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@0.3.2.0/bytes.bend as Bytes
4	import bend-kit-journal@0.2.0.0/journal.bend as J
5	import bend-kit-scheduler@0.3.0.0/queue.bend as Q
6	import bend-kit-resources@0.1.0.0/resources.bend as Res
7	import bend-kit-retry@0.1.0.0/retry.bend as Retry
8	import bend-kit-time@0.2.0.0/time.bend as Time
9	import bend-kit-collections@0.1.3.0/heap.bend as H
10	import bend-kit-collections@0.1.3.0/omap.bend as O
11	import bend-kit-files@0.2.0.0/files.bend as Files
12	
13	# Why retry gave up on a ticket: its budget was spent, its deadline passed, or its failure must not repeat.
14	type Why is Data:
15	  WhySpent{}
16	  WhyLate{}
17	  WhyUnsafe{}
18	
19	# A ticket that retry gave up on, with its payload. It stays until Revive or Cancel takes it.
20	type Dead<-A: Data> is Data:
21	  Dead{ticket: Q.Ticket<A>, why: Why}
22	
23	# One queue transition. Each one is one journal record, appended before the state changes.
24	# Select: the queue's select at now. Replay runs it at the same now, so it takes the same ticket.
25	# Requeue: the attempt failed; it counts one failure. Defer: the breaker held the ticket back; no attempt ran.
26	# Abandon: retry gave up; the running ticket becomes dead. Revive: a dead ticket is pending again at deadline, with a new budget.
27	type Op<-A: Data> is Data:
28	  Enqueue{deadline: Nat, payload: A}
29	  Select{now: Nat}
30	  Complete{id: Nat}
31	  Cancel{id: Nat}
32	  Requeue{id: Nat, deadline: Nat}
33	  Defer{id: Nat, deadline: Nat}
34	  Abandon{id: Nat, why: Why}
35	  Revive{id: Nat, deadline: Nat}
36	
37	# The failed attempts of a ticket, and its first deadline, from which the policy deadline counts.
38	type Try is Data:
39	  Try{fails: Nat, start: Nat}
40	
41	# A Try for each ticket that is not yet complete, cancelled, or dead, by ticket id.
42	def Tries() -> Data:
43	  O.OMap<Nat, Try>
44	
45	# The durable state: the queue, the tries, and the dead tickets, newest first.
46	type State<-A: Data> is Data:
47	  State{queue: Q.State<A>, tries: Tries(), dead: List<&2, Dead<A>>}
48	
49	def new(-A: Data) -> State<A>:
50	  State{Q.new(A), O.OMap.new(Nat, Try), Nil{}}
51	
52	def queue(-A: Data, s: State<A>) -> Q.State<A>:
53	  State{q, ts, ds} = s
54	  q
55	
56	def tries(-A: Data, s: State<A>) -> Tries():
57	  State{q, ts, ds} = s
58	  ts
59	
60	# The dead tickets, newest first. A Revive of one makes it pending again; a Cancel drops it.
61	def dead(-A: Data, s: State<A>) -> List<&2, Dead<A>>:
62	  State{q, ts, ds} = s
63	  ds
64	
65	# ---- dead tickets
66	
67	def dead.id(-A: Data, d: Dead<A>) -> Nat:
68	  match d:
69	    case Dead{t, w}:
70	      Q.id(A, t)
71	
72	def dead.remove.keep(-A: Data, b: Bool, d: Dead<A>, rest: List<&2, Dead<A>>) -> List<&2, Dead<A>>:
73	  match b:
74	    case True{}:
75	      rest
76	    case False{}:
77	      Con{d, rest}
78	
79	# The dead tickets without ticket i.
80	def dead.remove(-A: Data, ds: List<&2, Dead<A>>, +i: Nat) -> List<&2, Dead<A>>:
81	  match ds:
82	    case Nil{}:
83	      Nil{}
84	    case Con{+d, rest}:
85	      dead.remove.keep(A, Nat.is_eq(dead.id(A, d), i), d, dead.remove(A, rest, i))
86	
87	def dead.find.choose(-A: Data, b: Bool, d: Dead<A>, rest: Maybe<&2, Q.Ticket<A>>) -> Maybe<&2, Q.Ticket<A>>:
88	  match b:
89	    case True{}:
90	      match d:
91	        case Dead{t, w}:
92	          Some{t}
93	    case False{}:
94	      rest
95	
96	# The ticket of dead ticket i.
97	def dead.find(-A: Data, ds: List<&2, Dead<A>>, +i: Nat) -> Maybe<&2, Q.Ticket<A>>:
98	  match ds:
99	    case Nil{}:
100	      None{}
101	    case Con{+d, rest}:
102	      dead.find.choose(A, Nat.is_eq(dead.id(A, d), i), d, dead.find(A, rest, i))
103	
104	def dead.bury(-A: Data, m: Maybe<&2, Q.Ticket<A>>, ds: List<&2, Dead<A>>, w: Why) -> List<&2, Dead<A>>:
105	  match m:
106	    case None{}:
107	      ds
108	    case Some{t}:
109	      Con{Dead{t, w}, ds}
110	
111	# ---- tries
112	
113	def tries.drop(ts: Tries(), +i: Nat) -> Tries():
114	  O.OMap.del(~Nat, ~Try, ~Nat.cmp, ts, i)
115	
116	def tries.get(ts: Tries(), +i: Nat) -> Maybe<&2, Try>:
117	  O.OMap.get(~Nat, ~Try, ~Nat.cmp, ts, i)
118	
119	def tries.bump.got(m: Maybe<&2, Try>, ts: Tries(), +i: Nat) -> Tries():
120	  match m:
121	    case None{}:
122	      ts
123	    case Some{Try{f, st}}:
124	      O.OMap.put(~Nat, ~Try, ~Nat.cmp, ts, i, Try{1n+f, st})
125	
126	def tries.bump(+ts: Tries(), +i: Nat) -> Tries():
127	  tries.bump.got(tries.get(ts, i), ts, i)
128	
129	# ---- the transition
130	
131	def step.enqueued(-A: Data, r: Q.State<A> & Nat, ts: Tries(), ds: List<&2, Dead<A>>, d: Nat) -> State<A>:
132	  (q, i) = r
133	  State{q, O.OMap.put(~Nat, ~Try, ~Nat.cmp, ts, i, Try{0n, d}), ds}
134	
135	def step.selected(-A: Data, r: Q.State<A> & Maybe<&2, Q.Ticket<A>>, ts: Tries(), ds: List<&2, Dead<A>>) -> State<A>:
136	  (q, t) = r
137	  State{q, ts, ds}
138	
139	def step.requeued(-A: Data, r: Q.State<A> & Bool, ts: Tries(), ds: List<&2, Dead<A>>) -> State<A>:
140	  (q, b) = r
141	  State{q, ts, ds}
142	
143	# Complete takes the ticket from the running list only: O(running), not O(pending).
144	def step.completed(-A: Data, r: Q.State<A> & Maybe<&2, Q.Ticket<A>>, ts: Tries(), ds: List<&2, Dead<A>>) -> State<A>:
145	  (q, t) = r
146	  State{q, ts, ds}
147	
148	# Abandon takes the ticket from the running list, as Complete does, and keeps it with its payload.
149	def step.abandoned(-A: Data, r: Q.State<A> & Maybe<&2, Q.Ticket<A>>, ts: Tries(), ds: List<&2, Dead<A>>, w: Why) -> State<A>:
150	  (q, t) = r
151	  State{q, ts, dead.bury(A, t, ds, w)}
152	
153	def step.revived(~A: Data, m: Maybe<&2, Q.Ticket<A>>, n: Nat, h: H.Heap<Q.Ticket<A>>, r: List<&2, Q.Ticket<A>>, ts: Tries(), ds: List<&2, Dead<A>>, +d: Nat) -> State<A>:
154	  match m:
155	    case None{}:
156	      State{Q.State{n, h, r}, ts, ds}
157	    case Some{Q.Ticket{+i, old, x}}:
158	      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}
159	
160	# The state after op. Live and replay both run it, so they agree.
161	def step(~A: Data, s: State<A>, op: Op<A>) -> State<A>:
162	  match s op:
163	    case State{q, ts, ds} Enqueue{+d, x}:
164	      step.enqueued(A, Q.enqueue(~A, d, x, q), ts, ds, d)
165	    case State{q, ts, ds} Select{now}:
166	      step.selected(A, Q.select(~A, now, q), ts, ds)
167	    case State{q, ts, ds} Complete{+i}:
168	      step.completed(A, Q.complete(A, i, q), tries.drop(ts, i), ds)
169	    case State{q, ts, ds} Cancel{+i}:
170	      State{Q.cancel(~A, i, q), tries.drop(ts, i), dead.remove(A, ds, i)}
171	    case State{q, ts, ds} Requeue{+i, d}:
172	      step.requeued(A, Q.requeue(~A, i, d, q), tries.bump(ts, i), ds)
173	    case State{q, ts, ds} Defer{i, d}:
174	      step.requeued(A, Q.requeue(~A, i, d, q), ts, ds)
175	    case State{q, ts, ds} Abandon{+i, w}:
176	      step.abandoned(A, Q.complete(A, i, q), tries.drop(ts, i), ds, w)
177	    case State{Q.State{n, h, r}, ts, +ds} Revive{+i, d}:
178	      step.revived(~A, dead.find(A, ds, i), n, h, r, ts, dead.remove(A, ds, i), d)
179	
180	def apply.op(~A: Data, m: Maybe<&2, Op<A>>, s: State<A>) -> Maybe<&2, State<A>>:
181	  match m:
182	    case None{}:
183	      None{}
184	    case Some{op}:
185	      Some{step(~A, s, op)}
186	
187	# Folds one record into the replayed state. A record that does not decode ends the replay with None.
188	def apply(~A: Data, ~R: Type, ~dec: R -> Maybe<&2, Op<A>>, acc: Maybe<&2, State<A>>, r: R) -> Maybe<&2, State<A>>:
189	  match acc:
190	    case None{}:
191	      None{}
192	    case Some{s}:
193	      apply.op(~A, dec(r), s)
194	
195	# A journal record. Compaction writes one Snap of the whole state; every other record is one op.
196	type Rec<-A: Data> is Data:
197	  Snap{state: State<A>}
198	  Do{op: Op<A>}
199	
200	def load.rec(~A: Data, m: Maybe<&2, Rec<A>>, s: State<A>) -> Maybe<&2, State<A>>:
201	  match m:
202	    case None{}:
203	      None{}
204	    case Some{Snap{t}}:
205	      Some{t}
206	    case Some{Do{op}}:
207	      Some{step(~A, s, op)}
208	
209	# Folds one record into the replayed state: a Snap replaces it, and an op steps it, as apply does.
210	# A record that does not decode ends the replay with None.
211	def load(~A: Data, ~R: Type, ~dec: R -> Maybe<&2, Rec<A>>, acc: Maybe<&2, State<A>>, r: R) -> Maybe<&2, State<A>>:
212	  match acc:
213	    case None{}:
214	      None{}
215	    case Some{s}:
216	      load.rec(~A, dec(r), s)
217	
218	# ---- pool
219	
220	# Picked: a due ticket and one unit of the pool for it. Idle: no due ticket, or no free unit; the pool is unchanged.
221	type Pick<-A: Data> is Type:
222	  Picked{pool: Res.Pool, ticket: Q.Ticket<A>, grant: Res.Grant}
223	  Idle{pool: Res.Pool}
224	
225	def pick.took(-A: Data, r: Res.Reserve, t: Q.Ticket<A>) -> Pick<A>:
226	  match r:
227	    case Res.Took{p, g}:
228	      Picked{p, t, g}
229	    case Res.Refused{p}:
230	      Idle{p}
231	
232	def pick.due(-A: Data, m: Maybe<&2, Q.Ticket<A>>, p: Res.Pool) -> Pick<A>:
233	  match m:
234	    case None{}:
235	      Idle{p}
236	    case Some{t}:
237	      pick.took(A, Res.reserve(p, 1n), t)
238	
239	def pick.sel(-A: Data, r: Q.State<A> & Maybe<&2, Q.Ticket<A>>, p: Res.Pool) -> Pick<A>:
240	  (q, m) = r
241	  pick.due(A, m, p)
242	
243	# The ticket that Select{now} takes, with one unit of p, when there is a ticket due and a unit free.
244	def pick(~A: Data, p: Res.Pool, s: State<A>, now: Nat) -> Pick<A>:
245	  State{q, ts, ds} = s
246	  pick.sel(A, Q.select(~A, now, q), p)
247	
248	# ---- retry
249	
250	# The budget left for ticket i: the policy's attempts less its failures, until its start plus the policy deadline.
251	def budget.of(+attempts: Nat, t: Maybe<&2, Try>, d: Maybe<&2, Nat>) -> Retry.Budget:
252	  match t d:
253	    case None{} m:
254	      Retry.Budget{attempts, None{}}
255	    case Some{Try{f, st}} None{}:
256	      Retry.Budget{Nat.sub(attempts, f), None{}}
257	    case Some{Try{f, +st}} Some{ms}:
258	      Retry.Budget{Nat.sub(attempts, f), Some{Nat.add(st, ms)}}
259	
260	def budget(p: Retry.Policy, t: Maybe<&2, Try>) -> Retry.Budget:
261	  Retry.Policy{a, base, cap, d} = p
262	  budget.of(a, t, d)
263	
264	# The failures so far: the index of the next retry.
265	def fails(t: Maybe<&2, Try>) -> Nat:
266	  match t:
267	    case None{}:
268	      0n
269	    case Some{Try{f, st}}:
270	      f
271	
272	# Why retry stopped a transient failure: no attempt is left, or the next wait would reach the deadline.
273	def gave(b: Retry.Budget) -> Why:
274	  Retry.Budget{left, d} = b
275	  match left:
276	    case 0n:
277	      WhySpent{}
278	    case 1n+k:
279	      WhyLate{}
280	
281	# A settled attempt completes its ticket; retry gave up on any other.
282	def stopped(-A: Data, o: Retry.Outcome, b: Retry.Budget, +id: Nat) -> Op<A>:
283	  match o:
284	    case Retry.Settled{}:
285	      Complete{id}
286	    case Retry.Unsafe{}:
287	      Abandon{id, WhyUnsafe{}}
288	    case Retry.Transient{w}:
289	      Abandon{id, gave(b)}
290	
291	def after.plan(-A: Data, pl: Retry.Plan, o: Retry.Outcome, b: Retry.Budget, +id: Nat, +now: Nat) -> Op<A>:
292	  match pl:
293	    case Retry.PlanStop{}:
294	      stopped(A, o, b, id)
295	    case Retry.PlanWait{w}:
296	      Requeue{id, Nat.add(now, w)}
297	
298	# The record after an attempt of ticket id ends at now with o: requeue at now plus retry's wait,
299	# complete when o is settled, or abandon when retry gave up.
300	def after(-A: Data, +o: Retry.Outcome, p: Retry.Policy, +b: Retry.Budget, +id: Nat, +n: Nat, +now: Nat, +r: Nat) -> Op<A>:
301	  after.plan(A, Retry.plan(o, p, b, n, now, r), o, b, id, now)
302	
303	# When an open breaker lets attempts start again.
304	def until(br: Retry.Breaker, +now: Nat) -> Nat:
305	  match br:
306	    case Retry.BreakerOn{c, Retry.CircuitOpen{u}}:
307	      u
308	    case Retry.BreakerOn{c, Retry.CircuitClosed{fs}}:
309	      Nat.add(now, Retry.Circuit.cooldown(c))
310	    case Retry.BreakerOn{c, Retry.CircuitHalf{k}}:
311	      Nat.add(now, Retry.Circuit.cooldown(c))
312	    case Retry.BreakerOff{}:
313	      now
314	
315	# The record for a ticket that admit refused: Defer to when the breaker may admit, or Abandon when its budget is spent or late.
316	def held(-A: Data, why: Retry.Deny, br: Retry.Breaker, +id: Nat, +now: Nat) -> Op<A>:
317	  match why:
318	    case Retry.DenyOpen{}:
319	      Defer{id, until(br, now)}
320	    case Retry.DenyLate{}:
321	      Abandon{id, WhyLate{}}
322	    case Retry.DenySpent{}:
323	      Abandon{id, WhySpent{}}
324	
325	# ---- the record codec: a u32le tag, then u64le fields; an Enqueue then holds the payload. An Abandon's second field is why.code.
326	
327	def why.code(w: Why) -> Nat:
328	  match w:
329	    case WhySpent{}:
330	      0n
331	    case WhyLate{}:
332	      1n
333	    case WhyUnsafe{}:
334	      2n
335	
336	def W() -> Nat:
337	  Nat.mul(65536n, 65536n)
338	
339	# Nat fields hold values under 2^64.
340	def put.u64(b: Bytes.Bytes, +at: U32, +n: Nat) -> Bytes.Bytes:
341	  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())))
342	
343	def head(+tag: U32, +a: Nat, +c: Nat) -> Bytes.Bytes:
344	  put.u64(put.u64(Bytes.set.u32le(Bytes.new(20), 0, tag), 4, a), 12, c)
345	
346	def encode(~A: Data, ~penc: A -> Bytes.Bytes, op: Op<A>) -> Bytes.Bytes:
347	  match op:
348	    case Enqueue{+d, x}:
349	      Bytes.append(put.u64(Bytes.set.u32le(Bytes.new(12), 0, 1), 4, d), penc(x))
350	    case Select{+now}:
351	      head(2, now, 0n)
352	    case Complete{+i}:
353	      head(3, i, 0n)
354	    case Cancel{+i}:
355	      head(4, i, 0n)
356	    case Requeue{+i, +d}:
357	      head(5, i, d)
358	    case Defer{+i, +d}:
359	      head(6, i, d)
360	    case Abandon{+i, w}:
361	      head(8, i, why.code(w))
362	    case Revive{+i, +d}:
363	      head(9, i, d)
364	
365	def get.field(m: Maybe<&2, U32>) -> U32:
366	  match m:
367	    case None{}:
368	      0
369	    case Some{x}:
370	      x
371	
372	def get.u32(r: Bytes.Bytes & Maybe<&2, U32>) -> Bytes.Bytes & U32:
373	  (b, m) = r
374	  (b, get.field(m))
375	
376	def get.hi(+lo: U32, r: Bytes.Bytes & Maybe<&2, U32>) -> Bytes.Bytes & Nat:
377	  (b, m) = r
378	  (b, Nat.add(U32.to_nat(lo), Nat.mul(U32.to_nat(get.field(m)), W())))
379	
380	def get.lo(+at: U32, r: Bytes.Bytes & Maybe<&2, U32>) -> Bytes.Bytes & Nat:
381	  (b, m) = r
382	  +lo = get.field(m)
383	  get.hi(lo, Bytes.get.u32le(b, (at + 4 : U32)))
384	
385	def get.u64(b: Bytes.Bytes, +at: U32) -> Bytes.Bytes & Nat:
386	  get.lo(at, Bytes.get.u32le(b, at))
387	
388	# The Abandon of ticket a whose why.code is c, or None for any other code.
389	def decode.abandon(-A: Data, +a: Nat, +c: Nat) -> Maybe<&2, Op<A>>:
390	  Bool.pick(Maybe<&2, Op<A>>, Nat.is_eq(c, 0n), Some{Abandon{a, WhySpent{}}},
391	  Bool.pick(Maybe<&2, Op<A>>, Nat.is_eq(c, 1n), Some{Abandon{a, WhyLate{}}},
392	  Bool.pick(Maybe<&2, Op<A>>, Nat.is_eq(c, 2n), Some{Abandon{a, WhyUnsafe{}}}, None{})))
393	
394	def decode.small(-A: Data, ok: Bool, +tag: U32, +a: Nat, +c: Nat) -> Maybe<&2, Op<A>>:
395	  match ok:
396	    case False{}:
397	      None{}
398	    case True{}:
399	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 2), Some{Select{a}},
400	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 3), Some{Complete{a}},
401	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 4), Some{Cancel{a}},
402	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 5), Some{Requeue{a, c}},
403	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 6), Some{Defer{a, c}},
404	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 8), decode.abandon(A, a, c),
405	      Bool.pick(Maybe<&2, Op<A>>, U32.is_eq(tag, 9), Some{Revive{a, c}}, None{})))))))
406	
407	def decode.payload(-A: Data, m: Maybe<&2, A>, +d: Nat) -> Maybe<&2, Op<A>>:
408	  match m:
409	    case None{}:
410	      None{}
411	    case Some{x}:
412	      Some{Enqueue{d, x}}
413	
414	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>>:
415	  (b, +c) = r
416	  decode.small(A, U32.is_eq(len, 20), tag, a, c)
417	
418	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>>:
419	  match enq:
420	    case True{}:
421	      decode.payload(A, pdec(Bytes.snd(Bytes.slice(b, 12, (len - 12 : U32)))), a)
422	    case False{}:
423	      decode.c(~A, ~pdec, tag, len, a, get.u64(b, 12))
424	
425	def decode.a(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +tag: U32, +len: U32, r: Bytes.Bytes & Nat) -> Maybe<&2, Op<A>>:
426	  (b, +a) = r
427	  decode.kind(~A, ~pdec, U32.is_eq(tag, 1), tag, len, a, b)
428	
429	def decode.tag(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +len: U32, r: Bytes.Bytes & U32) -> Maybe<&2, Op<A>>:
430	  (b, +tag) = r
431	  decode.a(~A, ~pdec, tag, len, get.u64(b, 4))
432	
433	def decode.len(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, ok: Bool, +len: U32, b: Bytes.Bytes) -> Maybe<&2, Op<A>>:
434	  match ok:
435	    case False{}:
436	      None{}
437	    case True{}:
438	      decode.tag(~A, ~pdec, len, get.u32(Bytes.get.u32le(b, 0)))
439	
440	# The op of a record, or None when it is not one that encode writes.
441	def decode(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, b: Bytes.Bytes) -> Maybe<&2, Op<A>>:
442	  Bytes.Bytes{+len, buf} = b
443	  decode.len(~A, ~pdec, U32.is_ge(len, 12), len, Bytes.Bytes{len, buf})
444	
445	# ---- the snapshot codec: tag 7 and the next id, then the heap, the running list, the tries, and the dead tickets.
446	# A u32 flag starts each tree node and list cell (1) and marks each end (0), so decode rebuilds the same shape.
447	# A ticket is its u64 id and deadline, then a u32 length and the payload. A dead ticket is its ticket, then a u32 why.code.
448	
449	def snap.u32(out: Bytes.Bytes, +v: U32) -> Bytes.Bytes:
450	  Bytes.append(out, Bytes.set.u32le(Bytes.new(4), 0, v))
451	
452	def snap.u64(out: Bytes.Bytes, +n: Nat) -> Bytes.Bytes:
453	  Bytes.append(out, put.u64(Bytes.new(8), 0, n))
454	
455	def snap.payload(out: Bytes.Bytes, p: Bytes.Bytes) -> Bytes.Bytes:
456	  Bytes.Bytes{+len, buf} = p
457	  Bytes.append(snap.u32(out, len), Bytes.Bytes{len, buf})
458	
459	def snap.ticket(~A: Data, ~penc: A -> Bytes.Bytes, out: Bytes.Bytes, t: Q.Ticket<A>) -> Bytes.Bytes:
460	  Q.Ticket{+i, +d, x} = t
461	  snap.payload(snap.u64(snap.u64(out, i), d), penc(x))
462	
463	def snap.heap(~A: Data, ~penc: A -> Bytes.Bytes, h: H.Heap<Q.Ticket<A>>, out: Bytes.Bytes) -> Bytes.Bytes:
464	  match h:
465	    case H.HLeaf{}:
466	      snap.u32(out, 0)
467	    case H.HNode{+n, t, l, r}:
468	      snap.heap(~A, ~penc, r, snap.heap(~A, ~penc, l, snap.ticket(~A, ~penc, snap.u64(snap.u32(out, 1), n), t)))
469	
470	def snap.run(~A: Data, ~penc: A -> Bytes.Bytes, xs: List<&2, Q.Ticket<A>>, out: Bytes.Bytes) -> Bytes.Bytes:
471	  match xs:
472	    case Nil{}:
473	      snap.u32(out, 0)
474	    case Con{t, rest}:
475	      snap.run(~A, ~penc, rest, snap.ticket(~A, ~penc, snap.u32(out, 1), t))
476	
477	def snap.tries(ts: Tries(), out: Bytes.Bytes) -> Bytes.Bytes:
478	  match ts:
479	    case O.OTip{}:
480	      snap.u32(out, 0)
481	    case O.OBin{+n, +k, Try{+f, +st}, lo, hi}:
482	      snap.tries(hi, snap.tries(lo, snap.u64(snap.u64(snap.u64(snap.u32(snap.u32(out, 1), n), k), f), st)))
483	
484	def snap.dead(~A: Data, ~penc: A -> Bytes.Bytes, ds: List<&2, Dead<A>>, out: Bytes.Bytes) -> Bytes.Bytes:
485	  match ds:
486	    case Nil{}:
487	      snap.u32(out, 0)
488	    case Con{Dead{t, w}, rest}:
489	      snap.dead(~A, ~penc, rest, snap.u32(snap.ticket(~A, ~penc, snap.u32(out, 1), t), U32.from_nat(why.code(w))))
490	
491	# The Snap record of s.
492	def encode.snap(~A: Data, ~penc: A -> Bytes.Bytes, s: State<A>) -> Bytes.Bytes:
493	  State{Q.State{+n, h, run}, ts, ds} = s
494	  snap.dead(~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)))))
495	
496	def encode.rec(~A: Data, ~penc: A -> Bytes.Bytes, r: Rec<A>) -> Bytes.Bytes:
497	  match r:
498	    case Snap{s}:
499	      encode.snap(~A, ~penc, s)
500	    case Do{op}:
501	      encode(~A, ~penc, op)
502	
503	# A snapshot reader: the record, and the offset of the next field.
504	type Rd is Type:
505	  Rd{b: Bytes.Bytes, at: U32}
506	
507	# One step of the snapshot decoder. None: a field runs past the record or does not decode.
508	def Dec(-T: Type) -> Type:
509	  Rd -> Maybe<&1, Rd & T>
510	
511	def Dec.next(-A: Type, -B: Type, m: Maybe<&1, Rd & A>, f: A -> Dec(B)) -> Maybe<&1, Rd & B>:
512	  match m:
513	    case None{}:
514	      None{}
515	    case Some{(r, x)}:
516	      f(x)(r)
517	
518	def Dec.bind(-A: Type, -B: Type, m: Dec(A), f: A -> Dec(B)) -> Dec(B):
519	  r => Dec.next(A, B, m(r), f)
520	
521	def Dec.pure(-A: Type, x: A) -> Dec(A):
522	  r => Some{(r, x)}
523	
524	def Dec.fail(-A: Type) -> Dec(A):
525	  r => None{}
526	
527	def rd.u32.of(m: Maybe<&2, U32>, b: Bytes.Bytes, +at: U32) -> Maybe<&1, Rd & U32>:
528	  match m:
529	    case None{}:
530	      None{}
531	    case Some{x}:
532	      Some{(Rd{b, (at + 4 : U32)}, x)}
533	
534	def rd.u32.got(+at: U32, r: Bytes.Bytes & Maybe<&2, U32>) -> Maybe<&1, Rd & U32>:
535	  (b, m) = r
536	  rd.u32.of(m, b, at)
537	
538	def rd.u32.at(r: Rd) -> Maybe<&1, Rd & U32>:
539	  Rd{b, +at} = r
540	  rd.u32.got(at, Bytes.get.u32le(b, at))
541	
542	def rd.u32() -> Dec(U32):
543	  r => rd.u32.at(r)
544	
545	def rd.u64.of(+lo: U32, +hi: U32) -> Nat:
546	  Nat.add(U32.to_nat(lo), Nat.mul(U32.to_nat(hi), W()))
547	
548	def rd.u64() -> Dec(Nat):
549	  do Dec<Nat>:
550	    lo : U32 <- rd.u32()
551	    hi : U32 <- rd.u32()
552	    return rd.u64.of(lo, hi)
553	
554	def rd.cut.got(+at: U32, +n: U32, r: Bytes.Bytes & Bytes.Bytes) -> Maybe<&1, Rd & Bytes.Bytes>:
555	  (b, p) = r
556	  Some{(Rd{b, (at + n : U32)}, p)}
557	
558	def rd.cut.if(ok: Bool, b: Bytes.Bytes, +at: U32, +n: U32) -> Maybe<&1, Rd & Bytes.Bytes>:
559	  match ok:
560	    case False{}:
561	      None{}
562	    case True{}:
563	      rd.cut.got(at, n, Bytes.slice(b, at, n))
564	
565	def rd.cut.at(+n: U32, r: Rd) -> Maybe<&1, Rd & Bytes.Bytes>:
566	  Rd{Bytes.Bytes{+len, buf}, +at} = r
567	  rd.cut.if(Bytes.fits(len, at, n), Bytes.Bytes{len, buf}, at, n)
568	
569	# The next n octets.
570	def rd.cut(+n: U32) -> Dec(Bytes.Bytes):
571	  r => rd.cut.at(n, r)
572	
573	def rd.flag.of(+m: U32) -> Dec(Bool):
574	  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)))
575	
576	# A u32 flag: 1 is True, 0 is False, and any other value fails.
577	def rd.flag() -> Dec(Bool):
578	  Dec.bind(U32, Bool, rd.u32(), m => rd.flag.of(m))
579	
580	def rd.payload(-A: Data, +i: Nat, +d: Nat, m: Maybe<&2, A>) -> Dec(Q.Ticket<A>):
581	  match m:
582	    case None{}:
583	      Dec.fail(Q.Ticket<A>)
584	    case Some{x}:
585	      Dec.pure(Q.Ticket<A>, Q.Ticket{i, d, x})
586	
587	def rd.ticket(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>) -> Dec(Q.Ticket<A>):
588	  do Dec<Q.Ticket<A>>:
589	    i : Nat <- rd.u64()
590	    d : Nat <- rd.u64()
591	    n : U32 <- rd.u32()
592	    p : Bytes.Bytes <- rd.cut(n)
593	    rd.payload(A, i, d, pdec(p))
594	
595	# Each rd.* tree reader takes the flag already read. fuel bounds the depth: every node holds at least one 4-octet flag.
596	def rd.heap(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +fuel: Nat, node: Bool) -> Dec(H.Heap<Q.Ticket<A>>):
597	  match fuel node:
598	    case 0n b:
599	      Dec.fail(H.Heap<Q.Ticket<A>>)
600	    case 1n+f False{}:
601	      Dec.pure(H.Heap<Q.Ticket<A>>, H.HLeaf{})
602	    case 1n+f True{}:
603	      do Dec<H.Heap<Q.Ticket<A>>>:
604	        n : Nat <- rd.u64()
605	        t : Q.Ticket<A> <- rd.ticket(~A, ~pdec)
606	        lf : Bool <- rd.flag()
607	        l : H.Heap<Q.Ticket<A>> <- rd.heap(~A, ~pdec, f, lf)
608	        rf : Bool <- rd.flag()
609	        r : H.Heap<Q.Ticket<A>> <- rd.heap(~A, ~pdec, f, rf)
610	        return H.HNode{n, t, l, r}
611	
612	def rd.run(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +fuel: Nat, more: Bool) -> Dec(List<&2, Q.Ticket<A>>):
613	  match fuel more:
614	    case 0n b:
615	      Dec.fail(List<&2, Q.Ticket<A>>)
616	    case 1n+f False{}:
617	      Dec.pure(List<&2, Q.Ticket<A>>, Nil{})
618	    case 1n+f True{}:
619	      do Dec<List<&2, Q.Ticket<A>>>:
620	        t : Q.Ticket<A> <- rd.ticket(~A, ~pdec)
621	        m : Bool <- rd.flag()
622	        rest : List<&2, Q.Ticket<A>> <- rd.run(~A, ~pdec, f, m)
623	        return Con{t, rest}
624	
625	def rd.tries(+fuel: Nat, node: Bool) -> Dec(O.OMap<Nat, Try>):
626	  match fuel node:
627	    case 0n b:
628	      Dec.fail(O.OMap<Nat, Try>)
629	    case 1n+f False{}:
630	      Dec.pure(O.OMap<Nat, Try>, O.OTip{})
631	    case 1n+f True{}:
632	      do Dec<O.OMap<Nat, Try>>:
633	        n : U32 <- rd.u32()
634	        k : Nat <- rd.u64()
635	        fails : Nat <- rd.u64()
636	        st : Nat <- rd.u64()
637	        lf : Bool <- rd.flag()
638	        lo : O.OMap<Nat, Try> <- rd.tries(f, lf)
639	        hf : Bool <- rd.flag()
640	        hi : O.OMap<Nat, Try> <- rd.tries(f, hf)
641	        return O.OBin{n, k, Try{fails, st}, lo, hi}
642	
643	def rd.why(+c: U32) -> Dec(Why):
644	  Bool.pick(Dec(Why), U32.is_eq(c, 0), Dec.pure(Why, WhySpent{}),
645	  Bool.pick(Dec(Why), U32.is_eq(c, 1), Dec.pure(Why, WhyLate{}),
646	  Bool.pick(Dec(Why), U32.is_eq(c, 2), Dec.pure(Why, WhyUnsafe{}), Dec.fail(Why))))
647	
648	def rd.dead(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +fuel: Nat, more: Bool) -> Dec(List<&2, Dead<A>>):
649	  match fuel more:
650	    case 0n b:
651	      Dec.fail(List<&2, Dead<A>>)
652	    case 1n+f False{}:
653	      Dec.pure(List<&2, Dead<A>>, Nil{})
654	    case 1n+f True{}:
655	      do Dec<List<&2, Dead<A>>>:
656	        t : Q.Ticket<A> <- rd.ticket(~A, ~pdec)
657	        c : U32 <- rd.u32()
658	        w : Why <- rd.why(c)
659	        m : Bool <- rd.flag()
660	        rest : List<&2, Dead<A>> <- rd.dead(~A, ~pdec, f, m)
661	        return Con{Dead{t, w}, rest}
662	
663	def rd.state(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, +fuel: Nat) -> Dec(State<A>):
664	  do Dec<State<A>>:
665	    n : Nat <- rd.u64()
666	    hf : Bool <- rd.flag()
667	    h : H.Heap<Q.Ticket<A>> <- rd.heap(~A, ~pdec, fuel, hf)
668	    rf : Bool <- rd.flag()
669	    run : List<&2, Q.Ticket<A>> <- rd.run(~A, ~pdec, fuel, rf)
670	    tf : Bool <- rd.flag()
671	    ts : O.OMap<Nat, Try> <- rd.tries(fuel, tf)
672	    df : Bool <- rd.flag()
673	    ds : List<&2, Dead<A>> <- rd.dead(~A, ~pdec, fuel, df)
674	    return State{Q.State{n, h, run}, ts, ds}
675	
676	def decode.snap.end(-A: Data, +len: U32, m: Maybe<&1, Rd & State<A>>) -> Maybe<&2, State<A>>:
677	  match m:
678	    case None{}:
679	      None{}
680	    case Some{(Rd{b, +at}, +s)}:
681	      Bool.pick(Maybe<&2, State<A>>, U32.is_eq(at, len), Some{s}, None{})
682	
683	# The state of a Snap record whose tag is already checked. The decoder must end at the record's end.
684	def decode.snap(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, b: Bytes.Bytes) -> Maybe<&2, State<A>>:
685	  Bytes.Bytes{+len, buf} = b
686	  decode.snap.end(A, len, rd.state(~A, ~pdec, U32.to_nat((len / 4 + 1 : U32)))(Rd{Bytes.Bytes{len, buf}, 4}))
687	
688	def decode.snapped(-A: Data, m: Maybe<&2, State<A>>) -> Maybe<&2, Rec<A>>:
689	  match m:
690	    case None{}:
691	      None{}
692	    case Some{s}:
693	      Some{Snap{s}}
694	
695	def decode.did(-A: Data, m: Maybe<&2, Op<A>>) -> Maybe<&2, Rec<A>>:
696	  match m:
697	    case None{}:
698	      None{}
699	    case Some{op}:
700	      Some{Do{op}}
701	
702	def decode.rec.if(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, snap: Bool, b: Bytes.Bytes) -> Maybe<&2, Rec<A>>:
703	  match snap:
704	    case True{}:
705	      decode.snapped(A, decode.snap(~A, ~pdec, b))
706	    case False{}:
707	      decode.did(A, decode(~A, ~pdec, b))
708	
709	def decode.rec.tag(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, r: Bytes.Bytes & U32) -> Maybe<&2, Rec<A>>:
710	  (b, +tag) = r
711	  decode.rec.if(~A, ~pdec, U32.is_eq(tag, 7), b)
712	
713	# The Rec of a journal record, or None when it is neither a Snap nor an op that encode.rec writes.
714	def decode.rec(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, b: Bytes.Bytes) -> Maybe<&2, Rec<A>>:
715	  decode.rec.tag(~A, ~pdec, get.u32(Bytes.get.u32le(b, 0)))
716	
717	# ---- the runner
718	
719	# policy: retry's policy for each ticket. circuit: the breaker of each target. idle: ms that drain waits when no attempt started.
720	# compact: tick compacts the journal once records past this many octets were appended since open or the last compaction; 0 never.
721	type Config is Data:
722	  Config{policy: Retry.Policy, circuit: Retry.Circuit, idle: U32, compact: U32}
723	
724	# The journal, its path, its file cap, and its length at open or at the last compaction.
725	type Log is Type:
726	  Log{journal: J.Journal, path: String, limit: U32, base: U32}
727	
728	# An open runner. Replaying its journal gives its state; its pool bounds the attempts that run at once.
729	type Runner<-A: Data> is Type:
730	  Runner{log: Log, state: State<A>, pool: Res.Pool, breakers: Map<&2, Retry.Breaker>, config: Config}
731	
732	# An attempt that runs: its unit of the pool, the budget that admit left, and its target.
733	type Flight<-A: Data> is Type:
734	  Flight{ticket: Q.Ticket<A>, grant: Res.Grant, budget: Retry.Budget, target: String}
735	
736	def Out(-X: Type) -> Type:
737	  Result<&1, &1, U32 & String, X>
738	
739	# Each attempt sends its ticket id and outcome on the round's one channel, so tick settles them in the order they finish.
740	def Msg() -> Type:
741	  Nat & Retry.Outcome
742	
743	# The runner, the attempts that run, the claim fuel left, and the first error.
744	def Claimed(-A: Data) -> Type:
745	  Runner<A> & List<&1, Flight<A>> & Nat & Out(Unit)
746	
747	def Ran(-A: Data) -> Type:
748	  Runner<A> & Out(Unit)
749	
750	# Wall-clock ms since the Unix epoch. Deadlines must keep their meaning across a restart.
751	def clock() -> IO(Nat):
752	  do IO<Nat>:
753	    t : Time.Instant <- Time.now()
754	    return Time.Duration.to_ms_nat(Time.Instant.since_epoch(t))
755	
756	def first(a: Out(Unit), b: Out(Unit)) -> Out(Unit):
757	  match a:
758	    case Fail{e}:
759	      Fail{e}
760	    case Done{u}:
761	      b
762	
763	def log.appended(+path: String, +limit: U32, +base: U32, x: J.Journal & Out(Unit)) -> Log & Out(Unit):
764	  (j, o) = x
765	  (Log{j, path, limit, base}, o)
766	
767	def log.append(l: Log, p: Bytes.Bytes) -> IO(Log & Out(Unit)):
768	  Log{j, +path, +limit, +base} = l
769	  do IO<Log & Out(Unit)>:
770	    x : J.Journal & Out(Unit) <- J.append(j, p)
771	    return log.appended(path, limit, base, x)
772	
773	def log.close(l: Log) -> IO(Unit):
774	  Log{j, path, limit, base} = l
775	  J.close(j)
776	
777	# The journal reads with limit as its record cap, so that a snapshot fits; appends keep max.
778	def log.of(j: J.Journal, +max: U32, path: String, +limit: U32) -> Log:
779	  J.Journal{file, cap, +end, ok} = j
780	  Log{J.Journal{file, max, end, ok}, path, limit, end}
781	
782	def commit.done(~A: Data, x: Log & Out(Unit), s: State<A>, +op: Op<A>, p: Res.Pool, bs: Map<&2, Retry.Breaker>, c: Config) -> Ran(A):
783	  match x:
784	    case (j, Done{u}):
785	      (Runner{j, step(~A, s, op), p, bs, c}, Done{u})
786	    case (j, Fail{e}):
787	      (Runner{j, s, p, bs, c}, Fail{e})
788	
789	# Appends op, and applies it once the append is durable. After a failed append the state is as it was,
790	# and the journal refuses every later append until the runner is reopened.
791	def commit(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +op: Op<A>) -> IO(Ran(A)):
792	  Runner{j, s, p, bs, c} = r
793	  do IO<Ran(A)>:
794	    x : Log & Out(Unit) <- log.append(j, encode(~A, ~penc, op))
795	    return commit.done(~A, x, s, op, p, bs, c)
796	
797	# A grant of this runner's pool always fits it, so Kept does not happen.
798	def release.got(-A: Data, x: Res.Release, j: Log, s: State<A>, bs: Map<&2, Retry.Breaker>, c: Config) -> Runner<A>:
799	  match x:
800	    case Res.Released{p}:
801	      Runner{j, s, p, bs, c}
802	    case Res.Kept{p, g}:
803	      Runner{j, s, p, bs, c}
804	
805	def release(-A: Data, r: Runner<A>, g: Res.Grant) -> Runner<A>:
806	  Runner{j, s, p, bs, c} = r
807	  release.got(A, Res.release(p, g), j, s, bs, c)
808	
809	def claim.held(-A: Data, y: Ran(A), g: Res.Grant, fs: List<&1, Flight<A>>, next: Runner<A> -> List<&1, Flight<A>> -> IO(Claimed(A))) -> IO(Claimed(A)):
810	  match y:
811	    case (r, Fail{e}):
812	      IO.pure(Claimed(A), (release(A, r, g), fs, 0n, Fail{e}))
813	    case (r, Done{u}):
814	      next(release(A, r, g), fs)
815	
816	def claim.tag(+id: Nat, o: Retry.Outcome) -> IO(Msg()):
817	  IO.pure(Msg(), (id, o))
818	
819	def claim.go(~A: Data, ~penc: A -> Bytes.Bytes, ~work: A -> IO(Retry.Outcome), +ch: Chan(Msg()), +now: Nat, a: Retry.Breaker & Retry.Budget & Retry.Gate, +key: String,
820	    j: Log, s: State<A>, p: Res.Pool, bs: Map<&2, Retry.Breaker>, c: Config, +id: Nat, d: Nat, +x: A, g: Res.Grant,
821	    fs: List<&1, Flight<A>>, next: Runner<A> -> List<&1, Flight<A>> -> IO(Claimed(A))) -> IO(Claimed(A)):
822	  match a:
823	    case (+br, b, Retry.GateGo{}):
824	      do IO<Claimed(A)>:
825	        u : Chan(Msg()) <- IO.fork.go(Msg(), IO.bind(Retry.Outcome, Msg(), work(x), o => claim.tag(id, o)), ch)
826	        next(Runner{j, s, p, Map.set(&2, Retry.Breaker, bs, key, br), c}, Con{Flight{Q.Ticket{id, d, x}, g, b, key}, fs})
827	    case (+br, b, Retry.GateNo{why}):
828	      do IO<Claimed(A)>:
829	        y : Ran(A) <- commit(~A, ~penc, Runner{j, s, p, Map.set(&2, Retry.Breaker, bs, key, br), c}, held(A, why, br, id, now))
830	        claim.held(A, y, g, fs, next)
831	
832	def claim.gate(~A: Data, ~penc: A -> Bytes.Bytes, ~work: A -> IO(Retry.Outcome), +ch: Chan(Msg()), +now: Nat, got: Map<&2, Retry.Breaker> & Retry.Breaker, +key: String,
833	    j: Log, +s: State<A>, p: Res.Pool, +c: Config, +id: Nat, d: Nat, +x: A, g: Res.Grant,
834	    fs: List<&1, Flight<A>>, next: Runner<A> -> List<&1, Flight<A>> -> IO(Claimed(A))) -> IO(Claimed(A)):
835	  (bs, br) = got
836	  Config{pol, circ, idle, every} = c
837	  claim.go(~A, ~penc, ~work, ch, now, Retry.admit(br, budget(pol, tries.get(tries(A, s), id)), now), key, j, s, p, bs, c, id, d, x, g, fs, next)
838	
839	# The selected ticket asks its target's breaker and its own budget for an attempt.
840	def claim.admit(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +ch: Chan(Msg()), +now: Nat, r: Runner<A>, t: Q.Ticket<A>, g: Res.Grant,
841	    fs: List<&1, Flight<A>>, next: Runner<A> -> List<&1, Flight<A>> -> IO(Claimed(A))) -> IO(Claimed(A)):
842	  Runner{j, s, p, bs, +c} = r
843	  Config{pol, circ, idle, every} = c
844	  Q.Ticket{id, d, +x} = t
845	  +key = target(x)
846	  claim.gate(~A, ~penc, ~work, ch, now, Map.get(Retry.Breaker, Retry.Breaker.new(circ), bs, key), key, j, s, p, c, id, d, x, g, fs, next)
847	
848	def claim.selected(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +ch: Chan(Msg()), +now: Nat, y: Ran(A),
849	    t: Q.Ticket<A>, g: Res.Grant, fs: List<&1, Flight<A>>, next: Runner<A> -> List<&1, Flight<A>> -> IO(Claimed(A))) -> IO(Claimed(A)):
850	  match y:
851	    case (r, Fail{e}):
852	      IO.pure(Claimed(A), (release(A, r, g), fs, 0n, Fail{e}))
853	    case (r, Done{u}):
854	      claim.admit(~A, ~penc, ~target, ~work, ch, now, r, t, g, fs, next)
855	
856	def claim.pick(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), +ch: Chan(Msg()), +left: Nat, +now: Nat, k: Pick<A>, j: Log, s: State<A>,
857	    bs: Map<&2, Retry.Breaker>, c: Config, fs: List<&1, Flight<A>>, next: Runner<A> -> List<&1, Flight<A>> -> IO(Claimed(A))) -> IO(Claimed(A)):
858	  match k:
859	    case Idle{p}:
860	      IO.pure(Claimed(A), (Runner{j, s, p, bs, c}, fs, Nat.add(1n, left), Done{Unit{}}))
861	    case Picked{p, t, g}:
862	      do IO<Claimed(A)>:
863	        y : Ran(A) <- commit(~A, ~penc, Runner{j, s, p, bs, c}, Select{now})
864	        claim.selected(~A, ~penc, ~target, ~work, ch, now, y, t, g, fs, next)
865	
866	# Starts an attempt for each due ticket while the pool has a free unit; each attempt sends its outcome on ch.
867	# fuel bounds the tickets taken, and the fuel left comes back.
868	def claim(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), fuel: Nat, +ch: Chan(Msg()), +now: Nat, r: Runner<A>, fs: List<&1, Flight<A>>) ->
869	    IO(Claimed(A)):
870	  match fuel:
871	    case 0n:
872	      IO.pure(Claimed(A), (r, fs, 0n, Done{Unit{}}))
873	    case 1n+n:
874	      Runner{j, +s, p, bs, c} = r
875	      +m = n
876	      claim.pick(~A, ~penc, ~target, ~work, ch, m, now, pick(~A, p, s, now), j, s, bs, c, fs, (r2 => fs2 => claim(~A, ~penc, ~target, ~work, m, ch, now, r2, fs2)))
877	
878	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)):
879	  (r, res) = y
880	  next(release(A, r, g), first(err, res))
881	
882	def settle.br(~A: Data, ~penc: A -> Bytes.Bytes, +failed: Bool, +key: String, +now: Nat, got: Map<&2, Retry.Breaker> & Retry.Breaker, +op: Op<A>,
883	    j: Log, s: State<A>, p: Res.Pool, c: Config, g: Res.Grant, err: Out(Unit), next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
884	  (bs, br) = got
885	  do IO<Runner<A> & Out(Bool)>:
886	    y : Ran(A) <- commit(~A, ~penc, Runner{j, s, p, Map.set(&2, Retry.Breaker, bs, key, Retry.Breaker.done(br, now, failed)), c}, op)
887	    settle.done(A, y, g, err, next)
888	
889	# Tells the target's breaker, appends Complete or Requeue, and returns the attempt's unit to the pool.
890	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,
891	    err: Out(Unit), next: Runner<A> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
892	  Runner{j, +s, p, bs, +c} = r
893	  Config{pol, circ, idle, every} = c
894	  settle.br(~A, ~penc, Retry.failed(o), key, now, Map.get(Retry.Breaker, Retry.Breaker.new(circ), bs, key),
895	    after(A, o, pol, b, id, fails(tries.get(tries(A, s), id)), now, rnd), j, s, p, c, g, err, next)
896	
897	def random(r: Out(U32)) -> Nat:
898	  match r:
899	    case Done{x}:
900	      U32.to_nat(x)
901	    case Fail{e}:
902	      0n
903	
904	def take.keep(-A: Data, m: Maybe<&1, Flight<A>>, xs: List<&1, Flight<A>>) -> List<&1, Flight<A>>:
905	  match m:
906	    case None{}:
907	      xs
908	    case Some{f}:
909	      Con{f, xs}
910	
911	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>>:
912	  match eq:
913	    case True{}:
914	      (m, xs) = r
915	      (Some{f}, take.keep(A, m, xs))
916	    case False{}:
917	      (m, xs) = r
918	      (m, Con{f, xs})
919	
920	# The flight of ticket id, and the other flights.
921	def take(-A: Data, fs: List<&1, Flight<A>>, +id: Nat) -> Maybe<&1, Flight<A>> & List<&1, Flight<A>>:
922	  match fs:
923	    case Nil{}:
924	      (None{}, Nil{})
925	    case Con{Flight{Q.Ticket{+i, d, x}, g, b, k}, rest}:
926	      take.at(A, Nat.is_eq(i, id), Flight{Q.Ticket{i, d, x}, g, b, k}, take(A, rest, id))
927	
928	def settle.flight(~A: Data, ~penc: A -> Bytes.Bytes, mf: Maybe<&1, Flight<A>>, fs: List<&1, Flight<A>>, +o: Retry.Outcome, +now: Nat, +rnd: Nat, r: Runner<A>, err: Out(Unit),
929	    next: Runner<A> -> List<&1, Flight<A>> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
930	  match mf:
931	    case None{}:
932	      next(r, fs, err)
933	    case Some{Flight{Q.Ticket{id, d, x}, g, b, key}}:
934	      settle.op(~A, ~penc, o, b, id, key, now, rnd, r, g, err, (r2 => e2 => next(r2, fs, e2)))
935	
936	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, r: Runner<A>, err: Out(Unit),
937	    next: Runner<A> -> List<&1, Flight<A>> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
938	  (mf, fs) = t
939	  settle.flight(~A, ~penc, mf, fs, o, now, rnd, r, err, next)
940	
941	# Settles the flight that m names. A closed channel is an error, and no flight settles.
942	def settle(~A: Data, ~penc: A -> Bytes.Bytes, m: Maybe<&1, Msg()>, +now: Nat, +rnd: Nat, r: Runner<A>, fs: List<&1, Flight<A>>, err: Out(Unit),
943	    next: Runner<A> -> List<&1, Flight<A>> -> Out(Unit) -> IO(Runner<A> & Out(Bool))) -> IO(Runner<A> & Out(Bool)):
944	  match m:
945	    case None{}:
946	      next(r, fs, first(err, Fail{(32, "runner: the outcome channel closed")}))
947	    case Some{(+id, +o)}:
948	      settle.take(~A, ~penc, take(A, fs, id), o, now, rnd, r, err, next)
949	
950	def merged(-A: Data, e: Out(Unit), k: Claimed(A)) -> Claimed(A):
951	  (r, fs, n, err) = k
952	  (r, fs, n, first(e, err))
953	
954	def empty.heap(-A: Data, h: H.Heap<Q.Ticket<A>>) -> Bool:
955	  match h:
956	    case H.HLeaf{}:
957	      True{}
958	    case H.HNode{n, t, l, r}:
959	      False{}
960	
961	def empty.list(-A: Data, xs: List<&2, Q.Ticket<A>>) -> Bool:
962	  match xs:
963	    case Nil{}:
964	      True{}
965	    case Con{t, rest}:
966	      False{}
967	
968	# No ticket is pending or running. Dead tickets wait for a Revive, so they do not count.
969	def idle(-A: Data, s: State<A>) -> Bool:
970	  State{Q.State{n, h, run}, ts, ds} = s
971	  Bool.and(empty.heap(A, h), empty.list(A, run))
972	
973	def pending(-A: Data, s: State<A>) -> Nat:
974	  State{Q.State{n, h, run}, ts, ds} = s
975	  H.Heap.size(Q.Ticket<A>, h)
976	
977	# ---- compaction
978	
979	# Kept: compaction failed before the rename, and the old journal stands.
980	# Lost: the rename happened, but the new journal did not open. Moved: the journal at path is the snapshot.
981	type Swap is Type:
982	  Kept{err: U32 & String}
983	  Lost{err: U32 & String}
984	  Moved{journal: J.Journal}
985	
986	def swap.reopened(x: Out(J.Journal & Unit)) -> Swap:
987	  match x:
988	    case Fail{e}:
989	      Lost{e}
990	    case Done{(j, u)}:
991	      Moved{j}
992	
993	# The reopen fsyncs the directory, so the rename is durable before the next append.
994	def swap.renamed(+path: String, +limit: U32, x: Out(Unit)) -> IO(Swap):
995	  match x:
996	    case Fail{e}:
997	      IO.pure(Swap, Kept{e})
998	    case Done{u}:
999	      do IO<Swap>:
1000	        y : Out(J.Journal & Unit) <- J.open(~Unit, ~(a => b => a), path, limit, limit, Unit{})
1001	        return swap.reopened(y)
1002	
1003	def swap.written(+path: String, +tmp: String, +limit: U32, x: J.Journal & Out(Unit)) -> IO(Swap):
1004	  match x:
1005	    case (t, Fail{e}):
1006	      do IO<Swap>:
1007	        J.close(t)
1008	        return Kept{e}
1009	    case (t, Done{u}):
1010	      do IO<Swap>:
1011	        J.close(t)
1012	        y : Out(Unit) <- Files.rename(tmp, path)
1013	        swap.renamed(path, limit, y)
1014	
1015	def swap.opened(+path: String, +tmp: String, +limit: U32, x: Out(J.Journal & Unit), snap: Bytes.Bytes) -> IO(Swap):
1016	  match x:
1017	    case Fail{e}:
1018	      IO.pure(Swap, Kept{e})
1019	    case Done{(t, u)}:
1020	      do IO<Swap>:
1021	        y : J.Journal & Out(Unit) <- J.append(t, snap)
1022	        swap.written(path, tmp, limit, y)
1023	
1024	# Writes snap as the only record of path.compact, fsyncs it, and renames it over path.
1025	def swap(+path: String, +limit: U32, snap: Bytes.Bytes) -> IO(Swap):
1026	  +tmp = path ++ ".compact"
1027	  do IO<Swap>:
1028	    gone : Out(Unit) <- Files.remove(tmp)
1029	    x : Out(J.Journal & Unit) <- J.open(~Unit, ~(a => b => a), tmp, limit, limit, Unit{})
1030	    swap.opened(path, tmp, limit, x, snap)
1031	
1032	def log.stop(j: J.Journal) -> J.Journal:
1033	  J.Journal{file, max, end, ok} = j
1034	  J.Journal{file, max, end, False{}}
1035	
1036	def compact.moved(-A: Data, x: Swap, j: J.Journal, +max: U32, path: String, +limit: U32, +base: U32, s: State<A>, p: Res.Pool, bs: Map<&2, Retry.Breaker>, c: Config) ->
1037	    IO(Ran(A)):
1038	  match x:
1039	    case Kept{e}:
1040	      IO.pure(Ran(A), (Runner{Log{j, path, limit, base}, s, p, bs, c}, Fail{e}))
1041	    case Lost{e}:
1042	      IO.pure(Ran(A), (Runner{Log{log.stop(j), path, limit, base}, s, p, bs, c}, Fail{e}))
1043	    case Moved{k}:
1044	      do IO<Ran(A)>:
1045	        J.close(j)
1046	        return (Runner{log.of(k, max, path, limit), s, p, bs, c}, Done{Unit{}})
1047	
1048	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>, p: Res.Pool,
1049	    bs: Map<&2, Retry.Breaker>, c: Config) -> IO(Ran(A)):
1050	  match ok:
1051	    case False{}:
1052	      IO.pure(Ran(A), (Runner{Log{j, path, limit, base}, s, p, bs, c}, Fail{(5, "runner: an append failed; reopen the runner")}))
1053	    case True{}:
1054	      do IO<Ran(A)>:
1055	        x : Swap <- swap(path, limit, encode.snap(~A, ~penc, s))
1056	        compact.moved(A, x, j, max, path, limit, base, s, p, bs, c)
1057	
1058	# Writes the state as one Snap record into a new journal, fsyncs it, and renames it over the old one.
1059	# A crash leaves one of the two journals, and both replay to this state. After a failure the old
1060	# journal stands; when the failure came after the rename, appends stop until the runner is reopened.
1061	def compact(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>) -> IO(Ran(A)):
1062	  Runner{Log{j, path, limit, base}, s, p, bs, c} = r
1063	  J.Journal{file, +max, end, +ok} = j
1064	  compact.if(~A, ~penc, ok, J.Journal{file, max, end, ok}, max, path, limit, base, s, p, bs, c)
1065	
1066	def tick.ran(e: Out(Unit), b: Bool) -> Out(Bool):
1067	  match e:
1068	    case Fail{x}:
1069	      Fail{x}
1070	    case Done{u}:
1071	      Done{b}
1072	
1073	def tick.compacted(-A: Data, +b: Bool, y: Ran(A)) -> Runner<A> & Out(Bool):
1074	  (r, e) = y
1075	  (r, tick.ran(e, b))
1076	
1077	def tick.due(~A: Data, ~penc: A -> Bytes.Bytes, due: Bool, +b: Bool, r: Runner<A>) -> IO(Runner<A> & Out(Bool)):
1078	  match due:
1079	    case False{}:
1080	      IO.pure(Runner<A> & Out(Bool), (r, Done{b}))
1081	    case True{}:
1082	      do IO<Runner<A> & Out(Bool)>:
1083	        y : Ran(A) <- compact(~A, ~penc, r)
1084	        return tick.compacted(A, b, y)
1085	
1086	def tick.size(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +b: Bool) -> IO(Runner<A> & Out(Bool)):
1087	  Runner{Log{J.Journal{file, max, +end, ok}, path, limit, +base}, s, p, bs, +c} = r
1088	  Config{pol, circ, idle, +every} = c
1089	  tick.due(~A, ~penc, Bool.and(Bool.not(U32.is_eq(every, 0)), U32.is_gt((end - base : U32), every)), b,
1090	    Runner{Log{J.Journal{file, max, end, ok}, path, limit, base}, s, p, bs, c})
1091	
1092	# Compacts once the journal grew past Config.compact octets since open or the last compaction.
1093	def tick.grown(~A: Data, ~penc: A -> Bytes.Bytes, t: Runner<A> & Out(Bool)) -> IO(Runner<A> & Out(Bool)):
1094	  match t:
1095	    case (r, Fail{e}):
1096	      IO.pure(Runner<A> & Out(Bool), (r, Fail{e}))
1097	    case (r, Done{b}):
1098	      tick.size(~A, ~penc, r, b)
1099	
1100	# Waits for the next attempt to finish, settles it, then claims due tickets into the freed unit with the claim fuel left.
1101	# fuel bounds the settles. claim starts at most as many attempts as the same fuel, so every attempt settles before fuel ends.
1102	def tick.run(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), fuel: Nat, +ch: Chan(Msg()), +ran: Bool, k: Claimed(A)) ->
1103	    IO(Runner<A> & Out(Bool)):
1104	  match fuel:
1105	    case 0n:
1106	      match k:
1107	        case (r, Nil{}, left, err):
1108	          IO.pure(Runner<A> & Out(Bool), (r, tick.ran(err, ran)))
1109	        case (r, Con{f, fs}, left, err):
1110	          IO.pure(Runner<A> & Out(Bool), (r, tick.ran(first(err, Fail{(62, "runner: an attempt outlived the round")}), ran)))
1111	    case 1n+n:
1112	      match k:
1113	        case (r, Nil{}, left, err):
1114	          IO.pure(Runner<A> & Out(Bool), (r, tick.ran(err, ran)))
1115	        case (r, Con{f, fs}, +left, err):
1116	          do IO<Runner<A> & Out(Bool)>:
1117	            m : Maybe<&1, Msg()> <- Chan.recv(Msg(), ch)
1118	            +now : Nat <- clock()
1119	            rnd : Out(U32) <- IO.random_u32()
1120	            settle(~A, ~penc, m, now, random(rnd), r, Con{f, fs}, err, (r2 => fs2 => e2 =>
1121	              IO.bind(Claimed(A), Runner<A> & Out(Bool), claim(~A, ~penc, ~target, ~work, left, ch, now, r2, fs2),
1122	                k2 => tick.run(~A, ~penc, ~target, ~work, n, ch, True{}, merged(A, e2, k2)))))
1123	
1124	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)):
1125	  Runner{j, +s, +p, bs, c} = r
1126	  +fuel = Nat.add(1n, pending(A, s))
1127	  do IO<Runner<A> & Out(Bool)>:
1128	    +ch : Chan(Msg()) <- Chan.new(Msg(), (U32.from_nat(Res.capacity(p)) + 1 : U32))
1129	    k : Claimed(A) <- claim(~A, ~penc, ~target, ~work, fuel, ch, now, Runner{j, s, p, bs, c}, Nil{})
1130	    t : Runner<A> & Out(Bool) <- tick.run(~A, ~penc, ~target, ~work, fuel, ch, False{}, k)
1131	    Chan.close(Msg(), ch)
1132	    return t
1133	
1134	# One round: start an attempt for each due ticket the pool admits. As each attempt finishes, record its outcome,
1135	# return its unit, and start the next due ticket in that unit. The round ends when no attempt runs.
1136	# Then compact the journal when it grew past Config.compact. Done{True} when an attempt ran.
1137	# work runs on its own; target names the breaker that guards it.
1138	def tick(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), r: Runner<A>) -> IO(Runner<A> & Out(Bool)):
1139	  do IO<Runner<A> & Out(Bool)>:
1140	    now : Nat <- clock()
1141	    t : Runner<A> & Out(Bool) <- tick.at(~A, ~penc, ~target, ~work, now, r)
1142	    tick.grown(~A, ~penc, t)
1143	
1144	def drain.ticked(-A: Data, +idle: U32, t: Runner<A> & Out(Bool), next: Runner<A> -> IO(Ran(A))) -> IO(Ran(A)):
1145	  match t:
1146	    case (r, Fail{e}):
1147	      IO.pure(Ran(A), (r, Fail{e}))
1148	    case (r, Done{True{}}):
1149	      next(r)
1150	    case (r, Done{False{}}):
1151	      do IO<Ran(A)>:
1152	        IO.sleep(idle)
1153	        next(r)
1154	
1155	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)):
1156	  match done:
1157	    case True{}:
1158	      IO.pure(Ran(A), (r, Done{Unit{}}))
1159	    case False{}:
1160	      Runner{j, s, p, bs, +c} = r
1161	      Config{pol, circ, +idle, every} = c
1162	      do IO<Ran(A)>:
1163	        t : Runner<A> & Out(Bool) <- tick(~A, ~penc, ~target, ~work, Runner{j, s, p, bs, c})
1164	        drain.ticked(A, idle, t, next)
1165	
1166	# Ticks until no ticket is pending or running. It waits idle ms after a round that started no attempt.
1167	# Fails when fuel rounds leave work.
1168	def drain(~A: Data, ~penc: A -> Bytes.Bytes, ~target: A -> String, ~work: A -> IO(Retry.Outcome), fuel: Nat, r: Runner<A>) -> IO(Ran(A)):
1169	  match fuel:
1170	    case 0n:
1171	      Runner{j, +s, p, bs, c} = r
1172	      IO.pure(Ran(A), (Runner{j, s, p, bs, c}, Bool.pick(Out(Unit), idle(A, s), Done{Unit{}}, Fail{(62, "runner: drain ran out of rounds")})))
1173	    case 1n+n:
1174	      Runner{j, +s, p, bs, c} = r
1175	      drain.if(~A, ~penc, ~target, ~work, idle(A, s), Runner{j, s, p, bs, c}, (r2 => drain(~A, ~penc, ~target, ~work, n, r2)))
1176	
1177	def orphans.next(-A: Data, y: Ran(A), err: Out(Unit), next: Runner<A> -> Out(Unit) -> IO(Ran(A))) -> IO(Ran(A)):
1178	  (r, e) = y
1179	  next(r, first(err, e))
1180	
1181	# A running ticket at open lost its attempt in a crash: it is requeued at now, as a failed attempt.
1182	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)):
1183	  match ts:
1184	    case Nil{}:
1185	      IO.pure(Ran(A), (r, err))
1186	    case Con{Q.Ticket{id, d, x}, rest}:
1187	      do IO<Ran(A)>:
1188	        y : Ran(A) <- commit(~A, ~penc, r, Requeue{id, now})
1189	        orphans.next(A, y, err, (r2 => e2 => orphans(~A, ~penc, rest, now, r2, e2)))
1190	
1191	def open.done(-A: Data, y: Ran(A)) -> IO(Out(Runner<A>)):
1192	  match y:
1193	    case (Runner{j, s, p, bs, c}, Fail{e}):
1194	      do IO<Out(Runner<A>)>:
1195	        log.close(j)
1196	        return Fail{e}
1197	    case (r, Done{u}):
1198	      IO.pure(Out(Runner<A>), Done{r})
1199	
1200	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) ->
1201	    IO(Out(Runner<A>)):
1202	  match x:
1203	    case Fail{e}:
1204	      IO.pure(Out(Runner<A>), Fail{e})
1205	    case Done{(j, None{})}:
1206	      do IO<Out(Runner<A>)>:
1207	        J.close(j)
1208	        return Fail{(22, "runner: a journal record is not a runner record")}
1209	    case Done{(j, Some{+s})}:
1210	      do IO<Out(Runner<A>)>:
1211	        now : Nat <- clock()
1212	        y : Ran(A) <- orphans(~A, ~penc, Q.running(A, queue(A, s)), now, Runner{log.of(j, max, path, limit), s, Res.new(0n, capacity), Map.new(&2, Retry.Breaker), c}, Done{Unit{}})
1213	        open.done(A, y)
1214	
1215	def replay.step(~A: Data, ~pdec: Bytes.Bytes -> Maybe<&2, A>, acc: Maybe<&2, State<A>>, b: Bytes.Bytes) -> Maybe<&2, State<A>>:
1216	  load(~A, ~Bytes.Bytes, ~(y => decode.rec(~A, ~pdec, y)), acc, b)
1217	
1218	# Opens the runner at path: replays its journal into the state, then requeues each ticket that was running.
1219	# capacity bounds the attempts that run at once. max caps an op record's octets; limit caps the journal file and a snapshot.
1220	# Fails when the journal fails to open, or a committed record is not a snapshot or an op.
1221	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>)):
1222	  do IO<Out(Runner<A>)>:
1223	    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)})
1224	    open.got(~A, ~penc, x, max, path, limit, c, capacity)
1225	
1226	def enqueue.done(-A: Data, +id: Nat, y: Ran(A)) -> Runner<A> & Out(Nat):
1227	  match y:
1228	    case (r, Done{u}):
1229	      (r, Done{id})
1230	    case (r, Fail{e}):
1231	      (r, Fail{e})
1232	
1233	# Adds a job that is due at deadline (wall-clock ms) and answers its ticket id once the record is durable.
1234	def enqueue(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +deadline: Nat, x: A) -> IO(Runner<A> & Out(Nat)):
1235	  Runner{j, +s, p, bs, c} = r
1236	  do IO<Runner<A> & Out(Nat)>:
1237	    y : Ran(A) <- commit(~A, ~penc, Runner{j, s, p, bs, c}, Enqueue{deadline, x})
1238	    return enqueue.done(A, Q.next_id(A, queue(A, s)), y)
1239	
1240	# Cancels ticket id. A running attempt finishes, but its ticket is never selected again. A dead ticket is dropped.
1241	def cancel(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +id: Nat) -> IO(Ran(A)):
1242	  commit(~A, ~penc, r, Cancel{id})
1243	
1244	# Makes dead ticket id pending again at deadline (wall-clock ms), with its payload and a new retry budget.
1245	# A ticket that is not dead does not change.
1246	def revive(~A: Data, ~penc: A -> Bytes.Bytes, r: Runner<A>, +id: Nat, +deadline: Nat) -> IO(Ran(A)):
1247	  commit(~A, ~penc, r, Revive{id, deadline})
1248	
1249	# The runner back, and a copy of its state.
1250	def state(-A: Data, r: Runner<A>) -> Runner<A> & State<A>:
1251	  Runner{j, +s, p, bs, c} = r
1252	  (Runner{j, s, p, bs, c}, s)
1253	
1254	def close(-A: Data, r: Runner<A>) -> IO(Unit):
1255	  Runner{j, s, p, bs, c} = r
1256	  log.close(j)