The background worker's key window is 250 keys (was 25: PullCap's hourly subscription ran out after 19 payments with the app closed, "key window exhausted"), derived once per account and memoized; 60 receive preimages (was 15). When the worker can't act on a request it still wakes the user, and now leaves a "wait, open the app" error with the notifier that goes out only if nobody answers within 20 s, so the other app hears something instead of silence and a sibling's real payment is never shouted over. A request older than its two-minute window (a push that waited at APNs) is ignored quietly instead of showing "Payment request"

Co-Authored-By: Claude Fable 5.1 <[email protected]>

4 files changed, 86 insertions, 21 deletions

diff --git a/src/features/ark.js b/src/features/ark.js
index 0a529c5..b8e83a7 100644
--- a/src/features/ark.js
+++ b/src/features/ark.js
@@ -995,6 +995,26 @@ export function arkFeature(ctx) {
   // connections/offer belong to the nwc feature; when it isn't the caller
   // (an auto-withdraw setting changed, say) carry forward what's mirrored
   // rather than blanking someone's wallet connections.
+  // The mirror carries hundreds of pre-derived keys (BG_KEY_LOOKAHEAD) and
+  // is rewritten after every state change: derive each index once per
+  // account, not once per write.
+  const bgKeyMemo = new WeakMap(); // account node -> Map('chain:i' -> hex)
+  function bgKeyFor(account) {
+    let m = bgKeyMemo.get(account);
+    if (!m) { m = new Map(); bgKeyMemo.set(account, m); }
+    const chains = new Map();
+    return (chain, i) => {
+      const k = chain + ':' + i;
+      let v = m.get(k);
+      if (!v) {
+        let node = chains.get(chain);
+        if (!node) { node = account.deriveChild(chain); chains.set(chain, node); }
+        v = hex.encode(node.deriveChild(i).privateKey);
+        m.set(k, v);
+      }
+      return v;
+    };
+  }
   async function writeBgMirror(connections, offer, prefs) {
     const cfg = getArkConfig();
     if (!cfg || !ark || !ark.info || !ark.state) return;
@@ -1006,7 +1026,7 @@ export function arkFeature(ctx) {
       ark: { arkUrl: cfg.ark, esploraUrl: cfg.esplora, network: getNetwork(), serverPubkey: ark.info.serverPubkey, honorsExpired: !!cfg.honorsExpired },
       mgrState: ark.state,
       custody: getCustody(),
-      keyFor: (chain, i) => hex.encode(ark.account.deriveChild(chain).deriveChild(i).privateKey),
+      keyFor: bgKeyFor(ark.account),
       connections,
       offer,
       autowithdraw: await awMirror(prev),
diff --git a/src/nwc-bg.js b/src/nwc-bg.js
index ab083d9..9e7a303 100644
--- a/src/nwc-bg.js
+++ b/src/nwc-bg.js
@@ -133,8 +133,15 @@ export async function allBgs() {
 }
 
 // How many upcoming change/refund indices travel with the record: the worker
-// cannot derive keys, so its allocations must land inside this window.
-export const BG_KEY_LOOKAHEAD = 25;
+// cannot derive keys, so its allocations must land inside this window. Each
+// Lightning send burns a change and an HTLC index, so 25 keys lasted PullCap
+// about 19 hourly payments (2026-10-26, "key window exhausted" after a day
+// with the app closed). 250 keys are 16 KB in the record and cover weeks of
+// hourly payments, or years of monthly ones; the app tops the window up
+// every time it writes the mirror.
+export const BG_KEY_LOOKAHEAD = 250;
+// ...and the receive preimages a closed wallet can mint invoices with
+export const BG_RECV_LOOKAHEAD = 60;
 
 // { ark } endpoints, the manager state to mirror, and a keyFor(chain, index)
 // derive function (the caller owns the account node — it never enters this
@@ -153,7 +160,7 @@ export function buildBg({ ark, mgrState, keyFor, connections, offer, autowithdra
   idx.add(0); // the receive-address key: mailbox/refund defaults land on it
   for (const i of idx) keys[String(i)] = keyFor(3, i);
   const keys5 = {};
-  for (let i = 0; i < 15; i++) {
+  for (let i = 0; i < BG_RECV_LOOKAHEAD; i++) {
     const n = (mgrState.nextLnRecvIndex || 0) + i;
     keys5[String(n)] = keyFor(5, n);
   }
diff --git a/src/nwc-respond.js b/src/nwc-respond.js
index 0778813..6ea23ab 100644
--- a/src/nwc-respond.js
+++ b/src/nwc-respond.js
@@ -24,6 +24,13 @@ import { budgetRenewsAt, renewalPeriod, spentInWindow } from './nwc-budget.js';
 import { renewPolicy } from './ark/manager.js';
 
 const MAX_AGE_SEC = 120;
+// A request the worker can't act on (keys or coins short in the mirror, an
+// old mirror) wakes the user — and, unless someone answers first, tells the
+// other app so (PullCap read the silence as "no answer" and suspended the
+// subscription; with an error it can tell the payer their wallet is waiting
+// for them). The notifier holds the reply this long before sending it.
+const DECLINE_AFTER_MS = 20_000;
+const WAIT_MSG = 'Coinos is waiting for its owner to open the app and approve. Look this request up again later.';
 const PAY_WAIT_MS = 20_000; // inside the SW's ~30s budget, with reply margin
 const nowSec = () => Math.floor(Date.now() / 1000);
 const errRes = (t, code, message) => ({ result_type: t, error: { code, message } });
@@ -206,7 +213,10 @@ export async function respondFromBg(data, {
 } = {}) {
   const ev = data && data.event;
   if (!ev || !ev.id || !ev.pubkey) return false;
-  if (nowSec() - (ev.created_at || 0) > MAX_AGE_SEC) return false;
+  // Older than the request window (a push that sat at APNs while the phone
+  // was out of reach): the other app has moved on — nothing to answer, and
+  // no "Payment request" to show for it. The wake still tends what's in flight.
+  if (nowSec() - (ev.created_at || 0) > MAX_AGE_SEC) { log(`request ${String(ev.id).slice(0, 8)} is ${nowSec() - ev.created_at} s old — ignored`); return true; }
 
   const target = (ev.tags?.find((t) => t[0] === 'p') || [])[1];
   if (!target) return false;
@@ -250,6 +260,20 @@ export async function respondFromBg(data, {
   try { req = JSON.parse(await decrypt(scheme, conn.serviceSk, ev.pubkey, ev.content)); } catch { return false; }
   const method = req?.method;
   const params = req?.params || {};
+  // Wake the user, out loud: the wait reply is left with the notifier, which
+  // sends it only if nobody (a sibling device, the woken user) answers in
+  // time — so it can never land on top of a real payment.
+  const declineLater = async (why) => {
+    log(why + ' — waking the user; the app is told to wait');
+    try {
+      const evt = await buildReply(conn, ev, scheme, errRes(method || 'error', 'OTHER', WAIT_MSG));
+      await fetchFn(`${notifier}/decline`, {
+        method: 'POST', headers: { 'content-type': 'application/json' },
+        body: JSON.stringify({ event: evt, afterMs: DECLINE_AFTER_MS }),
+      });
+    } catch (e) { log('could not leave the wait reply: ' + (e && e.message)); }
+    return false;
+  };
   // the push's budget (on iOS about ten seconds from the push, not thirty)
   const deadline = until || Date.now() + PAY_WAIT_MS;
   let lookupNotice = null;
@@ -301,9 +325,9 @@ export async function respondFromBg(data, {
   if (method === 'make_invoice') {
     const sat = Math.floor((params.amount || 0) / 1000);
     if (!sat) { await publish(errRes('make_invoice', 'OTHER', 'amount required (msat)')); return true; }
-    if (!rec.keys5 || !rec.key4 || !rec.mgr) return false; // old mirror: wake the user
-    if (!rec.keys5[String(rec.mgr.nextLnRecvIndex || 0)]) return false; // preimage window exhausted
-    if (!rec.keys[String(rec.mgr.nextKeyIndex || 1)]) return false; // claim key window exhausted
+    if (!rec.keys5 || !rec.key4 || !rec.mgr) return declineLater('old mirror, no receive keys');
+    if (!rec.keys5[String(rec.mgr.nextLnRecvIndex || 0)]) return declineLater('preimage window exhausted');
+    if (!rec.keys[String(rec.mgr.nextKeyIndex || 1)]) return declineLater('key window exhausted');
     let a;
     try {
       const mgr = await bgManager(rec, walletKey, saveFn);
@@ -340,7 +364,7 @@ export async function respondFromBg(data, {
       tx = view.lnLookup(q);
       if (tx) { ({ walletKey, rec, conn } = m); break; }
     }
-    if (!tx && !rec.mgr) return false;
+    if (!tx && !rec.mgr) return declineLater('old mirror, no state to look up');
     // a payment of ours still on its way (incoming: claim it; outgoing: a
     // send an earlier wake began — drive it on, so the lookup itself can
     // answer 'settled' with the preimage instead of a second pay request)
@@ -512,13 +536,13 @@ export async function respondFromBg(data, {
   const withNotices = (out) => (notices.length && out ? { ...(out === true ? {} : out), notices } : out);
 
   const spendable = spendableSat(rec);
-  if (spendable < dec.amountSat) { log(`${spendable} sat spendable in the mirror, ${dec.amountSat} asked — waking the user`); return false; }
+  if (spendable < dec.amountSat) return declineLater(`${spendable} sat spendable in the mirror, ${dec.amountSat} asked`);
 
   // The ASP's own Lightning send, over the mirrored state. Change and HTLC
   // outputs allocate from nextKeyIndex, which must stay inside the key
   // window the app pre-derived; if it ran out, wake the user instead.
-  if (!rec.keys[String(rec.mgr.nextKeyIndex || 1)]) { log('key window exhausted'); return false; }
-  try { await ready(); } catch (e) { log('ark init failed: ' + e.message); return false; }
+  if (!rec.keys[String(rec.mgr.nextKeyIndex || 1)]) return declineLater('key window exhausted');
+  try { await ready(); } catch (e) { return declineLater('ark init failed: ' + e.message); }
 
   if (!(await claim())) { log('another device claimed this request — standing down'); return true; }
   // whose send this is, for the wake that finishes it should this one not
diff --git a/tools/nwc-sw-test.js b/tools/nwc-sw-test.js
index 5a15d48..376a12e 100644
--- a/tools/nwc-sw-test.js
+++ b/tools/nwc-sw-test.js
@@ -47,11 +47,16 @@ function makeReq(method, params = {}, scheme = 'nip44_v2', ageSec = 0) {
 
 function harness({ rec = baseRec(), recs = null, answered = false } = {}) {
   const published = [];
+  const declined = []; // wait replies left with the notifier (sent only if nobody answers)
   const fetchFn = async (url, opts) => {
     if (url.includes('/publish')) {
       published.push(JSON.parse(opts.body).event);
       return { ok: true, json: async () => ({ ok: true }) };
     }
+    if (url.includes('/decline')) {
+      declined.push(JSON.parse(opts.body).event);
+      return { ok: true, json: async () => ({ ok: true, scheduled: true }) };
+    }
     if (url.includes('/claim')) {
       return { ok: true, json: async () => ({ ok: true }) };
     }
@@ -73,7 +78,7 @@ function harness({ rec = baseRec(), recs = null, answered = false } = {}) {
       ? nip44.decrypt(ev.content, nip44.getConversationKey(clientSk, conn.servicePk))
       : nip04.decrypt(clientSk, conn.servicePk, ev.content));
   };
-  return { deps, published, decryptReply };
+  return { deps, published, declined, decryptReply };
 }
 
 const ev = (e) => JSON.parse(JSON.stringify(e)); // strip verified symbol like a push payload would
@@ -137,15 +142,19 @@ console.log('\n[merchant methods (BTCPay)]');
   check('…one still in flight (ASP unreachable here) → outgoing, pending, inside the push budget', r2.type === 'outgoing' && r2.state === 'pending' && Date.now() - t0 < 4000, JSON.stringify(r2) + ` ${Date.now() - t0} ms`);
 }
 {
-  const { deps } = harness(); // a mirror without receive keys
-  check('make_invoice on an old mirror → wake the user', (await respondFromBg({ type: 'nwc', event: ev(makeReq('make_invoice', { amount: 21000 })) }, deps)) === false);
+  const { deps, declined, decryptReply } = harness(); // a mirror without receive keys
+  const handled = await respondFromBg({ type: 'nwc', event: ev(makeReq('make_invoice', { amount: 21000 })) }, deps);
+  const r = declined.length ? decryptReply(declined[0]) : null;
+  check('make_invoice on an old mirror → wake the user, wait reply left behind', handled === false && declined.length === 1 && r.result_type === 'make_invoice' && r.error?.code === 'OTHER', JSON.stringify(r));
 }
 
 console.log('\n[guards]');
 {
-  const { deps } = harness();
+  const { deps, published, declined } = harness();
   const stale = ev(makeReq('get_balance', {}, 'nip44_v2', 300));
-  check('stale request refused', (await respondFromBg({ type: 'nwc', event: stale }, deps)) === false);
+  // a push that sat at APNs past the request window: nothing to answer, and
+  // no "Payment request" for it either (true = handled quietly)
+  check('stale request ignored quietly', (await respondFromBg({ type: 'nwc', event: stale }, deps)) === true && !published.length && !declined.length);
 }
 {
   const { deps } = harness();
@@ -185,16 +194,21 @@ const INV21 = 'lnbc210n1p4xuk2wpp506wkjr0xk3677nu7je9c55vq4lzlkyd0ztcq2mlvumap0z
 {
   const rec = baseRec();
   rec.mgr.vtxos = []; // nothing spendable in the mirror
-  const { deps } = harness({ rec });
+  const { deps, published, declined, decryptReply } = harness({ rec });
   const handled = await respondFromBg({ type: 'nwc', event: ev(makeReq('pay_invoice', { invoice: INV21 })) }, deps);
-  check('empty mirror → wake the user', handled === false);
+  const r = declined.length ? decryptReply(declined[0]) : null;
+  // the wait reply is NOT published by the worker: the notifier holds it and
+  // sends it only if nobody answers — a sibling's payment can't be shouted over
+  check('empty mirror → wake the user, wait reply left with the notifier', handled === false && published.length === 0 && declined.length === 1
+    && r.result_type === 'pay_invoice' && r.error?.code === 'OTHER' && /open the app/.test(r.error.message), JSON.stringify(r));
+  check('…addressed to the request', declined[0].kind === 23195 && declined[0].tags.some((t) => t[0] === 'e'));
 }
 {
   const rec = baseRec();
   delete rec.keys['5']; // change index outside the mirrored window
-  const { deps } = harness({ rec });
+  const { deps, declined } = harness({ rec });
   const handled = await respondFromBg({ type: 'nwc', event: ev(makeReq('pay_invoice', { invoice: INV21 })) }, deps);
-  check('exhausted key window → wake the user', handled === false);
+  check('exhausted key window → wake the user, wait reply left behind', handled === false && declined.length === 1);
 }
 
 console.log('\n[pay outcome]');