1	# Hairpin: an HTTP client with a base URL, default headers, a pool, cookies, mTLS, redirects, and proven retries. Source: https://github.com/paymog/bend-kit/tree/main/hairpin
2	import Base
3	import bend-kit-http@0.34.0.0/http.bend as Http
4	import bend-kit-time@0.2.0.0/time.bend as Time
5	import bend-kit-url@0.4.1.0/url.bend as Url
6	import bend-kit-json@0.6.0.0/json.bend as Json
7	import bend-kit-bytes@0.3.2.0/bytes.bend as Bytes
8	import bend-kit-retry@0.1.0.0/retry.bend as Retry
9	
10	# base: "" for none. headers: lowercase names. ms: the step timeout. policy: attempts, backoff, and deadline of each request.
11	type Opts is Data:
12	  Opts{base: String, headers: Map<&2, List<&2, String>>, ms: U32, mode: Http.Mode, policy: Retry.Policy, cert: Maybe<&2, Http.Cert>}
13	
14	# breaker: the circuit breaker of every request on this client.
15	type Client is Type:
16	  Client{opts: Opts, pool: Http.Pool, jar: Maybe<&2, Http.Jar>, breaker: Retry.Breaker}
17	
18	# ErrHttp: the last attempt failed. ErrDenied: no attempt started, for why.
19	type Err is Data:
20	  ErrHttp{error: Http.Err}
21	  ErrDenied{why: Retry.Deny}
22	
23	# One attempt, the first included; backoff from 500 ms, doubled to at most 30 s; no deadline.
24	def policy() -> Retry.Policy:
25	  Retry.Policy{1n, 500n, 30000, None{}}
26	
27	# A pool of up to cap idle sockets per origin, no base, no default headers, 30 s steps, follow redirects,
28	# no retries, and no circuit breaker.
29	def new.with(+cap: U32) -> Client:
30	  Client{Opts{"", Http.empty(), 30000, Http.ModeFollow{}, policy(), None{}}, Http.pool.new.with(cap), None{}, Retry.BreakerOff{}}
31	
32	def new() -> Client:
33	  new.with(8)
34	
35	def base(c: Client, url: String) -> Client:
36	  Client{Opts{b, h, ms, mode, p, cert}, pool, j, br} = c
37	  Client{Opts{url, h, ms, mode, p, cert}, pool, j, br}
38	
39	# A default header. It replaces an earlier default of the same name; a request header of that name replaces it.
40	def header(c: Client, k: String, v: String) -> Client:
41	  Client{Opts{b, h, ms, mode, p, cert}, pool, j, br} = c
42	  Client{Opts{b, Http.set(h, String.to_lower(k), v), ms, mode, p, cert}, pool, j, br}
43	
44	def timeout(c: Client, ms: U32) -> Client:
45	  Client{Opts{b, h, old, mode, p, cert}, pool, j, br} = c
46	  Client{Opts{b, h, ms, mode, p, cert}, pool, j, br}
47	
48	def redirect(c: Client, mode: Http.Mode) -> Client:
49	  Client{Opts{b, h, ms, old, p, cert}, pool, j, br} = c
50	  Client{Opts{b, h, ms, mode, p, cert}, pool, j, br}
51	
52	# Up to n retries after the first attempt: n + 1 attempts in all.
53	def retry(c: Client, n: U32) -> Client:
54	  Client{Opts{b, h, ms, mode, Retry.Policy{old, base, cap, d}, cert}, pool, j, br} = c
55	  Client{Opts{b, h, ms, mode, Retry.Policy{1n+U32.to_nat(n), base, cap, d}, cert}, pool, j, br}
56	
57	# No attempt of a request starts ms or more after the request starts.
58	def deadline(c: Client, ms: U32) -> Client:
59	  Client{Opts{b, h, t, mode, Retry.Policy{a, base, cap, old}, cert}, pool, j, br} = c
60	  Client{Opts{b, h, t, mode, Retry.Policy{a, base, cap, Some{U32.to_nat(ms)}}, cert}, pool, j, br}
61	
62	# A circuit breaker in front of every request, starting closed.
63	def circuit(c: Client, circ: Retry.Circuit) -> Client:
64	  Client{o, pool, j, old} = c
65	  Client{o, pool, j, Retry.Breaker.new(circ)}
66	
67	# The breaker, for inspection.
68	def breaker.of(c: Client) -> Client & Retry.Breaker:
69	  Client{o, pool, j, +br} = c
70	  (Client{o, pool, j, br}, br)
71	
72	# Paths to a PEM client chain and its private key, presented to every HTTPS origin.
73	def cert(c: Client, chain: String, key: String) -> Client:
74	  Client{Opts{b, h, ms, mode, p, old}, pool, j, br} = c
75	  Client{Opts{b, h, ms, mode, p, Some{Http.Cert{chain, key}}}, pool, j, br}
76	
77	# Every request stores and sends cookies through jar. Its clock moves to Time.now() before each try.
78	def jar(c: Client, jar: Http.Jar) -> Client:
79	  Client{o, pool, old, br} = c
80	  Client{o, pool, Some{jar}, br}
81	
82	# The jar, if the client has one.
83	def jar.of(c: Client) -> Client & Maybe<&2, Http.Jar>:
84	  Client{o, pool, +j, br} = c
85	  (Client{o, pool, j, br}, j)
86	
87	def close(c: Client) -> IO(Unit):
88	  Client{o, pool, j, br} = c
89	  Http.pool.close(pool)
90	
91	def target.pick(none: Bool, b: String, ref: String) -> Maybe<&2, String>:
92	  match none:
93	    case True{}:
94	      Some{ref}
95	    case False{}:
96	      Http.resolve(b, ref)
97	
98	# ref against the base (RFC 3986 §5.2); with no base, ref as given. None when the base is not absolute.
99	def target(+b: String, ref: String) -> Maybe<&2, String>:
100	  target.pick(String.is_empty(b), b, ref)
101	
102	def target.abs(m: Maybe<&2, String>) -> Maybe<&2, Url.Abs>:
103	  match m:
104	    case None{}:
105	      None{}
106	    case Some{s}:
107	      Url.absolute(s)
108	
109	# The defaults under the request headers, names lowercased; a request header replaces a default of its name.
110	# Host, Connection, Content-Length, and Transfer-Encoding are the client's own, so both sides drop them.
111	def merge(defaults: Map<&2, List<&2, String>>, headers: Map<&2, List<&2, String>>) -> Map<&2, List<&2, String>>:
112	  Http.req.put(Map.to_list(&2, List<&2, String>, headers), Http.req.put(Map.to_list(&2, List<&2, String>, defaults), Http.empty()))
113	
114	def jar.now(j: Maybe<&2, Http.Jar>) -> IO(Maybe<&2, Http.Jar>):
115	  match j:
116	    case None{}:
117	      IO.pure(Maybe<&2, Http.Jar>, None{})
118	    case Some{jar}:
119	      do IO<Maybe<&2, Http.Jar>>:
120	        now : Time.Instant <- Time.now()
121	        return Some{Http.jar.at(jar, now)}
122	
123	def once.final(+enc: String, x: Http.Pool & Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, Http.Jar>) ->
124	    IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>):
125	  (p, r, j) = x
126	  do IO<Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>>:
127	    d : Result<&1, &1, Http.Err, Http.Res> <- Http.fetch.final(enc, r)
128	    return (p, j, d)
129	
130	# One try: the redirect loop on the pool, with the jar and the certificate, then the body decoded.
131	def once(+ms: U32, +mode: Http.Mode, +cert: Maybe<&2, Http.Cert>, +enc: String, p: Http.Pool, j: Maybe<&2, Http.Jar>,
132	    +method: String, +u: Maybe<&2, Url.Abs>, +headers: Map<&2, List<&2, String>>, body: Bytes.Bytes) ->
133	    IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>):
134	  do IO<Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>>:
135	    now : Maybe<&2, Http.Jar> <- jar.now(j)
136	    x : Http.Pool & Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, Http.Jar> <-
137	      Http.pool.hops(mode, ms, Http.Hops{p, Http.fetch.start(method, headers, body, u, enc), cert, now, ""})
138	    once.final(enc, x)
139	
140	def hint.ms(m: Maybe<&2, U32>) -> Maybe<&2, Nat>:
141	  match m:
142	    case None{}:
143	      None{}
144	    case Some{ms}:
145	      Some{U32.to_nat(ms)}
146	
147	# m is Http.retry.judge's verdict for an idempotent request: Some{Retry-After} when another try may help.
148	# A non-idempotent request with that verdict is Unsafe: the breaker counts it, but it never repeats.
149	def outcome(+idem: Bool, m: Maybe<&2, String>, now: Time.Instant) -> Retry.Outcome:
150	  match m:
151	    case None{}:
152	      Retry.Settled{}
153	    case Some{+v}:
154	      Bool.pick(Retry.Outcome, idem, Retry.Transient{hint.ms(Http.retry.after(v, now))}, Retry.Unsafe{})
155	
156	def lift(-R: Type, r: Result<&1, &1, Http.Err, R>) -> Result<&1, &1, Err, R>:
157	  match r:
158	    case Fail{e}:
159	      Fail{ErrHttp{e}}
160	    case Done{res}:
161	      Done{res}
162	
163	# The live state of one request between attempts. R: a response or a stream. last: the result of the last
164	# attempt, None before the first.
165	type Step<-R: Type> is Type:
166	  Step{pool: Http.Pool, jar: Maybe<&2, Http.Jar>, breaker: Retry.Breaker, budget: Retry.Budget, body: Bytes.Bytes,
167	    last: Maybe<&1, Result<&1, &1, Err, R>>, plan: Retry.Plan}
168	
169	# The clock, read only when a decision depends on it; 0 otherwise, which no such decision reads.
170	def clock(need: Bool) -> IO(Nat):
171	  match need:
172	    case True{}:
173	      Time.mono.ms()
174	    case False{}:
175	      IO.pure(Nat, 0n)
176	
177	def breaker.on(br: Retry.Breaker) -> Bool:
178	  match br:
179	    case Retry.BreakerOff{}:
180	      False{}
181	    case Retry.BreakerOn{c, s}:
182	      True{}
183	
184	def transient(o: Retry.Outcome) -> Bool:
185	  match o:
186	    case Retry.Transient{after}:
187	      True{}
188	    case Retry.Settled{}:
189	      False{}
190	    case Retry.Unsafe{}:
191	      False{}
192	
193	def some(m: Maybe<&2, Nat>) -> Bool:
194	  match m:
195	    case None{}:
196	      False{}
197	    case Some{t}:
198	      True{}
199	
200	def deadline.some(b: Retry.Budget) -> Bool:
201	  Retry.Budget{l, d} = b
202	  some(d)
203	
204	# The wall clock dates a Retry-After, so it is read only for a transient failure.
205	def outcome.io(+idem: Bool, m: Maybe<&2, String>) -> IO(Retry.Outcome):
206	  match m:
207	    case None{}:
208	      IO.pure(Retry.Outcome, Retry.Settled{})
209	    case Some{v}:
210	      do IO<Retry.Outcome>:
211	        wall : Time.Instant <- Time.now()
212	        return outcome(idem, Some{v}, wall)
213	
214	# Jitter for the backoff, drawn only when a retry may follow.
215	def jitter.io(need: Bool) -> IO(Nat):
216	  match need:
217	    case True{}:
218	      do IO<Nat>:
219	        rnd : Result<&1, &1, U32 & String, U32> <- IO.random_u32()
220	        return U32.to_nat(Http.retry.jitter(rnd))
221	    case False{}:
222	      IO.pure(Nat, 0n)
223	
224	def settle.at(-R: Type, +now: Nat, +rnd: Nat, +p: Retry.Policy, +n: Nat, +o: Retry.Outcome, br: Retry.Breaker, +b: Retry.Budget,
225	    body: Bytes.Bytes, pool: Http.Pool, jar: Maybe<&2, Http.Jar>, r: Result<&1, &1, Http.Err, R>) -> Step<R>:
226	  Step{pool, jar, Retry.Breaker.done(br, now, Retry.failed(o)), b, body, Some{lift(R, r)}, Retry.plan(o, p, b, n, now, rnd)}
227	
228	# The attempt has ended: tell the breaker, and plan the next step.
229	def settle.outcome(-R: Type, +p: Retry.Policy, +n: Nat, +o: Retry.Outcome, +br: Retry.Breaker, b: Retry.Budget, body: Bytes.Bytes,
230	    pool: Http.Pool, jar: Maybe<&2, Http.Jar>, r: Result<&1, &1, Http.Err, R>) -> IO(Step<R>):
231	  do IO<Step<R>>:
232	    now : Nat <- clock(Bool.or(breaker.on(br), transient(o)))
233	    rnd : Nat <- jitter.io(transient(o))
234	    return settle.at(R, now, rnd, p, n, o, br, b, body, pool, jar, r)
235	
236	def settle.judged(-R: Type, +p: Retry.Policy, +idem: Bool, +n: Nat, br: Retry.Breaker, b: Retry.Budget, body: Bytes.Bytes,
237	    pool: Http.Pool, jar: Maybe<&2, Http.Jar>, j: Result<&1, &1, Http.Err, R> & Maybe<&2, String>) -> IO(Step<R>):
238	  (r, m) = j
239	  do IO<Step<R>>:
240	    o : Retry.Outcome <- outcome.io(idem, m)
241	    settle.outcome(R, p, n, o, br, b, body, pool, jar, r)
242	
243	# The default judge: Http.retry.judge, as if the request were idempotent; outcome then applies idem.
244	def judge(r: Result<&1, &1, Http.Err, Http.Res>) -> Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, String>:
245	  Http.retry.judge(True{}, r)
246	
247	def settle(~R: Type, ~judge: Result<&1, &1, Http.Err, R> -> Result<&1, &1, Http.Err, R> & Maybe<&2, String>,
248	    +p: Retry.Policy, +idem: Bool, +n: Nat, br: Retry.Breaker, b: Retry.Budget, body: Bytes.Bytes,
249	    x: Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R>) -> IO(Step<R>):
250	  (pool, jar, r) = x
251	  settle.judged(R, p, idem, n, br, b, body, pool, jar, judge(r))
252	
253	# The inputs of one attempt. u: the target, None when it is not absolute. headers: the merged headers.
254	type Call is Data:
255	  Call{enc: String, method: String, u: Maybe<&2, Url.Abs>, headers: Map<&2, List<&2, String>>}
256	
257	# One attempt that reads a whole response: the redirect loop on the pool, with the jar and the certificate.
258	def try.res(o: Opts, call: Call, pool: Http.Pool, jar: Maybe<&2, Http.Jar>, body: Bytes.Bytes) ->
259	    IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>):
260	  Opts{base, h, +ms, +mode, p, +cert} = o
261	  Call{+enc, +method, +u, +headers} = call
262	  once(ms, mode, cert, enc, pool, jar, method, u, headers, body)
263	
264	def try.stream.got(pool: Http.Pool, x: Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Stream>) ->
265	    Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Stream>:
266	  (jar, s) = x
267	  (pool, jar, s)
268	
269	# One attempt that opens a stream on its own connection, outside the pool: the redirect loop with the mode,
270	# the jar, and the certificate, then the body decoded by enc.
271	def try.stream(o: Opts, call: Call, pool: Http.Pool, jar: Maybe<&2, Http.Jar>, body: Bytes.Bytes) ->
272	    IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Stream>):
273	  Opts{base, h, +ms, mode, p, cert} = o
274	  Call{+enc, method, u, headers} = call
275	  do IO<Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Stream>>:
276	    now : Maybe<&2, Http.Jar> <- jar.now(jar)
277	    r : Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Stream> <-
278	      Http.stream.hops(mode, ms, Http.StreamHops{Http.stream.start(method, Http.accept.enc(headers, enc), body, u), cert, now})
279	    x : Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Stream> <- Http.stream.init.jar(enc, r)
280	    return try.stream.got(pool, x)
281	
282	def close.res(r: Http.Res) -> IO(Unit):
283	  IO.pure(Unit, Unit{})
284	
285	def close.last(~R: Type, ~close: R -> IO(Unit), last: Maybe<&1, Result<&1, &1, Err, R>>) -> IO(Unit):
286	  match last:
287	    case None{}:
288	      IO.pure(Unit, Unit{})
289	    case Some{r}:
290	      match r:
291	        case Fail{e}:
292	          IO.pure(Unit, Unit{})
293	        case Done{x}:
294	          close(x)
295	
296	def attempt.run(~R: Type, ~judge: Result<&1, &1, Http.Err, R> -> Result<&1, &1, Http.Err, R> & Maybe<&2, String>,
297	    ~try: Opts -> Call -> Http.Pool -> Maybe<&2, Http.Jar> -> Bytes.Bytes -> IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R>),
298	    +o: Opts, +call: Call, +idem: Bool, +n: Nat, pool: Http.Pool, jar: Maybe<&2, Http.Jar>, br: Retry.Breaker,
299	    b: Retry.Budget, r: Bytes.Bytes & Bytes.Bytes) -> IO(Step<R>):
300	  Opts{base, h, ms, mode, +p, cert} = o
301	  (body, spare) = r
302	  do IO<Step<R>>:
303	    x : Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R> <- try(o, call, pool, jar, spare)
304	    settle(~R, ~judge, p, idem, n, br, b, body, x)
305	
306	def denied(-R: Type, why: Retry.Deny, last: Maybe<&1, Result<&1, &1, Err, R>>) -> Maybe<&1, Result<&1, &1, Err, R>>:
307	  match last:
308	    case None{}:
309	      Some{Fail{ErrDenied{why}}}
310	    case Some{r}:
311	      Some{r}
312	
313	# A refused stream closes only when the next attempt is admitted, so a denied attempt still returns it.
314	def attempt.gate(~R: Type, ~judge: Result<&1, &1, Http.Err, R> -> Result<&1, &1, Http.Err, R> & Maybe<&2, String>,
315	    ~try: Opts -> Call -> Http.Pool -> Maybe<&2, Http.Jar> -> Bytes.Bytes -> IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R>),
316	    ~close: R -> IO(Unit),
317	    x: Retry.Breaker & Retry.Budget & Retry.Gate, +o: Opts, +call: Call, +idem: Bool, +n: Nat, pool: Http.Pool,
318	    jar: Maybe<&2, Http.Jar>, body: Bytes.Bytes, last: Maybe<&1, Result<&1, &1, Err, R>>) -> IO(Step<R>):
319	  (br, b, g) = x
320	  match g:
321	    case Retry.GateNo{why}:
322	      IO.pure(Step<R>, Step{pool, jar, br, b, body, denied(R, why, last), Retry.PlanStop{}})
323	    case Retry.GateGo{}:
324	      do IO<Step<R>>:
325	        close.last(~R, ~close, last)
326	        attempt.run(~R, ~judge, ~try, o, call, idem, n, pool, jar, br, b, Bytes.slice(body, 0, 4294967295))
327	
328	# Ask the gate; when it admits, make attempt n (0 first) of this layer. With no deadline and no breaker,
329	# admission does not read the clock.
330	def attempt(~R: Type, ~judge: Result<&1, &1, Http.Err, R> -> Result<&1, &1, Http.Err, R> & Maybe<&2, String>,
331	    ~try: Opts -> Call -> Http.Pool -> Maybe<&2, Http.Jar> -> Bytes.Bytes -> IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R>),
332	    ~close: R -> IO(Unit),
333	    +o: Opts, +call: Call, +idem: Bool, +n: Nat, s: Step<R>) -> IO(Step<R>):
334	  Step{pool, jar, +br, +b, body, last, plan} = s
335	  do IO<Step<R>>:
336	    now : Nat <- clock(Bool.or(breaker.on(br), deadline.some(b)))
337	    attempt.gate(~R, ~judge, ~try, ~close, Retry.admit(br, b, now), o, call, idem, n, pool, jar, body, last)
338	
339	def pause(ms: Nat) -> IO(Unit):
340	  match ms:
341	    case 0n:
342	      IO.pure(Unit, Unit{})
343	    case 1n+k:
344	      IO.sleep(U32.from_nat(1n+k))
345	
346	# left: attempts this layer may still make. n: attempts made so far. Each turn follows the plan of the last step.
347	def go(~R: Type, ~judge: Result<&1, &1, Http.Err, R> -> Result<&1, &1, Http.Err, R> & Maybe<&2, String>,
348	    ~try: Opts -> Call -> Http.Pool -> Maybe<&2, Http.Jar> -> Bytes.Bytes -> IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R>),
349	    ~close: R -> IO(Unit),
350	    left: Nat, +n: Nat, +o: Opts, +call: Call, +idem: Bool, s: Step<R>) -> IO(Step<R>):
351	  match left:
352	    case 0n:
353	      IO.pure(Step<R>, s)
354	    case 1n+rest:
355	      Step{pool, jar, br, b, body, last, plan} = s
356	      match plan:
357	        case Retry.PlanStop{}:
358	          IO.pure(Step<R>, Step{pool, jar, br, b, body, last, Retry.PlanStop{}})
359	        case Retry.PlanWait{ms}:
360	          do IO<Step<R>>:
361	            pause(ms)
362	            next : Step<R> <- attempt(~R, ~judge, ~try, ~close, o, call, idem, n, Step{pool, jar, br, b, body, last, Retry.PlanStop{}})
363	            go(~R, ~judge, ~try, ~close, rest, 1n+n, o, call, idem, next)
364	
365	def result.of(-R: Type, m: Maybe<&1, Result<&1, &1, Err, R>>) -> Result<&1, &1, Err, R>:
366	  match m:
367	    case None{}:
368	      Fail{ErrDenied{Retry.DenySpent{}}}
369	    case Some{r}:
370	      r
371	
372	def request.done(-R: Type, +o: Opts, s: Step<R>) -> Client & Retry.Budget & Result<&1, &1, Err, R>:
373	  Step{pool, jar, br, b, body, last, plan} = s
374	  (Client{o, pool, jar, br}, b, result.of(R, last))
375	
376	def attempts(p: Retry.Policy) -> Nat:
377	  Retry.Policy{a, base, cap, d} = p
378	  a
379	
380	# The budget of one plain request: the policy's attempts, and its deadline from now. The clock is read only for a deadline.
381	def budget(c: Client) -> IO(Client & Retry.Budget):
382	  Client{+o, pool, jar, br} = c
383	  Opts{base, h, ms, mode, Retry.Policy{+a, bs, cap, +d}, cert} = o
384	  do IO<Client & Retry.Budget>:
385	    now : Nat <- clock(some(d))
386	    return (Client{o, pool, jar, br}, Retry.Budget.new(a, now, d))
387	
388	def run(~R: Type, ~judge: Result<&1, &1, Http.Err, R> -> Result<&1, &1, Http.Err, R> & Maybe<&2, String>,
389	    ~try: Opts -> Call -> Http.Pool -> Maybe<&2, Http.Jar> -> Bytes.Bytes -> IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, R>),
390	    ~close: R -> IO(Unit),
391	    c: Client, b: Retry.Budget, +idem: Bool, enc: String, method: String, url: String,
392	    headers: Map<&2, List<&2, String>>, body: Bytes.Bytes) -> IO(Client & Retry.Budget & Result<&1, &1, Err, R>):
393	  Client{+o, pool, jar, br} = c
394	  Opts{+base, +h, ms, mode, +p, cert} = o
395	  +call = {Call{enc, method, target.abs(target(base, url)), merge(h, headers)} : Call}
396	  do IO<Client & Retry.Budget & Result<&1, &1, Err, R>>:
397	    s : Step<R> <- go(~R, ~judge, ~try, ~close, attempts(p), 0n, o, call, idem, Step{pool, jar, br, b, body, None{}, Retry.PlanWait{0n}})
398	    return request.done(R, o, s)
399	
400	# One request inside a caller's retry layer, with every choice explicit. Every attempt takes from b, the
401	# caller's shared budget, and obeys its deadline; the client's own policy caps the attempts of this call.
402	# The budget left comes back beside the result: pass it to the next call, so nested layers never reset or
403	# multiply it. The client's own deadline does not apply here; b's does.
404	# judge: Some{Retry-After} when another attempt may help. idem: the request is safe to repeat, so a judged
405	# failure is retried; else the breaker counts it and it never repeats. enc: the Accept-Encoding codings.
406	def request.as(~judge: Result<&1, &1, Http.Err, Http.Res> -> Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, String>,
407	    c: Client, b: Retry.Budget, +idem: Bool, +enc: String, +method: String, url: String,
408	    headers: Map<&2, List<&2, String>>, body: Bytes.Bytes) -> IO(Client & Retry.Budget & Result<&1, &1, Err, Http.Res>):
409	  run(~Http.Res, ~judge, ~try.res, ~close.res, c, b, idem, enc, method, url, headers, body)
410	
411	# A streamed request under the same budget, deadline, backoff, and breaker as request.as. The stream opens its
412	# own connection outside the pool. It follows the client's redirect mode, presents its certificate, and sends
413	# and stores cookies through its jar, as request.as does. judge reads the status and headers through
414	# Http.stream.res. A refused stream closes before the next attempt; the last one comes back open, so the
415	# caller can read its body.
416	def open.as(~judge: Result<&1, &1, Http.Err, Http.Stream> -> Result<&1, &1, Http.Err, Http.Stream> & Maybe<&2, String>,
417	    c: Client, b: Retry.Budget, +idem: Bool, +enc: String, +method: String, url: String,
418	    headers: Map<&2, List<&2, String>>, body: Bytes.Bytes) -> IO(Client & Retry.Budget & Result<&1, &1, Err, Http.Stream>):
419	  run(~Http.Stream, ~judge, ~try.stream, ~Http.stream.close, c, b, idem, enc, method, url, headers, body)
420	
421	# request.as with judge, the idempotency of method, and every coding Http decodes.
422	def request.in(c: Client, b: Retry.Budget, +method: String, url: String, headers: Map<&2, List<&2, String>>,
423	    body: Bytes.Bytes) -> IO(Client & Retry.Budget & Result<&1, &1, Err, Http.Res>):
424	  do IO<Client & Retry.Budget & Result<&1, &1, Err, Http.Res>>:
425	    enc : String <- Http.codings()
426	    request.as(~judge, c, b, Http.pool.idempotent(method), enc, method, url, headers, body)
427	
428	def request.drop(x: Client & Retry.Budget & Result<&1, &1, Err, Http.Res>) -> Client & Result<&1, &1, Err, Http.Res>:
429	  (c, b, r) = x
430	  (c, r)
431	
432	def request.budget(+method: String, url: String, headers: Map<&2, List<&2, String>>, body: Bytes.Bytes,
433	    x: Client & Retry.Budget) -> IO(Client & Retry.Budget & Result<&1, &1, Err, Http.Res>):
434	  (c, b) = x
435	  request.in(c, b, method, url, headers, body)
436	
437	# url resolves against the base; headers go over the defaults. Redirects follow the client's mode, at most 20.
438	# Each request gets a fresh budget from budget(c). Before each attempt the deadline, the
439	# budget, and the breaker must admit it. GET, HEAD, OPTIONS, TRACE, PUT, and DELETE try again after a connect
440	# error, a timeout, or 408, 429, 500, 502, 503, or 504, as Http.retry.judge says, on the same pool.
441	# When the attempts run out, the last result comes back; when none started, ErrDenied says why.
442	def request(c: Client, +method: String, url: String, headers: Map<&2, List<&2, String>>, body: Bytes.Bytes) ->
443	    IO(Client & Result<&1, &1, Err, Http.Res>):
444	  do IO<Client & Result<&1, &1, Err, Http.Res>>:
445	    y : Client & Retry.Budget <- budget(c)
446	    x : Client & Retry.Budget & Result<&1, &1, Err, Http.Res> <- request.budget(method, url, headers, body, y)
447	    return request.drop(x)
448	
449	def get(c: Client, url: String) -> IO(Client & Result<&1, &1, Err, Http.Res>):
450	  request(c, "GET", url, Http.empty(), Bytes.new(0))
451	
452	# POST v as compact JSON, with content-type: application/json.
453	def post.json(c: Client, url: String, v: Json.Val) -> IO(Client & Result<&1, &1, Err, Http.Res>):
454	  request(c, "POST", url, Http.set(Http.empty(), "content-type", "application/json"), Json.encode.bytes(v))