unlike the shared bare-key row rounds 1/2
      -- removed: the nonce that names this row was never handed to
      -- anyone but this one call() reply, so no other request -- a
      -- duplicate, a retry, anything -- can ever construct this exact
      -- key to read it. Deleting it removes the leak AND the
      -- nonce-wraparound collision (time.ticks() % 1_000_000_000 repeats
      -- every ~1000s; a lingering row from an earlier failed attempt
      -- landing on the same nonce would otherwise be there to collide
      -- with, or worse, poison the actor's OWN unguarded insert on the
      -- next failure for this key).
      delete row;
    }
    return result;
  }
}

-- JSON shape of IdempotencyKey.response. Allowlisted headers only:
-- content-type, never Set-Cookie or Date.
typedef IdempotentStoredResp = {
  status:  Int,
  headers: map<Text, Text>,
  body:    Text
}