1	# Durable job runner: a journaled scheduler queue, bounded by a resources pool, with retry and a breaker per target. Source: https://github.com/paymog/bend-kit/tree/main/runner
2	import Base
3	import bend-kit-bytes@0.3.3.0/bytes.bend as Bytes
4	import bend-kit-journal@0.3.0.1/journal.bend as J
5	import bend-kit-scheduler@0.4.1.0/queue.bend as Q
6	import bend-kit-resources@0.1.0.0/resources.bend as Res
7	import bend-kit-retry@0.3.0.0/retry.bend as Retry
8	import bend-kit-time@0.3.0.0/time.bend as Time
9	import bend-kit-collections@0.1.4.1/heap.bend as H
10	import bend-kit-collections@0.1.4.1/omap.bend as O
11	import bend-kit-collections@0.1.4.1/hmap.bend as M
12	import bend-kit-collections@0.1.4.1/LAWS.bend as CL
13	import bend-kit-collections@0.1.4.1/PROOF.bend as CP
14	import bend-kit-files@0.3.0.0/files.bend as Files
15	
16	# Why retry gave up on a ticket: its budget was spent, its deadline passed, its failure must not repeat,
17	# or another try would repeat its failure.
18	type Why is Data:
19	  WhySpent{}
20	  WhyLate{}
21	  WhyUnsafe{}
22	  WhyFailed{}
23	
24	# A ticket that retry gave up on, with its payload, and when it gave up (wall-clock ms).
25	# It stays until Revive, Cancel, or Purge takes it.
26	type Dead<-A: Data> is Data:
27	  Dead{ticket: Q.Ticket<A>, why: Why, at: Nat}
28	
29	# One durable transition. Each one is one journal record, appended before the state changes.
30	# Select: the queue's select at now. Replay runs it at the same now, so it takes the same ticket.
31	# Requeue: the attempt failed; it counts one failure. Defer: the breaker held the ticket back; no attempt ran.
32	# Abandon: retry gave up at at; the running ticket becomes dead. Revive: a dead ticket is pending again at deadline, with a new budget.
33	# Purge: the dead tickets abandoned before before are dropped. PurgeAll: every dead ticket is dropped.
34	# Break: the breaker of target changed to state.
35	type Op<-A: Data> is Data:
36	  Enqueue{deadline: Nat, payload: A}
37	  Select{now: Nat}
38	  Complete{id: Nat}
39	  Cancel{id: Nat}
40	  Requeue{id: Nat, deadline: Nat}
41	  Defer{id: Nat, deadline: Nat}
42	  Abandon{id: Nat, why: Why, at: Nat}
43	  Revive{id: Nat, deadline: Nat}
44	  Purge{before: Nat}
45	  PurgeAll{}
46	  Break{target: String, state: Retry.State}
47	
48	# The failed attempts of a ticket, and its first deadline, from which the policy deadline counts.
49	type Try is Data:
50	  Try{fails: Nat, start: Nat}
51	
52	# A Try for each ticket that is not yet complete, cancelled, or dead, by ticket id.
53	def Tries() -> Data:
54	  O.OMap<Nat, Try>
55	
56	# The breaker state of each target that has one, by target. An absent target is closed with no failures.
57	def Breakers() -> Data:
58	  Map<&2, Retry.State>
59	
60	def dead.hash(i: Nat) -> U32:
61	  U32.from_nat(i)
62	
63	# The dead tickets, by ticket id.
64	def Graves(-A: Data) -> Data:
65	  M.HMap<Nat, Dead<A>>
66	
67	# The durable state: the queue, the tries, the dead tickets, and the breakers.
68	type State<-A: Data> is Data:
69	  State{queue: Q.State<A>, tries: Tries(), dead: Graves(A), breakers: Breakers()}
70	
71	def new(-A: Data) -> State<A>:
72	  State{Q.new(A), O.OMap.new(Nat, Try), M.HMap.new(Nat, Dead<A>), Map.new(&2, Retry.State)}
73	
74	def queue(-A: Data, s: State<A>) -> Q.State<A>:
75	  State{q, ts, ds, bs} = s
76	  q
77	
78	def tries(-A: Data, s: State<A>) -> Tries():
79	  State{q, ts, ds, bs} = s
80	  ts
81	
82	# The dead tickets, by id. A Revive of one makes it pending again; a Cancel or a Purge drops it.
83	def dead(-A: Data, s: State<A>) -> Graves(A):
84	  State{q, ts, ds, bs} = s
85	  ds
86	
87	def breakers(-A: Data, s: State<A>) -> Breakers():
88	  State{q, ts, ds, bs} = s
89	  bs
90	
91	# ---- dead tickets
92	
93	def dead.get(~A: Data, ds: Graves(A), +i: Nat) -> Maybe<&2, Dead<A>>:
94	  M.HMap.get(~Nat, ~Dead<A>, ~dead.hash, ~Nat.is_eq, ds, i)
95	
96	def dead.drop(~A: Data, ds: Graves(A), +i: Nat) -> Graves(A):
97	  M.HMap.del(~Nat, ~Dead<A>, ~dead.hash, ~Nat.is_eq, ds, i)
98	
99	def dead.bury(~A: Data, m: Maybe<&2, Q.Ticket<A>>, ds: Graves(A), w: Why, +at: Nat) -> Graves(A):
100	  match m:
101	    case None{}:
102	      ds
103	    case Some{Q.Ticket{+i, d, x}}:
104	      M.HMap.put(~Nat, ~Dead<A>, ~dead.hash, ~Nat.is_eq, ds, i, Dead{Q.Ticket{i, d, x}, w, at})
105	
106	def dead.list.entries(-A: Data, es: List<&2, M.Entry<Nat, Dead<A>>>, acc: List<&2, Dead<A>>) -> List<&2, Dead<A>>:
107	  match es:
108	    case Nil{}:
109	      acc
110	    case Con{M.Entry{k, d}, rest}:
111	      dead.list.entries(A, rest, Con{d, acc})
112	
113	def dead.list.go(-A: Data, ds: Graves(A), acc: List<&2, Dead<A>>) -> List<&2, Dead<A>>:
114	  match ds:
115	    case M.HNil{}:
116	      acc
117	    case M.HLeaf{h, es}:
118	      dead.list.entries(A, es, acc)
119	    case M.HBranch{lo, hi}:
120	      dead.list.go(A, lo, dead.list.go(A, hi, acc))
121	
122	# The dead tickets, in no set order.
123	def dead.list(-A: Data, ds: Graves(A)) -> List<&2, Dead<A>>:
124	  dead.list.go(A, ds, Nil{})
125	
126	def dead.old.keep(-A: Data, old: Bool, +i: Nat, rest: List<&2, Nat>) -> List<&2, Nat>:
127	  match old:
128	    case True{}:
129	      Con{i, rest}
130	    case False{}:
131	      rest
132	
133	# The ids of the dead tickets abandoned before t.
134	def dead.old(-A: Data, ds: List<&2, Dead<A>>, +t: Nat) -> List<&2, Nat>:
135	  match ds:
136	    case Nil{}:
137	      Nil{}
138	    case Con{Dead{Q.Ticket{i, d, x}, w, +at}, rest}:
139	      dead.old.keep(A, Nat.is_lt(at, t), i, dead.old(A, rest, t))
140	
141	def dead.drops(~A: Data, ks: List<&2, Nat>, ds: Graves(A)) -> Graves(A):
142	  match ks:
143	    case Nil{}:
144	      ds
145	    case Con{k, rest}:
146	      dead.drops(~A, rest, dead.drop(~A, ds, k))
147	
148	# The dead tickets without those abandoned before t. It walks every dead ticket once.
149	def dead.purge(~A: Data, +ds: Graves(A), +t: Nat) -> Graves(A):
150	  dead.drops(~A, dead.old(A, dead.list(A, ds), t), ds)
151	
152	# ---- tries
153	
154	def tries.drop(ts: Tries(), +i: Nat) -> Tries():
155	  O.OMap.del(~Nat, ~Try, ~Nat.cmp, ts, i)
156	
157	def tries.get(ts: Tries(), +i: Nat) -> Maybe<&2, Try>:
158	  O.OMap.get(~Nat, ~Try, ~Nat.cmp, ts, i)
159	
160	def tries.bump.got(m: Maybe<&2, Try>, ts: Tries(), +i: Nat) -> Tries():
161	  match m:
162	    case None{}:
163	      ts
164	    case Some{Try{f, st}}:
165	      O.OMap.put(~Nat, ~Try, ~Nat.cmp, ts, i, Try{1n+f, st})
166	
167	def tries.bump(+ts: Tries(), +i: Nat) -> Tries():
168	  tries.bump.got(tries.get(ts, i), ts, i)
169	
170	# ---- breakers
171	
172	def brk.got(r: Breakers() & Retry.State) -> Retry.State:
173	  (m, b) = r
174	  b
175	
176	# The breaker state of target key.
177	def brk.get(-A: Data, s: State<A>, key: String) -> Retry.State:
178	  brk.got(Map.get(Retry.State, Retry.CircuitClosed{Nil{}}, breakers(A, s), key))
179	
180	def brk.state(br: Retry.Breaker, old: Retry.State) -> Retry.State:
181	  match br:
182	    case Retry.BreakerOff{}:
183	      old
184	    case Retry.BreakerOn{c, st}:
185	      st
186	
187	def brk.nats(a: List<&2, Nat>, b: List<&2, Nat>) -> Bool:
188	  match a b:
189	    case Nil{} Nil{}:
190	      True{}
191	    case Con{+x, xs} Con{+y, ys}:
192	      Bool.and(Nat.is_eq(x, y), brk.nats(xs, ys))
193	    case p q:
194	      False{}
195	
196	def brk.same(a: Retry.State, b: Retry.State) -> Bool:
197	  match a b:
198	    case Retry.CircuitClosed{xs} Retry.CircuitClosed{ys}:
199	      brk.nats(xs, ys)
200	    case Retry.CircuitOpen{+x} Retry.CircuitOpen{+y}:
201	      Nat.is_eq(x, y)
202	    case Retry.CircuitHalf{+x} Retry.CircuitHalf{+y}:
203	      Nat.is_eq(x, y)
204	    case p q:
205	      False{}
206	
207	def calm.state(b: Retry.State) -> Retry.State:
208	  match b:
209	    case Retry.CircuitClosed{fs}:
210	      Retry.CircuitClosed{fs}
211	    case Retry.CircuitOpen{u}:
212	      Retry.CircuitOpen{u}
213	    case Retry.CircuitHalf{k}:
214	      Retry.CircuitHalf{0n}
215	
216	def calm.map(m: Breakers()) -> Breakers():
217	  match m:
218	    case MTip{}:
219	      MTip{}
220	    case MLeaf{k, v}:
221	      MLeaf{k, calm.state(v)}
222	    case MNode{p, lo, hi}:
223	      MNode{p, calm.map(lo), calm.map(hi)}
224	
225	# The probes of a half-open breaker died with the process that ran them, so open counts none in flight.
226	def calm(-A: Data, s: State<A>) -> State<A>:
227	  State{q, ts, ds, bs} = s
228	  State{q, ts, ds, calm.map(bs)}
229	
230	# ---- the transition
231	
232	def step.enqueued(-A: Data, r: Q.State<A> & Nat, ts: Tries(), ds: Graves(A), bs: Breakers(), d: Nat) -> State<A>:
233	  (q, i) = r
234	  State{q, O.OMap.put(~Nat, ~Try, ~Nat.cmp, ts, i, Try{0n, d}), ds, bs}
235	
236	def step.selected(-A: Data, r: Q.State<A> & Maybe<&2, Q.Ticket<A>>, ts: Tries(), ds: Graves(A), bs: Breakers()) -> State<A>:
237	  (q, t) = r
238	  State{q, ts, ds, bs}
239	
240	def step.requeued(-A: Data, r: Q.State<A> & Bool, ts: Tries(), ds: Graves(A), bs: Breakers()) -> State<A>:
241	  (q, b) = r
242	  State{q, ts, ds, bs}
243	
244	# Complete takes the ticket from the running list only: O(running), not O(pending).
245	def step.completed(-A: Data, r: Q.State<A> & Maybe<&2, Q.Ticket<A>>, ts: Tries(), ds: Graves(A), bs: Breakers()) -> State<A>:
246	  (q, t) = r
247	  State{q, ts, ds, bs}
248	
249	# Abandon takes the ticket from the running list, as Complete does, and keeps it with its payload.
250	def step.abandoned(~A: Data, r: Q.State<A> & Maybe<&2, Q.Ticket<A>>, ts: Tries(), ds: Graves(A), bs: Breakers(), w: Why, +at: Nat) -> State<A>:
251	  (q, t) = r
252	  State{q, ts, dead.bury(~A, t, ds, w, at), bs}
253	
254	# The revived ticket keeps its payload and takes id i, the key it was dead under.
255	def step.revived(~A: Data, m: Maybe<&2, Dead<A>>, +i: Nat, n: Nat, h: H.Heap<Q.Ticket<A>>, r: List<&2, Q.Ticket<A>>, ts: Tries(), ds: Graves(A), bs: Breakers(), +d: Nat) -> State<A>:
256	  match m:
257	    case None{}:
258	      State{Q.State{n, h, r}, ts, ds, bs}
259	    case Some{Dead{Q.Ticket{k, old, x}, w, at}}:
260	      State{Q.State{n, H.Heap.push(~Q.Ticket<A>, ~(a => b => Q.le(A, a, b)), Q.Ticket{i, d, x}, h), r}, O.OMap.put(~Nat, ~Try, ~Nat.cmp, ts, i, Try{0n, d}), ds, bs}
261	
262	# A closed breaker with no failures reads the same as an absent one, so it leaves the map.
263	def step.broke(bs: Breakers(), k: String, b: Retry.State) -> Breakers():
264	  match b:
265	    case Retry.CircuitClosed{Nil{}}:
266	      Map.del(&2, Retry.State, bs, k)
267	    case b:
268	      Map.set(&2, Retry.State, bs, k, b)
269	
270	# The state after op. Live and replay both run it, so they agree.
271	def step(~A: Data, s: State<A>, op: Op<A>) -> State<A>:
272	  match s op:
273	    case State{q, ts, ds, bs} Enqueue{+d, x}:
274	      step.enqueued(A, Q.enqueue(~A, d, x, q), ts, ds, bs, d)
275	    case State{q, ts, ds, bs} Select{now}:
276	      step.selected(A, Q.select(~A, now, q), ts, ds, bs)
277	    case State{q, ts, ds, bs} Complete{+i}:
278	      step.completed(A, Q.complete(A, i, q), tries.drop(ts, i), ds, bs)
279	    case State{q, ts, ds, bs} Cancel{+i}:
280	      State{Q.cancel(~A, i, q), tries.drop(ts, i), dead.drop(~A, ds, i), bs}
281	    case State{q, ts, ds, bs} Requeue{+i, d}:
282	      step.requeued(A, Q.requeue(~A, i, d, q), tries.bump(ts, i), ds, bs)
283	    case State{q, ts, ds, bs} Defer{i, d}:
284	      step.requeued(A, Q.requeue(~A, i, d, q), ts, ds, bs)
285	    case State{q, ts, ds, bs} Abandon{+i, w, at}:
286	      step.abandoned(~A, Q.complete(A, i, q), tries.drop(ts, i), ds, bs, w, at)
287	    case State{Q.State{n, h, r}, ts, +ds, bs} Revive{+i, d}:
288	      step.revived(~A, dead.get(~A, ds, i), i, n, h, r, ts, dead.drop(~A, ds, i), bs, d)
289	    case State{q, ts, ds, bs} Purge{t}:
290	      State{q, ts, dead.purge(~A, ds, t), bs}
291	    case State{q, ts, ds, bs} PurgeAll{}:
292	      State{q, ts, M.HMap.new(Nat, Dead<A>), bs}
293	    case State{q, ts, ds, bs} Break{k, b}:
294	      State{q, ts, ds, step.broke(bs, k, b)}
295	
296	def apply.op(~A: Data, m: Maybe<&2, Op<A>>, s: State<A>) -> Maybe<&2, State<A>>:
297	  match m:
298	    case None{}:
299	      None{}
300	    case Some{op}:
301	      Some{step(~A, s, op)}
302	
303	# Folds one record into the replayed state. A record that does not decode ends the replay with None.
304	def apply(~A: Data, ~R: Type, ~dec: R -> Maybe<&2, Op<A>>, acc: Maybe<&2, State<A>>, r: R) -> Maybe<&2, State<A>>:
305	  match acc:
306	    case None{}:
307	      None{}
308	    case Some{s}:
309	      apply.op(~A, dec(r), s)
310	
311	# A journal record. Compaction writes one Snap of the whole state; every other record is one op.
312	type Rec<-A: Data> is Data:
313	  Snap{state: State<A>}
314	  Do{op: Op<A>}
315	
316	def load.rec(~A: Data, m: Maybe<&2, Rec<A>>, s: State<A>) -> Maybe<&2, State<A>>:
317	  match m:
318	    case None{}:
319	      None{}
320	    case Some{Snap{t}}:
321	      Some{t}
322	    case Some{Do{op}}:
323	      Some{step(~A, s, op)}
324	
325	# Folds one record into the replayed state: a Snap replaces it, and an op steps it, as apply does.
326	# A record that does not decode ends the replay with None.
327	def load(~A: Data, ~R: Type, ~dec: R -> Maybe<&2, Rec<A>>, acc: Maybe<&2, State<A>>, r: R) -> Maybe<&2, State<A>>:
328	  match acc:
329	    case None{}:
330	      None{}
331	    case Some{s}:
332	      load.rec(~A, dec(r), s)
333	
334	# ---- pool
335	
336	# Picked: a due ticket and one unit of the pool for it. Idle: no due ticket, or no free unit; the pool is unchanged.
337	type Pick<-A: Data> is Type:
338	  Picked{pool: Res.Pool, ticket: Q.Ticket<A>, grant: Res.Grant}
339	  Idle{pool: Res.Pool}
340	
341	def pick.took(-A: Data, r: Res.Reserve, t: Q.Ticket<A>) -> Pick<A>:
342	  match r:
343	    case Res.Took{p, g}:
344	      Picked{p, t, g}
345	    case Res.Refused{p}:
346	      Idle{p}
347	
348	def pick.due(-A: Data, m: Maybe<&2, Q.Ticket<A>>, p: Res.Pool) -> Pick<A>:
349	  match m:
350	    case None{}:
351	      Idle{p}
352	    case Some{t}:
353	      pick.took(A, Res.reserve(p, 1n), t)
354	
355	def pick.sel(-A: Data, r: Q.State<A> & Maybe<&2, Q.Ticket<A>>, p: Res.Pool) -> Pick<A>:
356	  (q, m) = r
357	  pick.due(A, m, p)
358	
359	# The ticket that Select{now} takes, with one unit of p, when there is a ticket due and a unit free.
360	def pick(~A: Data, p: Res.Pool, s: State<A>, now: Nat) -> Pick<A>:
361	  State{q, ts, ds, bs} = s
362	  pick.sel(A, Q.select(~A, now, q), p)
363	
364	# ---- retry
365	
366	# The budget left for ticket i: the policy's attempts less its failures, until its start plus the policy deadline.
367	def budget.of(+attempts: Nat, t: Maybe<&2, Try>, d: Maybe<&2, Nat>) -> Retry.Budget:
368	  match t d:
369	    case None{} m:
370	      Retry.Budget{attempts, None{}}
371	    case Some{Try{f, st}} None{}:
372	      Retry.Budget{Nat.sub(attempts, f), None{}}
373	    case Some{Try{f, +st}} Some{ms}:
374	      Retry.Budget{Nat.sub(attempts, f), Some{Nat.add(st, ms)}}
375	
376	def budget(p: Retry.Policy, t: Maybe<&2, Try>) -> Retry.Budget:
377	  Retry.Policy{a, base, cap, d} = p
378	  budget.of(a, t, d)
379	
380	# The failures so far: the index of the next retry.
381	def fails(t: Maybe<&2, Try>) -> Nat:
382	  match t:
383	    case None{}:
384	      0n
385	    case Some{Try{f, st}}:
386	      f
387	
388	# Why retry stopped a transient failure: no attempt is left, or the next wait would reach the deadline.
389	def gave(b: Retry.Budget) -> Why:
390	  Retry.Budget{left, d} = b
391	  match left:
392	    case 0n:
393	      WhySpent{}
394	    case 1n+k:
395	      WhyLate{}
396	
397	# A settled attempt completes its ticket; retry gave up at now on any other.
398	def stopped(-A: Data, o: Retry.Outcome, b: Retry.Budget, +id: Nat, +now: Nat) -> Op<A>:
399	  match o:
400	    case Retry.Settled{}:
401	      Complete{id}
402	    case Retry.Failed{}:
403	      Abandon{id, WhyFailed{}, now}
404	    case Retry.Rejected{}:
405	      Abandon{id, WhyFailed{}, now}
406	    case Retry.Unsafe{}:
407	      Abandon{id, WhyUnsafe{}, now}
408	    case Retry.Transient{w}:
409	      Abandon{id, gave(b), now}
410	
411	def after.plan(-A: Data, pl: Retry.Plan, o: Retry.Outcome, b: Retry.Budget, +id: Nat, +now: Nat) -> Op<A>:
412	  match pl:
413	    case Retry.PlanStop{}:
414	      stopped(A, o, b, id, now)
415	    case Retry.PlanWait{w}:
416	      Requeue{id, Nat.add(now, w)}
417	
418	# The record after an attempt of ticket id ends at now with o: requeue at now plus retry's wait,
419	# complete when o is settled, or abandon when retry gave up.
420	def after(-A: Data, +o: Retry.Outcome, p: Retry.Policy, +b: Retry.Budget, +id: Nat, +n: Nat, +now: Nat, +r: Nat) -> Op<A>:
421	  after.plan(A, Retry.plan(o, p, b, n, now, r), o, b, id, now)
422	
423	# A target's breaker after an attempt ends at now with o.
424	def brk.after(+circ: Retry.Circuit, old: Retry.State, +now: Nat, o: Retry.Outcome) -> Retry.Breaker:
425	  Retry.Breaker.done(Retry.BreakerOn{circ, old}, now, Retry.failed(o))
426	
427	# When an open breaker lets attempts start again.
428	def until(br: Retry.Breaker, +now: Nat) -> Nat:
429	  match br:
430	    case Retry.BreakerOn{c, Retry.CircuitOpen{u}}:
431	      u
432	    case Retry.BreakerOn{c, Retry.CircuitClosed{fs}}:
433	      Nat.add(now, Retry.Circuit.cooldown(c))
434	    case Retry.BreakerOn{c, Retry.CircuitHalf{k}}:
435	      Nat.add(now, Retry.Circuit.cooldown(c))
436	    case Retry.BreakerOff{}:
437	      now
438	
439	# The record for a ticket that admit refused: Defer to when the breaker may admit, or Abandon when its budget is spent or late.
440	def held(-A: Data, why: Retry.Deny, br: Retry.Breaker, +id: Nat, +now: Nat) -> Op<A>:
441	  match why:
442	    case Retry.DenyOpen{}:
443	      Defer{id, until(br, now)}
444	    case Retry.DenyLate{}:
445	      Abandon{id, WhyLate{}, now}
446	    case Retry.DenySpent{}:
447	      Abandon{id, WhySpent{}, now}
448	
449	# ---- the record codec: a u32le tag, then u64le fields; an Enqueue then holds the payload.
450	# An Abandon is tag 8, then id, why.code, and at. A Purge is tag 11 and before; a PurgeAll is tag 12.
451	# A Break is tag 10, then its target and its breaker state.
452	
453	def why.code(w: Why) -> Nat:
454	  match w:
455	    case WhySpent{}:
456	      0n
457	    case WhyLate{}:
458	      1n
459	    case WhyUnsafe{}:
460	      2n
461	    case WhyFailed{}:
462	      3n
463	
464	def W() -> Nat:
465	  Nat.mul(65536n, 65536n)
466	
467	# Nat fields hold values under 2^64.
468	def put.u64(b: Bytes.Bytes, +at: U32, +n: Nat) -> Bytes.Bytes:
469	  Bytes.set.u32le(Bytes.set.u32le(b, at, U32.from_nat(Nat.mod(n, W()))), (at + 4 : U32), U32.from_nat(Nat.div(n, W())))
470	
471	def head(+tag: U32, +a: Nat, +c: Nat) -> Bytes.Bytes:
472	  put.u64(put.u64(Bytes.set.u32le(Bytes.new(20), 0, tag), 4, a), 12, c)
473	
474	def snap.u32(out: Bytes.Bytes, +v: U32) -> Bytes.Bytes:
475	  Bytes.append(out, Bytes.set.u32le(Bytes.new(4), 0, v))
476	
477	def snap.u64(out: Bytes.Bytes, +n: Nat) -> Bytes.Bytes:
478	  Bytes.append(out, put.u64(Bytes.new(8), 0, n))
479	
480	def snap.chars(s: String, out: Bytes.Bytes) -> Bytes.Bytes:
481	  match s:
482	    case SNil{}:
483	      out
484	    case SCon{Chr{c}, t}:
485	      snap.chars(t, snap.u32(out, c))
486	
487	# A target is a u32 count of chars, then each char as a u32.
488	def snap.key(out: Bytes.Bytes, +k: String) -> Bytes.Bytes:
489	  snap.chars(k, snap.u32(out, Bytes.count(k, 0)))
490	
491	def snap.nats(xs: List<&2, Nat>, out: Bytes.Bytes) -> Bytes.Bytes:
492	  match xs:
493	    case Nil{}:
494	      out
495	    case Con{x, rest}:
496	      snap.nats(rest, snap.u64(out, x))
497	
498	# A breaker state is a u32 kind. 0 is closed: a u32 count, then each failure as a u64. 1 is open: until. 2 is half-open: busy.
499	def snap.brk(out: Bytes.Bytes, b: Retry.State) -> Bytes.Bytes:
500	  match b:
501	    case Retry.CircuitClosed{+fs}:
502	      snap.nats(fs, snap.u32(snap.u32(out, 0), U32.from_nat(List.length(&2, Nat, fs))))
503	    case Retry.CircuitOpen{u}:
504	      snap.u64(snap.u32(out, 1), u)
505	    case Retry.CircuitHalf{k}:
506	      snap.u64(snap.u32(out, 2), k)
507	
508	def encode(~A: Data, ~penc: A -> Bytes.Bytes, op: Op<A>) -> Bytes.Bytes:
509	  match op:
510	    case Enqueue{+d, x}:
511	      Bytes.append(put.u64(Bytes.set.u32le(Bytes.new(12), 0, 1), 4, d), penc(x))
512	    case Select{+now}:
513	      head(2, now, 0n)
514	    case Complete{+i}:
515	      head(3, i, 0n)
516	    case Cancel{+i}:
517	      head(4, i, 0n)
518	    case Requeue{+i, +d}:
519	      head(5, i, d)
520	    case Defer{+i, +d}:
521	      head(6, i, d)
522	    case Abandon{+i, w, +at}:
523	      Bytes.append(head(8, i, why.code(w)), put.u64(Bytes.new(8), 0, at))
524	    case Revive{+i, +d}:
525	      head(9, i, d)
526	    case Purge{+t}:
527	      head(11, t, 0n)
528	    case PurgeAll{}:
529	      head(12, 0n, 0n)
530	    case Break{k, b}:
531	      snap.brk(snap.key(snap.u32(Bytes.new(0), 10), k), b)
532	
533	def get.field(m: Maybe<&2, U32>) -> U32:
534	  match m:
535	    case None{}:
536	      0
537	    case Some{x}:
538	      x
539	
540	def get.u32(r: Bytes.Bytes & Maybe<&2, U32>) -> Bytes.Bytes & U32:
541	  (b, m) = r
542	  (b, get.field(m))
543	
544	def get.hi(+lo: U32, r: Bytes.Bytes & Maybe<&2, U32>) -> Bytes.Bytes & Nat:
545	  (b, m) = r
546	  (b, Nat.add(U32.to_nat(lo), Nat.mul(U32.to_nat(get.field(m)), W())))
547	
548	def get.lo(+at: U32, r: Bytes.Bytes & Maybe<&2, U32>) -> Bytes.Bytes & Nat:
549	  (b, m) = r
550	  +lo = get.field(m)
551	  get.hi(lo, Bytes.get.u32le(b, (at + 4 : U32)))
552	
553	def get.u64(b: Bytes.Bytes, +at: U32) -> Bytes.Bytes & Nat:
554	  get.lo(at, Bytes.get.u32le(b, at))
555	
556	# ---- the snapshot codec: tag 7 and the next id, then the heap, the running list, the tries, the dead tickets, and the breakers.
557	# A u32 flag starts each tree node and list cell (1) and marks each end (0), so decode rebuilds the same shape.
558	# A ticket is its u64 id and deadline, then a u32 length and the payload.
559	# The dead tickets are a trie: each node is a u32 kind, 0 empty, 1 a bucket, or 2 a branch, then its lo and hi nodes.
560	# A bucket is its u32 hash, then a list of entries: the u64 key, the ticket, a u32 why.code, and the u64 abandon time.
561	# A breaker is its target, then its state, as in a Break record.
562	
563	def snap.payload(out: Bytes.Bytes, p: Bytes.Bytes) -> Bytes.Bytes:
564	  Bytes.Bytes{+len, buf} = p
565	  Bytes.append(snap.u32(out, len), Bytes.Bytes{len, buf})
566	
567	def snap.ticket(~A: Data, ~penc: A -> Bytes.Bytes, out: Bytes.Bytes, t: Q.Ticket<A>) -> Bytes.Bytes:
568	  Q.Ticket{+i, +d, x} = t
569	  snap.payload(snap.u64(snap.u64(out, i), d), penc(x))
570	
571	def snap.heap(~A: Data, ~penc: A -> Bytes.Bytes, h: H.Heap<Q.Ticket<A>>, out: Bytes.Bytes) -> Bytes.Bytes:
572	  match h:
573	    case H.HLeaf{}:
574	      snap.u32(out, 0)
575	    case H.HNode{+n, t, l, r}:
576	      snap.heap(~A, ~penc, r, snap.heap(~A, ~penc, l, snap.ticket(~A, ~penc, snap.u64(snap.u32(out, 1), n), t)))
577	
578	def snap.run(~A: Data, ~penc: A -> Bytes.Bytes, xs: List<&2, Q.Ticket<A>>, out: Bytes.Bytes) -> Bytes.Bytes:
579	  match xs:
580	    case Nil{}:
581	      snap.u32(out, 0)
582	    case Con{t, rest}:
583	      snap.run(~A, ~penc, rest, snap.ticket(~A, ~penc, snap.u32(out, 1), t))
584	
585	def snap.tries(ts: Tries(), out: Bytes.Bytes) -> Bytes.Bytes:
586	  match ts:
587	    case O.OTip{}:
588	      snap.u32(out, 0)
589	    case O.OBin{+n, +k, Try{+f, +st}, lo, hi}:
590	      snap.tries(hi, snap.tries(lo, snap.u64(snap.u64(snap.u64(snap.u32(snap.u32(out, 1), n), k), f), st)))
591	
592	def snap.entries(~A: Data, ~penc: A -> Bytes.Bytes, es: List<&2, M.Entry<Nat, Dead<A>>>, out: Bytes.Bytes) -> Bytes.Bytes:
593	  match es:
594	    case Nil{}:
595	      snap.u32(out, 0)
596	    case Con{M.Entry{k, Dead{t, w, at}}, rest}:
597	      snap.entries(~A, ~penc, rest, snap.u64(snap.u32(snap.ticket(~A, ~penc, snap.u64(snap.u32(out, 1), k), t), U32.from_nat(why.code(w))), at))
598	
599	def snap.graves(~A: Data, ~penc: A -> Bytes.Bytes, ds: Graves(A), out: Bytes.Bytes) -> Bytes.Bytes:
600	  match ds:
601	    case M.HNil{}:
602	      snap.u32(out, 0)
603	    case M.HLeaf{h, es}:
604	      snap.entries(~A, ~penc, es, snap.u32(snap.u32(out, 1), h))
605	    case M.HBranch{lo, hi}:
606	      snap.graves(~A, ~penc, hi, snap.graves(~A, ~penc, lo, snap.u32(out, 2)))
607	
608	def snap.brks(xs: List<&2, Sigma<&2, &2, String, _ => Retry.State>>, out: Bytes.Bytes) -> Bytes.Bytes:
609	  match xs:
610	    case Nil{}:
611	      snap.u32(out, 0)
612	    case Con{(k, b), rest}:
613	      snap.brks(rest, snap.brk(snap.key(snap.u32(out, 1), k), b))
614	
615	# The Snap record of s.
616	def encode.snap(~A: Data, ~penc: A -> Bytes.Bytes, s: State<A>) -> Bytes.Bytes:
617	  State{Q.State{+n, h, run}, ts, ds, bs} = s
618	  snap.brks(Map.to_list(&2, Retry.State, bs),
619	    snap.graves(~A, ~penc, ds, snap.tries(ts, snap.run(~A, ~penc, run, snap.heap(~A, ~penc, h, snap.u64(snap.u32(Bytes.new(0), 7), n))))))
620	
621	def encode.rec(~A: Data, ~penc: A -> Bytes.Bytes, r: Rec<A>) -> Bytes.Bytes:
622	  match r:
623	    case Snap{s}:
624	      encode.snap(~A, ~penc, s)
625	    case Do{op}:
626	      encode(~A, ~penc, op)
627	
628	# A snapshot reader: the record, and the offset of the next field.
629	type Rd is Type:
630	  Rd{b: Bytes.Bytes, at: U32}
631	
632	# One step of the snapshot decoder. None: a field runs past the record or does not decode.
633	def Dec(-T: Type) -> Type:
634	  Rd -> Maybe<&1, Rd & T>
635	
636	def Dec.next(-A: Type, -B: Type, m: Maybe<&1, Rd & A>, f: A -> Dec(B)) -> Maybe<&1, Rd & B>:
637	  match m:
638	    case None{}:
639	      None{}
640	    case Some{(r, x)}:
641	      f(x)(r)
642	
643	def Dec.bind(-A: Type, -B: Type, m: Dec(A), f: A -> Dec(B)) -> Dec(B):
644	  r => Dec.next(A, B, m(r), f)
645	
646	def Dec.pure(-A: Type, x: A) -> Dec(A):
647	  r => Some{(r, x)}
648	
649	def Dec.fail(-A: Type) -> Dec(A):
650	  r => None{}
651	
652	def rd.u32.of(m: Maybe<&2, U32>, b: Bytes.Bytes, +at: U32) -> Maybe<&1, Rd & U32>:
653	  match m:
654	    case None{}:
655	      None{}
656	    case Some{x}:
657	      Some{(Rd{b, (at + 4 : U32)}, x)}
658	
659	def rd.u32.got(+at: U32, r: Bytes.Bytes & Maybe<&2, U32>) -> Maybe<&1, Rd & U32>:
660	  (b, m) = r
661	  rd.u32.of(m, b, at)
662	
663	def rd.u32.at(r: Rd) -> Maybe<&1, Rd & U32>:
664	  Rd{b, +at} = r
665	  rd.u32.got(at, Bytes.get.u32le(b, at))
666	
667	def rd.u32() -> Dec(U32):
668	  r => rd.u32.at(r)
669	
670	def rd.u64.of(+lo: U32, +hi: U32) -> Nat:
671	  Nat.add(U32.to_nat(lo), Nat.mul(U32.to_nat(hi), W()))
672	
673	def rd.u64() -> Dec(Nat):
674	  do Dec<Nat>:
675	    lo : U32 <- rd.u32()
676	    hi : U32 <- rd.u32()
677	    return rd.u64.of(lo, hi)
678	
679	def rd.cut.got(+at: U32, +n: U32, r: Bytes.Bytes & Bytes.Bytes) -> Maybe<&1, Rd & Bytes.Bytes>:
680	  (b, p) = r
681	  Some{(Rd{b, (at + n : U32)}, p)}
682	
683	def rd.cut.if(ok: Bool, b: Bytes.Bytes, +at: U32, +n: U32) -> Maybe<&1, Rd & Bytes.Bytes>:
684	  match ok:
685	    case False{}:
686	      None{}
687	    case True{}:
688	      rd.cut.got(at, n, Bytes.slice(b, at, n))
689	
690	def rd.cut.at(+n: U32, r: Rd) -> Maybe<&1, Rd & Bytes.Bytes>:
691	  Rd{Bytes.Bytes{+len, buf}, +at} = r
692	  rd.cut.if(Bytes.fits(len, at, n), Bytes.Bytes{len, buf}, at, n)
693	
694	# The next n octets.
695	def rd.cut(+n: U32) -> Dec(Bytes.Bytes):
696	  r => rd.cut.at(n, r)
697	
698	def rd.flag.of(+m: U32) -> Dec(Bool):
699	  Bool.pick(Dec(Bool), U32.is_eq(m, 0), Dec.pure(Bool, False{}), Bool.pick(Dec(Bool), U32.is_eq(m, 1), Dec.pure(Bool, True{}), Dec.fail(Bool)))
700	
701	# A u32 flag: 1 is True, 0 is False, and any other value fails.
702	def rd.flag() -> Dec(Bool):
703	  Dec.bind(U32, Bool, rd.u32(), m => rd.flag.of(m))
704	
705	def rd.payload(-A: Data, +i: Nat, +d: Nat, m: Maybe<&2, A>) -> Dec(Q.Ticket<A>):
706	  match m:
707	    case None{}:
708	      Dec.fail(Q.Ticket<A>)
709	    case Some{x}:
710	      Dec.pure(Q.Ticket<A>, Q.Ticket{i, d, x})
711	
712	def rd.ticket(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>) -> Dec(Q.Ticket<A>):
713	  do Dec<Q.Ticket<A>>:
714	    i : Nat <- rd.u64()
715	    d : Nat <- rd.u64()
716	    n : U32 <- rd.u32()
717	    p : Bytes.Bytes <- rd.cut(n)
718	    rd.payload(A, i, d, pdec(p))
719	
720	# Each rd.* tree reader takes the flag already read. fuel bounds the depth: every node holds at least one 4-octet flag.
721	def rd.heap(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +fuel: Nat, node: Bool) -> Dec(H.Heap<Q.Ticket<A>>):
722	  match fuel node:
723	    case 0n b:
724	      Dec.fail(H.Heap<Q.Ticket<A>>)
725	    case 1n+f False{}:
726	      Dec.pure(H.Heap<Q.Ticket<A>>, H.HLeaf{})
727	    case 1n+f True{}:
728	      do Dec<H.Heap<Q.Ticket<A>>>:
729	        n : Nat <- rd.u64()
730	        t : Q.Ticket<A> <- rd.ticket(~A, ~pdec)
731	        lf : Bool <- rd.flag()
732	        l : H.Heap<Q.Ticket<A>> <- rd.heap(~A, ~pdec, f, lf)
733	        rf : Bool <- rd.flag()
734	        r : H.Heap<Q.Ticket<A>> <- rd.heap(~A, ~pdec, f, rf)
735	        return H.HNode{n, t, l, r}
736	
737	def rd.run(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +fuel: Nat, more: Bool) -> Dec(List<&2, Q.Ticket<A>>):
738	  match fuel more:
739	    case 0n b:
740	      Dec.fail(List<&2, Q.Ticket<A>>)
741	    case 1n+f False{}:
742	      Dec.pure(List<&2, Q.Ticket<A>>, Nil{})
743	    case 1n+f True{}:
744	      do Dec<List<&2, Q.Ticket<A>>>:
745	        t : Q.Ticket<A> <- rd.ticket(~A, ~pdec)
746	        m : Bool <- rd.flag()
747	        rest : List<&2, Q.Ticket<A>> <- rd.run(~A, ~pdec, f, m)
748	        return Con{t, rest}
749	
750	def rd.tries(+fuel: Nat, node: Bool) -> Dec(O.OMap<Nat, Try>):
751	  match fuel node:
752	    case 0n b:
753	      Dec.fail(O.OMap<Nat, Try>)
754	    case 1n+f False{}:
755	      Dec.pure(O.OMap<Nat, Try>, O.OTip{})
756	    case 1n+f True{}:
757	      do Dec<O.OMap<Nat, Try>>:
758	        n : U32 <- rd.u32()
759	        k : Nat <- rd.u64()
760	        fails : Nat <- rd.u64()
761	        st : Nat <- rd.u64()
762	        lf : Bool <- rd.flag()
763	        lo : O.OMap<Nat, Try> <- rd.tries(f, lf)
764	        hf : Bool <- rd.flag()
765	        hi : O.OMap<Nat, Try> <- rd.tries(f, hf)
766	        return O.OBin{n, k, Try{fails, st}, lo, hi}
767	
768	def rd.why(+c: U32) -> Dec(Why):
769	  Bool.pick(Dec(Why), U32.is_eq(c, 0), Dec.pure(Why, WhySpent{}),
770	  Bool.pick(Dec(Why), U32.is_eq(c, 1), Dec.pure(Why, WhyLate{}),
771	  Bool.pick(Dec(Why), U32.is_eq(c, 2), Dec.pure(Why, WhyUnsafe{}),
772	  Bool.pick(Dec(Why), U32.is_eq(c, 3), Dec.pure(Why, WhyFailed{}), Dec.fail(Why)))))
773	
774	def rd.entries(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +fuel: Nat, more: Bool) -> Dec(List<&2, M.Entry<Nat, Dead<A>>>):
775	  match fuel more:
776	    case 0n b:
777	      Dec.fail(List<&2, M.Entry<Nat, Dead<A>>>)
778	    case 1n+f False{}:
779	      Dec.pure(List<&2, M.Entry<Nat, Dead<A>>>, Nil{})
780	    case 1n+f True{}:
781	      do Dec<List<&2, M.Entry<Nat, Dead<A>>>>:
782	        k : Nat <- rd.u64()
783	        t : Q.Ticket<A> <- rd.ticket(~A, ~pdec)
784	        c : U32 <- rd.u32()
785	        w : Why <- rd.why(c)
786	        at : Nat <- rd.u64()
787	        m : Bool <- rd.flag()
788	        rest : List<&2, M.Entry<Nat, Dead<A>>> <- rd.entries(~A, ~pdec, f, m)
789	        return Con{M.Entry{k, Dead{t, w, at}}, rest}
790	
791	# A trie node whose kind is already read: nil is kind 0, leaf kind 1, and branch kind 2. Any other kind fails.
792	def rd.graves(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +fuel: Nat, nil: Bool, leaf: Bool, branch: Bool) -> Dec(Graves(A)):
793	  match fuel nil leaf branch:
794	    case 0n a b c:
795	      Dec.fail(Graves(A))
796	    case 1n+f True{} b c:
797	      Dec.pure(Graves(A), M.HNil{})
798	    case 1n+f False{} True{} c:
799	      do Dec<Graves(A)>:
800	        h : U32 <- rd.u32()
801	        m : Bool <- rd.flag()
802	        es : List<&2, M.Entry<Nat, Dead<A>>> <- rd.entries(~A, ~pdec, f, m)
803	        return M.HLeaf{h, es}
804	    case 1n+f False{} False{} True{}:
805	      do Dec<Graves(A)>:
806	        +lk : U32 <- rd.u32()
807	        lo : Graves(A) <- rd.graves(~A, ~pdec, f, U32.is_eq(lk, 0), U32.is_eq(lk, 1), U32.is_eq(lk, 2))
808	        +hk : U32 <- rd.u32()
809	        hi : Graves(A) <- rd.graves(~A, ~pdec, f, U32.is_eq(hk, 0), U32.is_eq(hk, 1), U32.is_eq(hk, 2))
810	        return M.HBranch{lo, hi}
811	    case 1n+f False{} False{} False{}:
812	      Dec.fail(Graves(A))
813	
814	# The trie, when it is well formed as the never-selected laws assume.
815	def rd.wf(-A: Data, ok: Bool, ds: Graves(A)) -> Dec(Graves(A)):
816	  match ok:
817	    case True{}:
818	      Dec.pure(Graves(A), ds)
819	    case False{}:
820	      Dec.fail(Graves(A))
821	
822	# Each rd.* sequence reader takes the count or flag already read. fuel bounds its length: every item holds at least 4 octets.
823	def rd.chars(+fuel: Nat, end: Bool, +n: U32) -> Dec(String):
824	  match fuel end:
825	    case 0n b:
826	      Dec.fail(String)
827	    case 1n+f True{}:
828	      Dec.pure(String, SNil{})
829	    case 1n+f False{}:
830	      +k = (n - 1 : U32)
831	      do Dec<String>:
832	        c : U32 <- rd.u32()
833	        rest : String <- rd.chars(f, U32.is_eq(k, 0), k)
834	        return SCon{Chr{c}, rest}
835	
836	def rd.key(+fuel: Nat) -> Dec(String):
837	  do Dec<String>:
838	    +n : U32 <- rd.u32()
839	    rd.chars(fuel, U32.is_eq(n, 0), n)
840	
841	def rd.nats(+fuel: Nat, end: Bool, +n: U32) -> Dec(List<&2, Nat>):
842	  match fuel end:
843	    case 0n b:
844	      Dec.fail(List<&2, Nat>)
845	    case 1n+f True{}:
846	      Dec.pure(List<&2, Nat>, Nil{})
847	    case 1n+f False{}:
848	      +k = (n - 1 : U32)
849	      do Dec<List<&2, Nat>>:
850	        x : Nat <- rd.u64()
851	        rest : List<&2, Nat> <- rd.nats(f, U32.is_eq(k, 0), k)
852	        return Con{x, rest}
853	
854	def rd.closed(+fuel: Nat) -> Dec(Retry.State):
855	  do Dec<Retry.State>:
856	    +n : U32 <- rd.u32()
857	    fs : List<&2, Nat> <- rd.nats(fuel, U32.is_eq(n, 0), n)
858	    return Retry.CircuitClosed{fs}
859	
860	def rd.brk.of(+fuel: Nat, +kind: U32) -> Dec(Retry.State):
861	  Bool.pick(Dec(Retry.State), U32.is_eq(kind, 0), rd.closed(fuel),
862	  Bool.pick(Dec(Retry.State), U32.is_eq(kind, 1), Dec.bind(Nat, Retry.State, rd.u64(), u => Dec.pure(Retry.State, Retry.CircuitOpen{u})),
863	  Bool.pick(Dec(Retry.State), U32.is_eq(kind, 2), Dec.bind(Nat, Retry.State, rd.u64(), k => Dec.pure(Retry.State, Retry.CircuitHalf{k})), Dec.fail(Retry.State))))
864	
865	def rd.brk(+fuel: Nat) -> Dec(Retry.State):
866	  Dec.bind(U32, Retry.State, rd.u32(), k => rd.brk.of(fuel, k))
867	
868	def rd.break(-A: Data, +fuel: Nat) -> Dec(Op<A>):
869	  do Dec<Op<A>>:
870	    k : String <- rd.key(fuel)
871	    b : Retry.State <- rd.brk(fuel)
872	    return Break{k, b}
873	
874	def rd.brks(+fuel: Nat, more: Bool, acc: Map<&2, Retry.State>) -> Dec(Map<&2, Retry.State>):
875	  match fuel more:
876	    case 0n b:
877	      Dec.fail(Map<&2, Retry.State>)
878	    case 1n+f False{}:
879	      Dec.pure(Map<&2, Retry.State>, acc)
880	    case 1n+f True{}:
881	      do Dec<Map<&2, Retry.State>>:
882	        k : String <- rd.key(f)
883	        b : Retry.State <- rd.brk(f)
884	        m : Bool <- rd.flag()
885	        rd.brks(f, m, Map.set(&2, Retry.State, acc, k, b))
886	
887	def rd.state(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +fuel: Nat) -> Dec(State<A>):
888	  do Dec<State<A>>:
889	    n : Nat <- rd.u64()
890	    hf : Bool <- rd.flag()
891	    h : H.Heap<Q.Ticket<A>> <- rd.heap(~A, ~pdec, fuel, hf)
892	    rf : Bool <- rd.flag()
893	    run : List<&2, Q.Ticket<A>> <- rd.run(~A, ~pdec, fuel, rf)
894	    tf : Bool <- rd.flag()
895	    ts : O.OMap<Nat, Try> <- rd.tries(fuel, tf)
896	    +dk : U32 <- rd.u32()
897	    +dt : Graves(A) <- rd.graves(~A, ~pdec, fuel, U32.is_eq(dk, 0), U32.is_eq(dk, 1), U32.is_eq(dk, 2))
898	    ds : Graves(A) <- rd.wf(A, CL.hmap_wf(~Nat, ~Dead<A>, ~dead.hash, ~Nat.is_eq, dt), dt)
899	    bf : Bool <- rd.flag()
900	    bs : Map<&2, Retry.State> <- rd.brks(fuel, bf, Map.new(&2, Retry.State))
901	    return State{Q.State{n, h, run}, ts, ds, bs}
902	
903	# The value read, when the reader ended at len, the record's end.
904	def rd.done(-T: Data, +len: U32, m: Maybe<&1, Rd & T>) -> Maybe<&2, T>:
905	  match m:
906	    case None{}:
907	      None{}
908	    case Some{(Rd{b, +at}, +x)}:
909	      Bool.pick(Maybe<&2, T>, U32.is_eq(at, len), Some{x}, None{})
910	
911	# The Abandon of ticket a at e whose why.code is c, or None for any other code.
912	def decode.abandon(-A: Data, +a: Nat, +c: Nat, +e: Nat) -> Maybe<&2, Op<A>>:
913	  Bool.pick(Maybe<&2, Op<A>>, Nat.is_eq(c, 0n), Some{Abandon{a, WhySpent{}, e}},
914	  Bool.pick(Maybe<&2, Op<A>>, Nat.is_eq(c, 1n), Some{Abandon{a, WhyLate{}, e}},
915	  Bool.pick(Maybe<&2, Op<A>>, Nat.is_eq(c, 2n), Some{Abandon{a, WhyUnsafe{}, e}},
916	  Bool.pick(Maybe<&2, Op<A>>, Nat.is_eq(c, 3n), Some{Abandon{a, WhyFailed{}, e}}, None{}))))
917	
918	def decode.small(-A: Data, ok: Bool, +tag: U32, +a: Nat, +c: Nat) -> Maybe<&2, Op<A>>:
919	  match ok:
920	    case False{}:
921	      None{}
922	    case True{}:
923	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 2), Some{Select{a}},
924	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 3), Some{Complete{a}},
925	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 4), Some{Cancel{a}},
926	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 5), Some{Requeue{a, c}},
927	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 6), Some{Defer{a, c}},
928	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 9), Some{Revive{a, c}},
929	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 11), Some{Purge{a}},
930	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 12), Some{PurgeAll{}}, None{}))))))))
931	
932	def decode.long(-A: Data, ok: Bool, +a: Nat, +c: Nat, r: Bytes.Bytes & Nat) -> Maybe<&2, Op<A>>:
933	  match ok:
934	    case False{}:
935	      None{}
936	    case True{}:
937	      (b, +e) = r
938	      decode.abandon(A, a, c, e)
939	
940	def decode.payload(-A: Data, m: Maybe<&2, A>, +d: Nat) -> Maybe<&2, Op<A>>:
941	  match m:
942	    case None{}:
943	      None{}
944	    case Some{x}:
945	      Some{Enqueue{d, x}}
946	
947	def decode.c.if(-A: Data, abandon: Bool, +tag: U32, +len: U32, +a: Nat, +c: Nat, b: Bytes.Bytes) -> Maybe<&2, Op<A>>:
948	  match abandon:
949	    case True{}:
950	      decode.long(A, U32.is_eq(len, 28), a, c, get.u64(b, 20))
951	    case False{}:
952	      decode.small(A, U32.is_eq(len, 20), tag, a, c)
953	
954	# An Abandon is 28 octets; every other op without a payload is 20.
955	def decode.c(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +tag: U32, +len: U32, +a: Nat, r: Bytes.Bytes & Nat) -> Maybe<&2, Op<A>>:
956	  (b, +c) = r
957	  decode.c.if(A, U32.is_eq(tag, 8), tag, len, a, c, b)
958	
959	def decode.kind(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, enq: Bool, +tag: U32, +len: U32, +a: Nat, b: Bytes.Bytes) -> Maybe<&2, Op<A>>:
960	  match enq:
961	    case True{}:
962	      decode.payload(A, pdec(Bytes.snd(Bytes.slice(b, 12, (len - 12 : U32)))), a)
963	    case False{}:
964	      decode.c(~A, ~pdec, tag, len, a, get.u64(b, 12))
965	
966	def decode.a(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +tag: U32, +len: U32, r: Bytes.Bytes & Nat) -> Maybe<&2, Op<A>>:
967	  (b, +a) = r
968	  decode.kind(~A, ~pdec, U32.is_eq(tag, 1), tag, len, a, b)
969	
970	def decode.tag.if(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, brk: Bool, +tag: U32, +len: U32, b: Bytes.Bytes) -> Maybe<&2, Op<A>>:
971	  match brk:
972	    case True{}:
973	      rd.done(Op<A>, len, rd.break(A, U32.to_nat((len / 4 + 1 : U32)))(Rd{b, 4}))
974	    case False{}:
975	      decode.a(~A, ~pdec, tag, len, get.u64(b, 4))
976	
977	def decode.tag(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +len: U32, r: Bytes.Bytes & U32) -> Maybe<&2, Op<A>>:
978	  (b, +tag) = r
979	  decode.tag.if(~A, ~pdec, U32.is_eq(tag, 10), tag, len, b)
980	
981	def decode.len(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, ok: Bool, +len: U32, b: Bytes.Bytes) -> Maybe<&2, Op<A>>:
982	  match ok:
983	    case False{}:
984	      None{}
985	    case True{}:
986	      decode.tag(~A, ~pdec, len, get.u32(Bytes.get.u32le(b, 0)))
987	
988	# The op of a record, or None when it is not one that encode writes.
989	def decode(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, b: Bytes.Bytes) -> Maybe<&2, Op<A>>:
990	  Bytes.Bytes{+len, buf} = b
991	  decode.len(~A, ~pdec, U32.is_ge(len, 12), len, Bytes.Bytes{len, buf})
992	
993	# The state of a Snap record whose tag is already checked. The decoder must end at the record's end.
994	def decode.snap(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, b: Bytes.Bytes) -> Maybe<&2, State<A>>:
995	  Bytes.Bytes{+len, buf} = b
996	  rd.done(State<A>, len, rd.state(~A, ~pdec, U32.to_nat((len / 4 + 1 : U32)))(Rd{Bytes.Bytes{len, buf}, 4}))
997	
998	def decode.snapped(-A: Data, m: Maybe<&2, State<A>>) -> Maybe<&2, Rec<A>>:
999	  match m:
1000	    case None{}:
1001	      None{}
1002	    case Some{s}:
1003	      Some{Snap{s}}
1004	
1005	def decode.did(-A: Data, m: Maybe<&2, Op<A>>) -> Maybe<&2, Rec<A>>:
1006	  match m:
1007	    case None{}:
1008	      None{}
1009	    case Some{op}:
1010	      Some{Do{op}}
1011	
1012	def decode.rec.if(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, snap: Bool, b: Bytes.Bytes) -> Maybe<&2, Rec<A>>:
1013	  match snap:
1014	    case True{}:
1015	      decode.snapped(A, decode.snap(~A, ~pdec, b))
1016	    case False{}:
1017	      decode.did(A, decode(~A, ~pdec, b))
1018	
1019	def decode.rec.tag(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, r: Bytes.Bytes & U32) -> Maybe<&2, Rec<A>>:
1020	  (b, +tag) = r
1021	  decode.rec.if(~A, ~pdec, U32.is_eq(tag, 7), b)
1022	
1023	# The Rec of a journal record, or None when it is neither a Snap nor an op that encode.rec writes.
1024	def decode.rec(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, b: Bytes.Bytes) -> Maybe<&2, Rec<A>>:
1025	  decode.rec.tag(~A, ~pdec, get.u32(Bytes.get.u32le(b, 0)))
1026	
1027	# ---- the runner
1028	
1029	# policy: retry's policy for each ticket. circuit: the breaker of each target. idle: ms that drain waits when no attempt started.
1030	# compact: tick compacts the journal once records past this many octets were appended since open or the last compaction; 0 never.
1031	type Config is Data:
1032	  Config{policy: Retry.Policy, circuit: Retry.Circuit, idle: U32, compact: U32}
1033	
1034	# The journal, its path, its file cap, and its length at open or at the last compaction.
1035	type Log is Type:
1036	  Log{journal: J.Journal, path: String, limit: U32, base: U32}
1037	
1038	# An attempt that runs: its unit of the pool, the budget that admit left, and its target.
1039	type Flight<-A: Data> is Type:
1040	  Flight{ticket: Q.Ticket<A>, grant: Res.Grant, budget: Retry.Budget, target: String}
1041	
1042	def Out(-X: Type) -> Type:
1043	  Result<&1, &1, U32 & String, X>
1044	
1045	# A message to the fiber that owns the runner. Ended: an attempt finished. Add and Drop: another fiber asks for an enqueue or a cancel.
1046	type Msg<-A: Data> is Type:
1047	  Ended{id: Nat, outcome: Retry.Outcome}
1048	  Add{deadline: Nat, payload: A, reply: Chan(Out(Nat))}
1049	  Drop{id: Nat, reply: Chan(Out(Unit))}
1050	
1051	# An open runner. Replaying its journal gives its state; its pool bounds the attempts that run at once.
1052	# Each attempt sends its outcome, and each other fiber its ask, on inbox. flights are the attempts that run.
1053	type Runner<-A: Data> is Type:
1054	  Runner{log: Log, state: State<A>, pool: Res.Pool, config: Config, inbox: Chan(Msg<A>), flights: List<&1, Flight<A>>}
1055	
1056	# The runner, the claim fuel left, and the first error.
1057	def Claimed(-A: Data) -> Type:
1058	  Runner<A> & Nat & Out(Unit)
1059	
1060	def Ran(-A: Data) -> Type:
1061	  Runner<A> & Out(Unit)
1062	
1063	# Wall-clock ms since the Unix epoch. Deadlines must keep their meaning across a restart.
1064	def clock() -> IO(Nat):
1065	  do IO<Nat>:
1066	    t : Time.Instant <- Time.now()
1067	    return Time.Duration.to_ms_nat(Time.Instant.since_epoch(t))
1068	
1069	def first(a: Out(Unit), b: Out(Unit)) -> Out(Unit):
1070	  match a:
1071	    case Fail{e}:
1072	      Fail{e}
1073	    case Done{u}:
1074	      b
1075	
1076	def log.appended(+path: String, +limit: U32, +base: U32, x: J.Journal & Out(Unit)) -> Log & Out(Unit):
1077	  (j, o) = x
1078	  (Log{j, path, limit, base}, o)
1079	
1080	def log.append(l: Log, p: Bytes.Bytes) -> IO(Log & Out(Unit)):
1081	  Log{j, +path, +limit, +base} = l
1082	  do IO<Log & Out(Unit)>:
1083	    x : J.Journal & Out(Unit) <- J.append(j, p)
1084	    return log.appended(path, limit, base, x)
1085	
1086	def log.close(l: Log) -> IO(Unit):
1087	  Log{j, path, limit, base} = l
1088	  J.close(j)
1089	
1090	# The journal reads with limit as its record cap, so that a snapshot fits; appends keep max.
1091	def log.of(j: J.Journal, +max: U32, path: String, +limit: U32) -> Log:
1092	  J.Journal{file, cap, +end, ok} = j
1093	  Log{J.Journal{file, max, end, ok}, path, limit, end}
1094	
1095	def commit.done(~A: Data, x: Log & Out(Unit), s: State<A>, +op: Op<A>, p: Res.Pool, c: Config, ib: Chan(Msg<A>), fs: List<&1, Flight<A>>) -> Ran(A):
1096	  match x:
1097	    case (j, Done{u}):
1098	      (Runner{j, step(~A, s, op), p, c, ib, fs}, Done{u})
1099	    case (j, Fail{e}):
1100	      (Runner{j, s, p, c, ib, fs}, Fail{e})
1101	
1102	# Appends op, and applies it once the append is durable. After a failed append the state is as it was,
1103	# and the journal refuses every later append until the runner is reopened.
1104	def commit(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +op: Op<A>) -> IO(Ran(A)):
1105	  Runner{j, s, p, c, ib, fs} = r
1106	  do IO<Ran(A)>:
1107	    x : Log & Out(Unit) <- log.append(j, encode(~A, ~penc, op))
1108	    return commit.done(~A, x, s, op, p, c, ib, fs)
1109	
1110	def enqueue.done(-A: Data, +id: Nat, y: Ran(A)) -> Runner<A> & Out(Nat):
1111	  match y:
1112	    case (r, Done{u}):
1113	      (r, Done{id})
1114	    case (r, Fail{e}):
1115	      (r, Fail{e})
1116	
1117	# Adds a job that is due at deadline (wall-clock ms) and answers its ticket id once the record is durable.
1118	def enqueue(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +deadline: Nat, x: A) -> IO(Runner<A> & Out(Nat)):
1119	  Runner{j, +s, p, c, ib, fs} = r
1120	  do IO<Runner<A> & Out(Nat)>:
1121	    y : Ran(A) <- commit(~A, ~penc, Runner{j, s, p, c, ib, fs}, Enqueue{deadline, x})
1122	    return enqueue.done(A, Q.next_id(A, queue(A, s)), y)
1123	
1124	# Cancels ticket id. A running attempt finishes, but its ticket is never selected again. A dead ticket is dropped.
1125	def cancel(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +id: Nat) -> IO(Ran(A)):
1126	  commit(~A, ~penc, r, Cancel{id})
1127	
1128	# Makes dead ticket id pending again at deadline (wall-clock ms), with its payload and a new retry budget.
1129	# A ticket that is not dead does not change.
1130	def revive(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +id: Nat, +deadline: Nat) -> IO(Ran(A)):
1131	  commit(~A, ~penc, r, Revive{id, deadline})
1132	
1133	# Drops, as one record, the dead tickets that retry gave up on before `before` (wall-clock ms).
1134	def purge(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +before: Nat) -> IO(Ran(A)):
1135	  commit(~A, ~penc, r, Purge{before})
1136	
1137	# Drops every dead ticket, as one record.
1138	def purge.all(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>) -> IO(Ran(A)):
1139	  commit(~A, ~penc, r, PurgeAll{})
1140	
1141	# ---- asks from other fibers
1142	
1143	# The runner back, and its inbox. Another fiber passes the inbox to ask.enqueue and ask.cancel.
1144	def inbox(-A: Data, r: Runner<A>) -> Runner<A> & Chan(Msg<A>):
1145	  Runner{j, s, p, c, +ib, fs} = r
1146	  (Runner{j, s, p, c, ib, fs}, ib)
1147	
1148	def ask.answer(-X: Type, m: Maybe<&1, Out(X)>) -> Out(X):
1149	  match m:
1150	    case None{}:
1151	      Fail{(32, "runner: the runner closed before it answered")}
1152	    case Some{o}:
1153	      o
1154	
1155	def ask.sent(-A: Data, -X: Type, x: Result<&1, &1, Msg<A>, Unit>, +rep: Chan(Out(X))) -> IO(Out(X)):
1156	  match x:
1157	    case Fail{m}:
1158	      IO.pure(Out(X), Fail{(32, "runner: the inbox is closed")})
1159	    case Done{u}:
1160	      do IO<Out(X)>:
1161	        a : Maybe<&1, Out(X)> <- Chan.recv(Out(X), rep)
1162	        return ask.answer(X, a)
1163	
1164	# Sends m, built with a new reply channel, on the inbox and waits for the reply.
1165	def ask(-A: Data, -X: Type, +ib: Chan(Msg<A>), m: Chan(Out(X)) -> Msg<A>) -> IO(Out(X)):
1166	  do IO<Out(X)>:
1167	    +rep : Chan(Out(X)) <- Chan.new(Out(X), 1)
1168	    x : Result<&1, &1, Msg<A>, Unit> <- Chan.send(Msg<A>, ib, m(rep))
1169	    ask.sent(A, X, x, rep)
1170	
1171	# From another fiber: adds a job due at deadline, and answers its ticket id once the record is durable.
1172	# The answer comes when the fiber that owns the runner reads the inbox, in tick or drain.
1173	def ask.enqueue(-A: Data, +ib: Chan(Msg<A>), +deadline: Nat, x: A) -> IO(Out(Nat)):
1174	  ask(A, Nat, ib, rep => Add{deadline, x, rep})
1175	
1176	# From another fiber: cancels ticket id, as cancel does. The answer comes when the owner reads the inbox.
1177	def ask.cancel(-A: Data, +ib: Chan(Msg<A>), +id: Nat) -> IO(Out(Unit)):
1178	  ask(A, Unit, ib, rep => Drop{id, rep})
1179	
1180	# ---- one round
1181	
1182	# A grant of this runner's pool always fits it, so Kept does not happen.
1183	def release.got(-A: Data, x: Res.Release, j: Log, s: State<A>, c: Config, ib: Chan(Msg<A>), fs: List<&1, Flight<A>>) -> Runner<A>:
1184	  match x:
1185	    case Res.Released{p}:
1186	      Runner{j, s, p, c, ib, fs}
1187	    case Res.Kept{p, g}:
1188	      Runner{j, s, p, c, ib, fs}
1189	
1190	def release(-A: Data, r: Runner<A>, g: Res.Grant) -> Runner<A>:
1191	  Runner{j, s, p, c, ib, fs} = r
1192	  release.got(A, Res.release(p, g), j, s, c, ib, fs)
1193	
1194	def claim.held(-A: Data, y: Ran(A), g: Res.Grant, next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1195	  match y:
1196	    case (r, Fail{e}):
1197	      IO.pure(Claimed(A), (release(A, r, g), 0n, Fail{e}))
1198	    case (r, Done{u}):
1199	      next(release(A, r, g))
1200	
1201	def claim.tag(-A: Data, +id: Nat, o: Retry.Outcome) -> IO(Msg<A>):
1202	  IO.pure(Msg<A>, Ended{id, o})
1203	
1204	def trip.if(~A: Data, ~penc: A -> Bytes.Bytes, same: Bool, r: Runner<A>, +key: String, st: Retry.State) -> IO(Ran(A)):
1205	  match same:
1206	    case True{}:
1207	      IO.pure(Ran(A), (r, Done{Unit{}}))
1208	    case False{}:
1209	      commit(~A, ~penc, r, Break{key, st})
1210	
1211	# Appends a Break when the target's breaker left old, so that a reopen restores it.
1212	def trip(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +key: String, +old: Retry.State, br: Retry.Breaker) -> IO(Ran(A)):
1213	  +st = brk.state(br, old)
1214	  trip.if(~A, ~penc, brk.same(old, st), r, key, st)
1215	
1216	def claim.go(~A: Data, ~penc: A -> Bytes.Bytes, ~work: A -> IO(Retry.Outcome), +now: Nat, gate: Retry.Gate, +br: Retry.Breaker, b: Retry.Budget, +key: String,
1217	    r: Runner<A>, +id: Nat, d: Nat, +x: A, g: Res.Grant, next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1218	  match gate:
1219	    case Retry.GateGo{}:
1220	      Runner{j, s, p, c, +ib, fs} = r
1221	      do IO<Claimed(A)>:
1222	        u : Chan(Msg<A>) <- IO.fork.go(Msg<A>, IO.bind(Retry.Outcome, Msg<A>, work(x), o => claim.tag(A, id, o)), ib)
1223	        next(Runner{j, s, p, c, ib, Con{Flight{Q.Ticket{id, d, x}, g, b, key}, fs}})
1224	    case Retry.GateNo{why}:
1225	      do IO<Claimed(A)>:
1226	        y : Ran(A) <- commit(~A, ~penc, r, held(A, why, br, id, now))
1227	        claim.held(A, y, g, next)
1228	
1229	def claim.tripped(~A: Data, ~penc: A -> Bytes.Bytes, ~work: A -> IO(Retry.Outcome), +now: Nat, y: Ran(A), gate: Retry.Gate, +br: Retry.Breaker, b: Retry.Budget, +key: String,
1230	    +id: Nat, d: Nat, +x: A, g: Res.Grant, next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1231	  match y:
1232	    case (r, Fail{e}):
1233	      IO.pure(Claimed(A), (release(A, r, g), 0n, Fail{e}))
1234	    case (r, Done{u}):
1235	      claim.go(~A, ~penc, ~work, now, gate, br, b, key, r, id, d, x, g, next)
1236	
1237	# A change to the breaker is durable before the attempt starts.
1238	def claim.gate(~A: Data, ~penc: A -> Bytes.Bytes, ~work: A -> IO(Retry.Outcome), +now: Nat, a: Retry.Breaker & Retry.Budget & Retry.Gate, +key: String, +old: Retry.State,
1239	    r: Runner<A>, +id: Nat, d: Nat, +x: A, g: Res.Grant, next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1240	  (+br, b, gate) = a
1241	  do IO<Claimed(A)>:
1242	    y : Ran(A) <- trip(~A, ~penc, r, key, old, br)
1243	    claim.tripped(~A, ~penc, ~work, now, y, gate, br, b, key, id, d, x, g, next)
1244	
1245	# The selected ticket asks its target's breaker and its own budget for an attempt.
1246	def claim.admit(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +now: Nat, r: Runner<A>, t: Q.Ticket<A>, g: Res.Grant,
1247	    next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1248	  Runner{j, +s, p, +c, ib, fs} = r
1249	  Config{pol, circ, idle, every} = c
1250	  Q.Ticket{+id, d, +x} = t
1251	  +key = target(x)
1252	  +old = brk.get(A, s, key)
1253	  claim.gate(~A, ~penc, ~work, now, Retry.admit(Retry.BreakerOn{circ, old}, budget(pol, tries.get(tries(A, s), id)), now), key, old,
1254	    Runner{j, s, p, c, ib, fs}, id, d, x, g, next)
1255	
1256	def claim.selected(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +now: Nat, y: Ran(A),
1257	    t: Q.Ticket<A>, g: Res.Grant, next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1258	  match y:
1259	    case (r, Fail{e}):
1260	      IO.pure(Claimed(A), (release(A, r, g), 0n, Fail{e}))
1261	    case (r, Done{u}):
1262	      claim.admit(~A, ~penc, ~target, ~work, now, r, t, g, next)
1263	
1264	def claim.pick(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +left: Nat, +now: Nat, k: Pick<A>, j: Log, s: State<A>,
1265	    c: Config, ib: Chan(Msg<A>), fs: List<&1, Flight<A>>, next: Runner<A> -> IO(Claimed(A))) -> IO(Claimed(A)):
1266	  match k:
1267	    case Idle{p}:
1268	      IO.pure(Claimed(A), (Runner{j, s, p, c, ib, fs}, Nat.add(1n, left), Done{Unit{}}))
1269	    case Picked{p, t, g}:
1270	      do IO<Claimed(A)>:
1271	        y : Ran(A) <- commit(~A, ~penc, Runner{j, s, p, c, ib, fs}, Select{now})
1272	        claim.selected(~A, ~penc, ~target, ~work, now, y, t, g, next)
1273	
1274	# Starts an attempt for each due ticket while the pool has a free unit; each attempt sends its outcome on the inbox.
1275	# fuel bounds the tickets taken, and the fuel left comes back.
1276	def claim(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), fuel: Nat, +now: Nat, r: Runner<A>) -> IO(Claimed(A)):
1277	  match fuel:
1278	    case 0n:
1279	      IO.pure(Claimed(A), (r, 0n, Done{Unit{}}))
1280	    case 1n+n:
1281	      Runner{j, +s, p, c, ib, fs} = r
1282	      +m = n
1283	      claim.pick(~A, ~penc, ~target, ~work, m, now, pick(~A, p, s, now), j, s, c, ib, fs, (r2 => claim(~A, ~penc, ~target, ~work, m, now, r2)))
1284	
1285	def settle.done(-A: Data, y: Ran(A), g: Res.Grant, err: Out(Unit), next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1286	  (r, res) = y
1287	  next(release(A, r, g), first(err, res))
1288	
1289	def settle.br(~A: Data, ~penc: A -> Bytes.Bytes, +op: Op<A>, y: Ran(A), g: Res.Grant, err: Out(Unit),
1290	    next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1291	  match y:
1292	    case (r, Fail{e}):
1293	      next(release(A, r, g), first(err, Fail{e}))
1294	    case (r, Done{u}):
1295	      do IO<Runner<A> & Out(Bool)>:
1296	        z : Ran(A) <- commit(~A, ~penc, r, op)
1297	        settle.done(A, z, g, err, next)
1298	
1299	# Tells the target's breaker, appends the outcome's op, and returns the attempt's unit to the pool.
1300	# The op of a cancelled ticket changes no state: the queue no longer holds it.
1301	def settle.op(~A: Data, ~penc: A -> Bytes.Bytes, +o: Retry.Outcome, b: Retry.Budget, +id: Nat, +key: String, +now: Nat, +rnd: Nat, r: Runner<A>, g: Res.Grant,
1302	    err: Out(Unit), next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1303	  Runner{j, +s, p, +c, ib, fs} = r
1304	  Config{pol, circ, idle, every} = c
1305	  +old = brk.get(A, s, key)
1306	  do IO<Runner<A> & Out(Bool)>:
1307	    y : Ran(A) <- trip(~A, ~penc, Runner{j, s, p, c, ib, fs}, key, old, brk.after(circ, old, now, o))
1308	    settle.br(~A, ~penc, after(A, o, pol, b, id, fails(tries.get(tries(A, s), id)), now, rnd), y, g, err, next)
1309	
1310	def random(r: Out(U32)) -> Nat:
1311	  match r:
1312	    case Done{x}:
1313	      U32.to_nat(x)
1314	    case Fail{e}:
1315	      0n
1316	
1317	def take.keep(-A: Data, m: Maybe<&1, Flight<A>>, xs: List<&1, Flight<A>>) -> List<&1, Flight<A>>:
1318	  match m:
1319	    case None{}:
1320	      xs
1321	    case Some{f}:
1322	      Con{f, xs}
1323	
1324	def take.at(-A: Data, eq: Bool, f: Flight<A>, r: Maybe<&1, Flight<A>> & List<&1, Flight<A>>) -> Maybe<&1, Flight<A>> & List<&1, Flight<A>>:
1325	  match eq:
1326	    case True{}:
1327	      (m, xs) = r
1328	      (Some{f}, take.keep(A, m, xs))
1329	    case False{}:
1330	      (m, xs) = r
1331	      (m, Con{f, xs})
1332	
1333	# The flight of ticket id, and the other flights.
1334	def take(-A: Data, fs: List<&1, Flight<A>>, +id: Nat) -> Maybe<&1, Flight<A>> & List<&1, Flight<A>>:
1335	  match fs:
1336	    case Nil{}:
1337	      (None{}, Nil{})
1338	    case Con{Flight{Q.Ticket{+i, d, x}, g, b, k}, rest}:
1339	      take.at(A, Nat.is_eq(i, id), Flight{Q.Ticket{i, d, x}, g, b, k}, take(A, rest, id))
1340	
1341	def settle.flight(~A: Data, ~penc: A -> Bytes.Bytes, mf: Maybe<&1, Flight<A>>, +o: Retry.Outcome, +now: Nat, +rnd: Nat, r: Runner<A>, err: Out(Unit),
1342	    next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1343	  match mf:
1344	    case None{}:
1345	      next(r, err)
1346	    case Some{Flight{Q.Ticket{id, d, x}, g, b, key}}:
1347	      settle.op(~A, ~penc, o, b, id, key, now, rnd, r, g, err, next)
1348	
1349	def settle.take(~A: Data, ~penc: A -> Bytes.Bytes, t: Maybe<&1, Flight<A>> & List<&1, Flight<A>>, +o: Retry.Outcome, +now: Nat, +rnd: Nat,
1350	    j: Log, s: State<A>, p: Res.Pool, c: Config, ib: Chan(Msg<A>), err: Out(Unit),
1351	    next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1352	  (mf, fs) = t
1353	  settle.flight(~A, ~penc, mf, o, now, rnd, Runner{j, s, p, c, ib, fs}, err, next)
1354	
1355	# Settles the flight of ticket id with outcome o.
1356	def settle(~A: Data, ~penc: A -> Bytes.Bytes, +id: Nat, +o: Retry.Outcome, +now: Nat, +rnd: Nat, r: Runner<A>, err: Out(Unit),
1357	    next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1358	  Runner{j, s, p, c, ib, fs} = r
1359	  settle.take(~A, ~penc, take(A, fs, id), o, now, rnd, j, s, p, c, ib, err, next)
1360	
1361	# Sends the failure to the asker, and keeps it as the round's error.
1362	def asked.failed(-A: Data, -X: Type, +rep: Chan(Out(X)), r: Runner<A>, +c: U32, +m: String, err: Out(Unit),
1363	    next: Runner<A> -> Nat -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1364	  do IO<Runner<A> & Out(Bool)>:
1365	    x : Result<&1, &1, Out(X), Unit> <- Chan.send(Out(X), rep, Fail{(c, m)})
1366	    next(r, 0n, first(err, Fail{(c, m)}))
1367	
1368	def asked.fail(-A: Data, -X: Type, +rep: Chan(Out(X)), r: Runner<A>, e: U32 & String, err: Out(Unit),
1369	    next: Runner<A> -> Nat -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1370	  (c, m) = e
1371	  asked.failed(A, X, rep, r, c, m, err, next)
1372	
1373	def added(-A: Data, +rep: Chan(Out(Nat)), y: Runner<A> & Out(Nat), err: Out(Unit),
1374	    next: Runner<A> -> Nat -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1375	  match y:
1376	    case (r, Done{+i}):
1377	      do IO<Runner<A> & Out(Bool)>:
1378	        x : Result<&1, &1, Out(Nat), Unit> <- Chan.send(Out(Nat), rep, Done{i})
1379	        next(r, 1n, err)
1380	    case (r, Fail{e}):
1381	      asked.fail(A, Nat, rep, r, e, err, next)
1382	
1383	def dropped(-A: Data, +rep: Chan(Out(Unit)), y: Ran(A), err: Out(Unit),
1384	    next: Runner<A> -> Nat -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1385	  match y:
1386	    case (r, Done{u}):
1387	      do IO<Runner<A> & Out(Bool)>:
1388	        x : Result<&1, &1, Out(Unit), Unit> <- Chan.send(Out(Unit), rep, Done{Unit{}})
1389	        next(r, 0n, err)
1390	    case (r, Fail{e}):
1391	      asked.fail(A, Unit, rep, r, e, err, next)
1392	
1393	# Settles an ended attempt, or appends an ask's op and answers it. next gets the claim fuel the message adds: 1 for an enqueue.
1394	def handle(~A: Data, ~penc: A -> Bytes.Bytes, m: Msg<A>, +now: Nat, +rnd: Nat, r: Runner<A>, err: Out(Unit),
1395	    next: Runner<A> -> Nat -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1396	  match m:
1397	    case Ended{+id, +o}:
1398	      settle(~A, ~penc, id, o, now, rnd, r, err, (r2 => e2 => next(r2, 0n, e2)))
1399	    case Add{+d, x, +rep}:
1400	      IO.bind(Runner<A> & Out(Nat), Runner<A> & Out(Bool), enqueue(~A, ~penc, r, d, x), y => added(A, rep, y, err, next))
1401	    case Drop{+i, +rep}:
1402	      IO.bind(Ran(A), Runner<A> & Out(Bool), cancel(~A, ~penc, r, i), y => dropped(A, rep, y, err, next))
1403	
1404	def merged(-A: Data, e: Out(Unit), k: Claimed(A)) -> Claimed(A):
1405	  (r, n, err) = k
1406	  (r, n, first(e, err))
1407	
1408	def empty.heap(-A: Data, h: H.Heap<Q.Ticket<A>>) -> Bool:
1409	  match h:
1410	    case H.HLeaf{}:
1411	      True{}
1412	    case H.HNode{n, t, l, r}:
1413	      False{}
1414	
1415	def empty.list(-A: Data, xs: List<&2, Q.Ticket<A>>) -> Bool:
1416	  match xs:
1417	    case Nil{}:
1418	      True{}
1419	    case Con{t, rest}:
1420	      False{}
1421	
1422	# No ticket is pending or running. Dead tickets wait for a Revive, so they do not count.
1423	def idle(-A: Data, s: State<A>) -> Bool:
1424	  State{Q.State{n, h, run}, ts, ds, bs} = s
1425	  Bool.and(empty.heap(A, h), empty.list(A, run))
1426	
1427	def pending(-A: Data, s: State<A>) -> Nat:
1428	  State{Q.State{n, h, run}, ts, ds, bs} = s
1429	  H.Heap.size(Q.Ticket<A>, h)
1430	
1431	# The flights back, and whether none runs.
1432	def flights.none(-A: Data, fs: List<&1, Flight<A>>) -> List<&1, Flight<A>> & Bool:
1433	  match fs:
1434	    case Nil{}:
1435	      (Nil{}, True{})
1436	    case Con{f, rest}:
1437	      (Con{f, rest}, False{})
1438	
1439	# ---- compaction
1440	
1441	# Kept: compaction failed before the rename, and the old journal stands.
1442	# Lost: the rename happened, but the new journal did not open. Moved: the journal at path is the snapshot.
1443	type Swap is Type:
1444	  Kept{err: U32 & String}
1445	  Lost{err: U32 & String}
1446	  Moved{journal: J.Journal}
1447	
1448	def swap.reopened(x: Out(J.Journal & Unit)) -> Swap:
1449	  match x:
1450	    case Fail{e}:
1451	      Lost{e}
1452	    case Done{(j, u)}:
1453	      Moved{j}
1454	
1455	# The reopen fsyncs the directory, so the rename is durable before the next append.
1456	def swap.renamed(+path: String, +limit: U32, x: Out(Unit)) -> IO(Swap):
1457	  match x:
1458	    case Fail{e}:
1459	      IO.pure(Swap, Kept{e})
1460	    case Done{u}:
1461	      do IO<Swap>:
1462	        y : Out(J.Journal & Unit) <- J.open(~Unit, ~(a => b => a), path, limit, limit, Unit{})
1463	        return swap.reopened(y)
1464	
1465	def swap.written(+path: String, +tmp: String, +limit: U32, x: J.Journal & Out(Unit)) -> IO(Swap):
1466	  match x:
1467	    case (t, Fail{e}):
1468	      do IO<Swap>:
1469	        J.close(t)
1470	        return Kept{e}
1471	    case (t, Done{u}):
1472	      do IO<Swap>:
1473	        J.close(t)
1474	        y : Out(Unit) <- Files.rename(tmp, path)
1475	        swap.renamed(path, limit, y)
1476	
1477	def swap.opened(+path: String, +tmp: String, +limit: U32, x: Out(J.Journal & Unit), snap: Bytes.Bytes) -> IO(Swap):
1478	  match x:
1479	    case Fail{e}:
1480	      IO.pure(Swap, Kept{e})
1481	    case Done{(t, u)}:
1482	      do IO<Swap>:
1483	        y : J.Journal & Out(Unit) <- J.append(t, snap)
1484	        swap.written(path, tmp, limit, y)
1485	
1486	# Writes snap as the only record of path.compact, fsyncs it, and renames it over path.
1487	def swap(+path: String, +limit: U32, snap: Bytes.Bytes) -> IO(Swap):
1488	  +tmp = path ++ ".compact"
1489	  do IO<Swap>:
1490	    gone : Out(Unit) <- Files.remove(tmp)
1491	    x : Out(J.Journal & Unit) <- J.open(~Unit, ~(a => b => a), tmp, limit, limit, Unit{})
1492	    swap.opened(path, tmp, limit, x, snap)
1493	
1494	def log.stop(j: J.Journal) -> J.Journal:
1495	  J.Journal{file, max, end, ok} = j
1496	  J.Journal{file, max, end, False{}}
1497	
1498	def compact.moved(x: Swap, j: J.Journal, +max: U32, path: String, +limit: U32, +base: U32) -> IO(Log & Out(Unit)):
1499	  match x:
1500	    case Kept{e}:
1501	      IO.pure(Log & Out(Unit), (Log{j, path, limit, base}, Fail{e}))
1502	    case Lost{e}:
1503	      IO.pure(Log & Out(Unit), (Log{log.stop(j), path, limit, base}, Fail{e}))
1504	    case Moved{k}:
1505	      do IO<Log & Out(Unit)>:
1506	        J.close(j)
1507	        return (log.of(k, max, path, limit), Done{Unit{}})
1508	
1509	def compact.if(~A: Data, ~penc: A -> Bytes.Bytes, ok: Bool, j: J.Journal, +max: U32, +path: String, +limit: U32, +base: U32, s: State<A>) -> IO(Log & Out(Unit)):
1510	  match ok:
1511	    case False{}:
1512	      IO.pure(Log & Out(Unit), (Log{j, path, limit, base}, Fail{(5, "runner: an append failed; reopen the runner")}))
1513	    case True{}:
1514	      do IO<Log & Out(Unit)>:
1515	        x : Swap <- swap(path, limit, encode.snap(~A, ~penc, s))
1516	        compact.moved(x, j, max, path, limit, base)
1517	
1518	def compact.done(-A: Data, x: Log & Out(Unit), s: State<A>, p: Res.Pool, c: Config, ib: Chan(Msg<A>), fs: List<&1, Flight<A>>) -> Ran(A):
1519	  (l, o) = x
1520	  (Runner{l, s, p, c, ib, fs}, o)
1521	
1522	# Writes the state as one Snap record into a new journal, fsyncs it, and renames it over the old one.
1523	# A crash leaves one of the two journals, and both replay to this state. After a failure the old
1524	# journal stands; when the failure came after the rename, appends stop until the runner is reopened.
1525	def compact(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>) -> IO(Ran(A)):
1526	  Runner{Log{j, path, limit, base}, +s, p, c, ib, fs} = r
1527	  J.Journal{file, +max, end, +ok} = j
1528	  do IO<Ran(A)>:
1529	    x : Log & Out(Unit) <- compact.if(~A, ~penc, ok, J.Journal{file, max, end, ok}, max, path, limit, base, s)
1530	    return compact.done(A, x, s, p, c, ib, fs)
1531	
1532	def tick.ran(e: Out(Unit), b: Bool) -> Out(Bool):
1533	  match e:
1534	    case Fail{x}:
1535	      Fail{x}
1536	    case Done{u}:
1537	      Done{b}
1538	
1539	def tick.compacted(-A: Data, +b: Bool, y: Ran(A)) -> Runner<A> & Out(Bool):
1540	  (r, e) = y
1541	  (r, tick.ran(e, b))
1542	
1543	def tick.due(~A: Data, ~penc: A -> Bytes.Bytes, due: Bool, +b: Bool, r: Runner<A>) -> IO(Runner<A> & Out(Bool)):
1544	  match due:
1545	    case False{}:
1546	      IO.pure(Runner<A> & Out(Bool), (r, Done{b}))
1547	    case True{}:
1548	      do IO<Runner<A> & Out(Bool)>:
1549	        y : Ran(A) <- compact(~A, ~penc, r)
1550	        return tick.compacted(A, b, y)
1551	
1552	def tick.size(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +b: Bool) -> IO(Runner<A> & Out(Bool)):
1553	  Runner{Log{J.Journal{file, max, +end, ok}, path, limit, +base}, s, p, +c, ib, fs} = r
1554	  Config{pol, circ, idle, +every} = c
1555	  tick.due(~A, ~penc, Bool.and(Bool.not(U32.is_eq(every, 0)), U32.is_gt((end - base : U32), every)), b,
1556	    Runner{Log{J.Journal{file, max, end, ok}, path, limit, base}, s, p, c, ib, fs})
1557	
1558	# Compacts once the journal grew past Config.compact octets since open or the last compaction.
1559	def tick.grown(~A: Data, ~penc: A -> Bytes.Bytes, t: Runner<A> & Out(Bool)) -> IO(Runner<A> & Out(Bool)):
1560	  match t:
1561	    case (r, Fail{e}):
1562	      IO.pure(Runner<A> & Out(Bool), (r, Fail{e}))
1563	    case (r, Done{b}):
1564	      tick.size(~A, ~penc, r, b)
1565	
1566	def tick.polled(-A: Data, x: Poll<&1, &1, Maybe<&1, Msg<A>>, Unit>) -> Maybe<&1, Msg<A>>:
1567	  match x:
1568	    case Ready{m}:
1569	      m
1570	    case Wait{u}:
1571	      None{}
1572	
1573	def tick.wait(-A: Data, t: List<&1, Flight<A>> & Bool, j: Log, s: State<A>, p: Res.Pool, c: Config, +ib: Chan(Msg<A>)) ->
1574	    IO(Runner<A> & Maybe<&1, Msg<A>>):
1575	  (fs, none) = t
1576	  match none:
1577	    case False{}:
1578	      do IO<Runner<A> & Maybe<&1, Msg<A>>>:
1579	        m : Maybe<&1, Msg<A>> <- Chan.recv(Msg<A>, ib)
1580	        return (Runner{j, s, p, c, ib, fs}, m)
1581	    case True{}:
1582	      do IO<Runner<A> & Maybe<&1, Msg<A>>>:
1583	        x : Poll<&1, &1, Maybe<&1, Msg<A>>, Unit> <- Chan.try_recv(Msg<A>, ib, 0)
1584	        return (Runner{j, s, p, c, ib, fs}, tick.polled(A, x))
1585	
1586	# The next message. While an attempt runs, it waits for one; when none runs, it takes one only when the inbox holds one.
1587	def tick.next(-A: Data, r: Runner<A>) -> IO(Runner<A> & Maybe<&1, Msg<A>>):
1588	  Runner{j, s, p, c, ib, fs} = r
1589	  tick.wait(A, flights.none(A, fs), j, s, p, c, ib)
1590	
1591	# Handles the message, then claims due tickets into free units with the claim fuel left. None ends the round.
1592	def tick.got(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), x: Runner<A> & Maybe<&1, Msg<A>>, +ran: Bool, +left: Nat,
1593	    err: Out(Unit), next: Claimed(A) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
1594	  (r, m) = x
1595	  match m:
1596	    case None{}:
1597	      IO.pure(Runner<A> & Out(Bool), (r, tick.ran(err, ran)))
1598	    case Some{msg}:
1599	      do IO<Runner<A> & Out(Bool)>:
1600	        +now : Nat <- clock()
1601	        rnd : Out(U32) <- IO.random_u32()
1602	        handle(~A, ~penc, msg, now, random(rnd), r, err, (r2 => more => e2 =>
1603	          IO.bind(Claimed(A), Runner<A> & Out(Bool), claim(~A, ~penc, ~target, ~work, Nat.add(left, more), now, r2), k2 => next(merged(A, e2, k2)))))
1604	
1605	# Handles messages until no attempt runs and the inbox is empty. fuel bounds the messages; attempts that still run
1606	# when it ends stay in the runner, and the next tick settles them.
1607	def tick.run(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), fuel: Nat, +ran: Bool, k: Claimed(A)) ->
1608	    IO(Runner<A> & Out(Bool)):
1609	  match fuel:
1610	    case 0n:
1611	      (r, left, err) = k
1612	      IO.pure(Runner<A> & Out(Bool), (r, tick.ran(err, ran)))
1613	    case 1n+n:
1614	      (r, +left, err) = k
1615	      do IO<Runner<A> & Out(Bool)>:
1616	        x : Runner<A> & Maybe<&1, Msg<A>> <- tick.next(A, r)
1617	        tick.got(~A, ~penc, ~target, ~work, x, ran, left, err, (k2 => tick.run(~A, ~penc, ~target, ~work, n, True{}, k2)))
1618	
1619	# Fuel for each pending ticket, each attempt that may already run, and the last look at the inbox.
1620	def tick.at(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +now: Nat, r: Runner<A>) -> IO(Runner<A> & Out(Bool)):
1621	  Runner{j, +s, +p, c, ib, fs} = r
1622	  +fuel = Nat.add(2n, Nat.add(pending(A, s), Res.capacity(p)))
1623	  do IO<Runner<A> & Out(Bool)>:
1624	    k : Claimed(A) <- claim(~A, ~penc, ~target, ~work, fuel, now, Runner{j, s, p, c, ib, fs})
1625	    tick.run(~A, ~penc, ~target, ~work, fuel, False{}, k)
1626	
1627	# One round: start an attempt for each due ticket the pool admits. Then read the inbox: as each attempt finishes, record its outcome,
1628	# return its unit, and start the next due ticket in that unit; append and answer each ask from another fiber.
1629	# The round ends when no attempt runs and the inbox is empty. Then compact the journal when it grew past Config.compact.
1630	# Done{True} when an attempt ran or an ask was answered. work runs on its own; target names the breaker that guards it.
1631	def tick(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), r: Runner<A>) -> IO(Runner<A> & Out(Bool)):
1632	  do IO<Runner<A> & Out(Bool)>:
1633	    now : Nat <- clock()
1634	    t : Runner<A> & Out(Bool) <- tick.at(~A, ~penc, ~target, ~work, now, r)
1635	    tick.grown(~A, ~penc, t)
1636	
1637	def finished.of(-A: Data, +quiet: Bool, t: List<&1, Flight<A>> & Bool, j: Log, s: State<A>, p: Res.Pool, c: Config, ib: Chan(Msg<A>)) ->
1638	    Runner<A> & Bool:
1639	  (fs, none) = t
1640	  (Runner{j, s, p, c, ib, fs}, Bool.and(quiet, none))
1641	
1642	# The runner back, and whether no ticket is pending or running and no attempt runs.
1643	def finished(-A: Data, r: Runner<A>) -> Runner<A> & Bool:
1644	  Runner{j, +s, p, c, ib, fs} = r
1645	  finished.of(A, idle(A, s), flights.none(A, fs), j, s, p, c, ib)
1646	
1647	def drain.handled(-A: Data, t: Runner<A> & Out(Bool), next: Runner<A> -> IO(Ran(A))) -> IO(Ran(A)):
1648	  match t:
1649	    case (r, Fail{e}):
1650	      IO.pure(Ran(A), (r, Fail{e}))
1651	    case (r, Done{b}):
1652	      next(r)
1653	
1654	def drain.woke(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +idle: U32,
1655	    x: Poll<&1, &1, Maybe<&1, Msg<A>>, Unit>, r: Runner<A>, next: Runner<A> -> IO(Ran(A))) -> IO(Ran(A)):
1656	  match x:
1657	    case Ready{Some{m}}:
1658	      do IO<Ran(A)>:
1659	        t : Runner<A> & Out(Bool) <- tick.got(~A, ~penc, ~target, ~work, (r, Some{m}), True{}, 0n, Done{Unit{}},
1660	          (k => tick.run(~A, ~penc, ~target, ~work, 0n, True{}, k)))
1661	        drain.handled(A, t, next)
1662	    case Ready{None{}}:
1663	      do IO<Ran(A)>:
1664	        IO.sleep(idle)
1665	        next(r)
1666	    case Wait{u}:
1667	      next(r)
1668	
1669	def drain.wait(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +idle: U32, r: Runner<A>,
1670	    next: Runner<A> -> IO(Ran(A))) -> IO(Ran(A)):
1671	  Runner{j, s, p, c, +ib, fs} = r
1672	  do IO<Ran(A)>:
1673	    x : Poll<&1, &1, Maybe<&1, Msg<A>>, Unit> <- Chan.try_recv(Msg<A>, ib, idle)
1674	    drain.woke(~A, ~penc, ~target, ~work, idle, x, Runner{j, s, p, c, ib, fs}, next)
1675	
1676	def drain.ticked(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +idle: U32,
1677	    t: Runner<A> & Out(Bool), next: Runner<A> -> IO(Ran(A))) -> IO(Ran(A)):
1678	  match t:
1679	    case (r, Fail{e}):
1680	      IO.pure(Ran(A), (r, Fail{e}))
1681	    case (r, Done{True{}}):
1682	      next(r)
1683	    case (r, Done{False{}}):
1684	      drain.wait(~A, ~penc, ~target, ~work, idle, r, next)
1685	
1686	def drain.if(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), done: Bool, r: Runner<A>, next: Runner<A> -> IO(Ran(A))) -> IO(Ran(A)):
1687	  match done:
1688	    case True{}:
1689	      IO.pure(Ran(A), (r, Done{Unit{}}))
1690	    case False{}:
1691	      Runner{j, s, p, +c, ib, fs} = r
1692	      Config{pol, circ, +idle, every} = c
1693	      do IO<Ran(A)>:
1694	        t : Runner<A> & Out(Bool) <- tick(~A, ~penc, ~target, ~work, Runner{j, s, p, c, ib, fs})
1695	        drain.ticked(~A, ~penc, ~target, ~work, idle, t, next)
1696	
1697	def drain.at(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), x: Runner<A> & Bool, next: Runner<A> -> IO(Ran(A))) -> IO(Ran(A)):
1698	  (r, done) = x
1699	  drain.if(~A, ~penc, ~target, ~work, done, r, next)
1700	
1701	def drain.end(-A: Data, x: Runner<A> & Bool) -> Ran(A):
1702	  (r, done) = x
1703	  (r, Bool.pick(Out(Unit), done, Done{Unit{}}, Fail{(62, "runner: drain ran out of rounds")}))
1704	
1705	# Ticks until no ticket is pending or running and no attempt runs. After a round that did nothing, it waits up to idle ms
1706	# for a message, and handles one that arrives at once.
1707	# Fails when fuel rounds leave work.
1708	def drain(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), fuel: Nat, r: Runner<A>) -> IO(Ran(A)):
1709	  match fuel:
1710	    case 0n:
1711	      IO.pure(Ran(A), drain.end(A, finished(A, r)))
1712	    case 1n+n:
1713	      drain.at(~A, ~penc, ~target, ~work, finished(A, r), (r2 => drain(~A, ~penc, ~target, ~work, n, r2)))
1714	
1715	def orphans.next(-A: Data, y: Ran(A), err: Out(Unit), next: Runner<A> -> Out(Unit) -> IO(Ran(A))) -> IO(Ran(A)):
1716	  (r, e) = y
1717	  next(r, first(err, e))
1718	
1719	# A running ticket at open lost its attempt in a crash: it is requeued at now, as a failed attempt.
1720	def orphans(~A: Data, ~penc: A -> Bytes.Bytes, ts: List<&2, Q.Ticket<A>>, +now: Nat, r: Runner<A>, err: Out(Unit)) -> IO(Ran(A)):
1721	  match ts:
1722	    case Nil{}:
1723	      IO.pure(Ran(A), (r, err))
1724	    case Con{Q.Ticket{id, d, x}, rest}:
1725	      do IO<Ran(A)>:
1726	        y : Ran(A) <- commit(~A, ~penc, r, Requeue{id, now})
1727	        orphans.next(A, y, err, (r2 => e2 => orphans(~A, ~penc, rest, now, r2, e2)))
1728	
1729	def close.answer(-A: Data, m: Msg<A>) -> IO(Unit):
1730	  match m:
1731	    case Ended{id, o}:
1732	      IO.pure(Unit, Unit{})
1733	    case Add{d, x, rep}:
1734	      do IO<Unit>:
1735	        y : Result<&1, &1, Out(Nat), Unit> <- Chan.send(Out(Nat), rep, Fail{(32, "runner: the runner closed")})
1736	        return Unit{}
1737	    case Drop{i, rep}:
1738	      do IO<Unit>:
1739	        y : Result<&1, &1, Out(Unit), Unit> <- Chan.send(Out(Unit), rep, Fail{(32, "runner: the runner closed")})
1740	        return Unit{}
1741	
1742	def close.polled(-A: Data, x: Poll<&1, &1, Maybe<&1, Msg<A>>, Unit>, next: Unit -> IO(Unit)) -> IO(Unit):
1743	  match x:
1744	    case Ready{Some{m}}:
1745	      do IO<Unit>:
1746	        close.answer(A, m)
1747	        next(Unit{})
1748	    case Ready{None{}}:
1749	      IO.pure(Unit, Unit{})
1750	    case Wait{u}:
1751	      IO.pure(Unit, Unit{})
1752	
1753	# Answers each ask left in the closed inbox with a failure. fuel bounds the messages it reads.
1754	def close.drain(-A: Data, fuel: Nat, +ib: Chan(Msg<A>)) -> IO(Unit):
1755	  match fuel:
1756	    case 0n:
1757	      IO.pure(Unit, Unit{})
1758	    case 1n+n:
1759	      do IO<Unit>:
1760	        x : Poll<&1, &1, Maybe<&1, Msg<A>>, Unit> <- Chan.try_recv(Msg<A>, ib, 0)
1761	        close.polled(A, x, (u => close.drain(A, n, ib)))
1762	
1763	# Closes the inbox and the journal. Each ask still in the inbox gets a failure, and no op is appended for it.
1764	def close(-A: Data, r: Runner<A>) -> IO(Unit):
1765	  Runner{j, s, +p, c, +ib, fs} = r
1766	  do IO<Unit>:
1767	    Chan.close(Msg<A>, ib)
1768	    close.drain(A, Nat.add(2n, Res.capacity(p)), ib)
1769	    log.close(j)
1770	
1771	def open.done(-A: Data, y: Ran(A)) -> IO(Out(Runner<A>)):
1772	  match y:
1773	    case (r, Fail{e}):
1774	      do IO<Out(Runner<A>)>:
1775	        close(A, r)
1776	        return Fail{e}
1777	    case (r, Done{u}):
1778	      IO.pure(Out(Runner<A>), Done{r})
1779	
1780	def open.got(~A: Data, ~penc: A -> Bytes.Bytes, x: Out(J.Journal & Maybe<&2, State<A>>), +max: U32, path: String, +limit: U32, c: Config, +capacity: Nat) ->
1781	    IO(Out(Runner<A>)):
1782	  match x:
1783	    case Fail{e}:
1784	      IO.pure(Out(Runner<A>), Fail{e})
1785	    case Done{(j, None{})}:
1786	      do IO<Out(Runner<A>)>:
1787	        J.close(j)
1788	        return Fail{(22, "runner: a journal record is not a runner record")}
1789	    case Done{(j, Some{+s})}:
1790	      do IO<Out(Runner<A>)>:
1791	        now : Nat <- clock()
1792	        ib : Chan(Msg<A>) <- Chan.new(Msg<A>, (U32.from_nat(capacity) + 1 : U32))
1793	        y : Ran(A) <- orphans(~A, ~penc, Q.running(A, queue(A, s)), now, Runner{log.of(j, max, path, limit), calm(A, s), Res.new(0n, capacity), c, ib, Nil{}}, Done{Unit{}})
1794	        open.done(A, y)
1795	
1796	def replay.step(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, acc: Maybe<&2, State<A>>, b: Bytes.Bytes) -> Maybe<&2, State<A>>:
1797	  load(~A, ~Bytes.Bytes, ~(y => decode.rec(~A, ~pdec, y)), acc, b)
1798	
1799	# Opens the runner at path: replays its journal into the state, then requeues each ticket that was running.
1800	# capacity bounds the attempts that run at once. max caps an op record's octets; limit caps the journal file and a snapshot.
1801	# Fails when the journal fails to open, or a committed record is not a snapshot or an op.
1802	def open(~A: Data, ~penc: A -> Bytes.Bytes, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +path: String, +max: U32, +limit: U32, c: Config, +capacity: Nat) -> IO(Out(Runner<A>)):
1803	  do IO<Out(Runner<A>)>:
1804	    x : Out(J.Journal & Maybe<&2, State<A>>) <- J.open(~Maybe<&2, State<A>>, ~(acc => b => replay.step(~A, ~pdec, acc, b)), path, limit, limit, Some{new(A)})
1805	    open.got(~A, ~penc, x, max, path, limit, c, capacity)
1806	
1807	# The runner back, and a copy of its state.
1808	def state(-A: Data, r: Runner<A>) -> Runner<A> & State<A>:
1809	  Runner{j, +s, p, c, ib, fs} = r
1810	  (Runner{j, s, p, c, ib, fs}, s)