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@0.23.0.1/http.bend as Http
4	import bend-kit-time@0.1.0.0/time.bend as Time
5	# By hash, as http imports them: bend-kit-url@0.4.1.0, -json@0.5.0.1, -bytes@0.3.0.0.
6	import 0x1f2d80f53f971b16c6de6a65cb1918ae/url.bend as Url
7	import 0x584fc27920487ceab242392391418d7f/json.bend as Json
8	import 0x49814d83de8f70993a43e1002be29ecd/bytes.bend as Bytes
9	import ./retry.bend as Retry
10	
11	# base: "" for none. headers: lowercase names. ms: the step timeout. policy: attempts, backoff, and deadline of each request.
12	type Opts is Data:
13	  Opts{base: String, headers: Map<&2, List<&2, String>>, ms: U32, mode: Http.Mode, policy: Retry.Policy, cert: Maybe<&2, Http.Cert>}
14	
15	# breaker: the circuit breaker of every request on this client.
16	type Client is Type:
17	  Client{opts: Opts, pool: Http.Pool, jar: Maybe<&2, Http.Jar>, breaker: Retry.Breaker}
18	
19	# ErrHttp: the last attempt failed. ErrDenied: no attempt started, for why.
20	type Err is Data:
21	  ErrHttp{error: Http.Err}
22	  ErrDenied{why: Retry.Deny}
23	
24	# One attempt, the first included; backoff from 500 ms, doubled to at most 30 s; no deadline.
25	def policy() -> Retry.Policy:
26	  Retry.Policy{1n, 500n, 30000, None{}}
27	
28	# A pool of up to cap idle sockets per origin, no base, no default headers, 30 s steps, follow redirects,
29	# no retries, and no circuit breaker.
30	def new.with(+cap: U32) -> Client:
31	  Client{Opts{"", Http.empty(), 30000, Http.ModeFollow{}, policy(), None{}}, Http.pool.new.with(cap), None{}, Retry.BreakerOff{}}
32	
33	def new() -> Client:
34	  new.with(8)
35	
36	def base(c: Client, url: String) -> Client:
37	  Client{Opts{b, h, ms, mode, p, cert}, pool, j, br} = c
38	  Client{Opts{url, h, ms, mode, p, cert}, pool, j, br}
39	
40	# A default header. It replaces an earlier default of the same name; a request header of that name replaces it.
41	def header(c: Client, k: String, v: String) -> Client:
42	  Client{Opts{b, h, ms, mode, p, cert}, pool, j, br} = c
43	  Client{Opts{b, Http.set(h, String.to_lower(k), v), ms, mode, p, cert}, pool, j, br}
44	
45	def timeout(c: Client, ms: U32) -> Client:
46	  Client{Opts{b, h, old, mode, p, cert}, pool, j, br} = c
47	  Client{Opts{b, h, ms, mode, p, cert}, pool, j, br}
48	
49	def redirect(c: Client, mode: Http.Mode) -> Client:
50	  Client{Opts{b, h, ms, old, p, cert}, pool, j, br} = c
51	  Client{Opts{b, h, ms, mode, p, cert}, pool, j, br}
52	
53	# Up to n retries after the first attempt: n + 1 attempts in all.
54	def retry(c: Client, n: U32) -> Client:
55	  Client{Opts{b, h, ms, mode, Retry.Policy{old, base, cap, d}, cert}, pool, j, br} = c
56	  Client{Opts{b, h, ms, mode, Retry.Policy{1n+U32.to_nat(n), base, cap, d}, cert}, pool, j, br}
57	
58	# No attempt of a request starts ms or more after the request starts.
59	def deadline(c: Client, ms: U32) -> Client:
60	  Client{Opts{b, h, t, mode, Retry.Policy{a, base, cap, old}, cert}, pool, j, br} = c
61	  Client{Opts{b, h, t, mode, Retry.Policy{a, base, cap, Some{U32.to_nat(ms)}}, cert}, pool, j, br}
62	
63	# A circuit breaker in front of every request, starting closed.
64	def circuit(c: Client, circ: Retry.Circuit) -> Client:
65	  Client{o, pool, j, old} = c
66	  Client{o, pool, j, Retry.Breaker.new(circ)}
67	
68	# The breaker, for inspection.
69	def breaker.of(c: Client) -> Client & Retry.Breaker:
70	  Client{o, pool, j, +br} = c
71	  (Client{o, pool, j, br}, br)
72	
73	# Paths to a PEM client chain and its private key, presented to every HTTPS origin.
74	def cert(c: Client, chain: String, key: String) -> Client:
75	  Client{Opts{b, h, ms, mode, p, old}, pool, j, br} = c
76	  Client{Opts{b, h, ms, mode, p, Some{Http.Cert{chain, key}}}, pool, j, br}
77	
78	# Every request stores and sends cookies through jar. Its clock moves to Time.now() before each try.
79	def jar(c: Client, jar: Http.Jar) -> Client:
80	  Client{o, pool, old, br} = c
81	  Client{o, pool, Some{jar}, br}
82	
83	# The jar, if the client has one.
84	def jar.of(c: Client) -> Client & Maybe<&2, Http.Jar>:
85	  Client{o, pool, +j, br} = c
86	  (Client{o, pool, j, br}, j)
87	
88	def close(c: Client) -> IO(Unit):
89	  Client{o, pool, j, br} = c
90	  Http.pool.close(pool)
91	
92	def ms.of(r: U32 & U32 & U32) -> Nat:
93	  (hi, lo, ns) = r
94	  Nat.add(Nat.mul(Nat.add(Nat.mul(U32.to_nat(hi), 1n+4294967295n), U32.to_nat(lo)), 1000n), U32.to_nat((ns / 1000000 : U32)))
95	
96	# Milliseconds on the monotonic clock that Retry budgets and breakers use.
97	def now.ms() -> IO(Nat):
98	  do IO<Nat>:
99	    r : U32 & U32 & U32 <- Time.mono.raw()
100	    return ms.of(r)
101	
102	def target.pick(none: Bool, b: String, ref: String) -> Maybe<&2, String>:
103	  match none:
104	    case True{}:
105	      Some{ref}
106	    case False{}:
107	      Http.resolve(b, ref)
108	
109	# ref against the base (RFC 3986 §5.2); with no base, ref as given. None when the base is not absolute.
110	def target(+b: String, ref: String) -> Maybe<&2, String>:
111	  target.pick(String.is_empty(b), b, ref)
112	
113	def target.abs(m: Maybe<&2, String>) -> Maybe<&2, Url.Abs>:
114	  match m:
115	    case None{}:
116	      None{}
117	    case Some{s}:
118	      Url.absolute(s)
119	
120	# The defaults under the request headers, names lowercased; a request header replaces a default of its name.
121	# Host, Connection, Content-Length, and Transfer-Encoding are the client's own, so both sides drop them.
122	def merge(defaults: Map<&2, List<&2, String>>, headers: Map<&2, List<&2, String>>) -> Map<&2, List<&2, String>>:
123	  Http.req.put(Map.to_list(&2, List<&2, String>, headers), Http.req.put(Map.to_list(&2, List<&2, String>, defaults), Http.empty()))
124	
125	def jar.now(j: Maybe<&2, Http.Jar>) -> IO(Maybe<&2, Http.Jar>):
126	  match j:
127	    case None{}:
128	      IO.pure(Maybe<&2, Http.Jar>, None{})
129	    case Some{jar}:
130	      do IO<Maybe<&2, Http.Jar>>:
131	        now : Time.Instant <- Time.now()
132	        return Some{Http.jar.at(jar, now)}
133	
134	def once.final(+enc: String, x: Http.Pool & Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, Http.Jar>) ->
135	    IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>):
136	  (p, r, j) = x
137	  do IO<Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>>:
138	    d : Result<&1, &1, Http.Err, Http.Res> <- Http.fetch.final(enc, r)
139	    return (p, j, d)
140	
141	# One try: the redirect loop on the pool, with the jar and the certificate, then the body decoded.
142	def once(+ms: U32, +mode: Http.Mode, +cert: Maybe<&2, Http.Cert>, +enc: String, p: Http.Pool, j: Maybe<&2, Http.Jar>,
143	    +method: String, +u: Maybe<&2, Url.Abs>, +headers: Map<&2, List<&2, String>>, body: Bytes.Bytes) ->
144	    IO(Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>):
145	  do IO<Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>>:
146	    now : Maybe<&2, Http.Jar> <- jar.now(j)
147	    x : Http.Pool & Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, Http.Jar> <-
148	      Http.pool.hops(mode, ms, Http.Hops{p, Http.fetch.start(method, headers, body, u, enc), cert, now})
149	    once.final(enc, x)
150	
151	def hint.ms(m: Maybe<&2, U32>) -> Maybe<&2, Nat>:
152	  match m:
153	    case None{}:
154	      None{}
155	    case Some{ms}:
156	      Some{U32.to_nat(ms)}
157	
158	# m is Http.retry.judge's verdict for an idempotent request: Some{Retry-After} when another try may help.
159	# A non-idempotent request with that verdict is Unsafe: the breaker counts it, but it never repeats.
160	def outcome(+idem: Bool, m: Maybe<&2, String>, now: Time.Instant) -> Retry.Outcome:
161	  match m:
162	    case None{}:
163	      Retry.Settled{}
164	    case Some{+v}:
165	      Bool.pick(Retry.Outcome, idem, Retry.Transient{hint.ms(Http.retry.after(v, now))}, Retry.Unsafe{})
166	
167	def lift(r: Result<&1, &1, Http.Err, Http.Res>) -> Result<&1, &1, Err, Http.Res>:
168	  match r:
169	    case Fail{e}:
170	      Fail{ErrHttp{e}}
171	    case Done{res}:
172	      Done{res}
173	
174	# The live state of one request between attempts. last: the result of the last attempt, None before the first.
175	type Step is Type:
176	  Step{pool: Http.Pool, jar: Maybe<&2, Http.Jar>, breaker: Retry.Breaker, budget: Retry.Budget, body: Bytes.Bytes,
177	    last: Maybe<&1, Result<&1, &1, Err, Http.Res>>, plan: Retry.Plan}
178	
179	# The clock, read only when a decision depends on it; 0 otherwise, which no such decision reads.
180	def clock(need: Bool) -> IO(Nat):
181	  match need:
182	    case True{}:
183	      now.ms()
184	    case False{}:
185	      IO.pure(Nat, 0n)
186	
187	def breaker.on(br: Retry.Breaker) -> Bool:
188	  match br:
189	    case Retry.BreakerOff{}:
190	      False{}
191	    case Retry.BreakerOn{c, s}:
192	      True{}
193	
194	def transient(o: Retry.Outcome) -> Bool:
195	  match o:
196	    case Retry.Transient{after}:
197	      True{}
198	    case Retry.Settled{}:
199	      False{}
200	    case Retry.Unsafe{}:
201	      False{}
202	
203	def some(m: Maybe<&2, Nat>) -> Bool:
204	  match m:
205	    case None{}:
206	      False{}
207	    case Some{t}:
208	      True{}
209	
210	def deadline.some(b: Retry.Budget) -> Bool:
211	  Retry.Budget{l, d} = b
212	  some(d)
213	
214	# The wall clock dates a Retry-After, so it is read only for a transient failure.
215	def outcome.io(+idem: Bool, m: Maybe<&2, String>) -> IO(Retry.Outcome):
216	  match m:
217	    case None{}:
218	      IO.pure(Retry.Outcome, Retry.Settled{})
219	    case Some{v}:
220	      do IO<Retry.Outcome>:
221	        wall : Time.Instant <- Time.now()
222	        return outcome(idem, Some{v}, wall)
223	
224	# Jitter for the backoff, drawn only when a retry may follow.
225	def jitter.io(need: Bool) -> IO(Nat):
226	  match need:
227	    case True{}:
228	      do IO<Nat>:
229	        rnd : Result<&1, &1, U32 & String, U32> <- IO.random_u32()
230	        return U32.to_nat(Http.retry.jitter(rnd))
231	    case False{}:
232	      IO.pure(Nat, 0n)
233	
234	def settle.at(+now: Nat, +rnd: Nat, +p: Retry.Policy, +n: Nat, +o: Retry.Outcome, br: Retry.Breaker, +b: Retry.Budget,
235	    body: Bytes.Bytes, pool: Http.Pool, jar: Maybe<&2, Http.Jar>, r: Result<&1, &1, Http.Err, Http.Res>) -> Step:
236	  Step{pool, jar, Retry.Breaker.done(br, now, Retry.failed(o)), b, body, Some{lift(r)}, Retry.plan(o, p, b, n, now, rnd)}
237	
238	# The attempt has ended: tell the breaker, and plan the next step.
239	def settle.outcome(+p: Retry.Policy, +n: Nat, +o: Retry.Outcome, +br: Retry.Breaker, b: Retry.Budget, body: Bytes.Bytes,
240	    pool: Http.Pool, jar: Maybe<&2, Http.Jar>, r: Result<&1, &1, Http.Err, Http.Res>) -> IO(Step):
241	  do IO<Step>:
242	    now : Nat <- clock(Bool.or(breaker.on(br), transient(o)))
243	    rnd : Nat <- jitter.io(transient(o))
244	    return settle.at(now, rnd, p, n, o, br, b, body, pool, jar, r)
245	
246	def settle.judged(+p: Retry.Policy, +idem: Bool, +n: Nat, br: Retry.Breaker, b: Retry.Budget, body: Bytes.Bytes,
247	    pool: Http.Pool, jar: Maybe<&2, Http.Jar>, j: Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, String>) -> IO(Step):
248	  (r, m) = j
249	  do IO<Step>:
250	    o : Retry.Outcome <- outcome.io(idem, m)
251	    settle.outcome(p, n, o, br, b, body, pool, jar, r)
252	
253	# The default judge: Http.retry.judge, as if the request were idempotent; outcome then applies idem.
254	def judge(r: Result<&1, &1, Http.Err, Http.Res>) -> Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, String>:
255	  Http.retry.judge(True{}, r)
256	
257	def settle(~judge: Result<&1, &1, Http.Err, Http.Res> -> Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, String>,
258	    +p: Retry.Policy, +idem: Bool, +n: Nat, br: Retry.Breaker, b: Retry.Budget, body: Bytes.Bytes,
259	    x: Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res>) -> IO(Step):
260	  (pool, jar, r) = x
261	  settle.judged(p, idem, n, br, b, body, pool, jar, judge(r))
262	
263	def attempt.run(~judge: Result<&1, &1, Http.Err, Http.Res> -> Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, String>,
264	    +o: Opts, +enc: String, +idem: Bool, +method: String, +u: Maybe<&2, Url.Abs>,
265	    +headers: Map<&2, List<&2, String>>, +n: Nat, pool: Http.Pool, jar: Maybe<&2, Http.Jar>, br: Retry.Breaker,
266	    b: Retry.Budget, r: Bytes.Bytes & Bytes.Bytes) -> IO(Step):
267	  Opts{base, h, +ms, +mode, +p, +cert} = o
268	  (body, spare) = r
269	  do IO<Step>:
270	    x : Http.Pool & Maybe<&2, Http.Jar> & Result<&1, &1, Http.Err, Http.Res> <- once(ms, mode, cert, enc, pool, jar, method, u, headers, spare)
271	    settle(~judge, p, idem, n, br, b, body, x)
272	
273	def denied(why: Retry.Deny, last: Maybe<&1, Result<&1, &1, Err, Http.Res>>) -> Maybe<&1, Result<&1, &1, Err, Http.Res>>:
274	  match last:
275	    case None{}:
276	      Some{Fail{ErrDenied{why}}}
277	    case Some{r}:
278	      Some{r}
279	
280	def attempt.gate(~judge: Result<&1, &1, Http.Err, Http.Res> -> Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, String>,
281	    x: Retry.Breaker & Retry.Budget & Retry.Gate, +o: Opts, +enc: String, +idem: Bool, +method: String,
282	    +u: Maybe<&2, Url.Abs>, +headers: Map<&2, List<&2, String>>, +n: Nat, pool: Http.Pool, jar: Maybe<&2, Http.Jar>,
283	    body: Bytes.Bytes, last: Maybe<&1, Result<&1, &1, Err, Http.Res>>) -> IO(Step):
284	  (br, b, g) = x
285	  match g:
286	    case Retry.GateNo{why}:
287	      IO.pure(Step, Step{pool, jar, br, b, body, denied(why, last), Retry.PlanStop{}})
288	    case Retry.GateGo{}:
289	      attempt.run(~judge, o, enc, idem, method, u, headers, n, pool, jar, br, b, Bytes.slice(body, 0, 4294967295))
290	
291	# Ask the gate; when it admits, make attempt n (0 first) of this layer. With no deadline and no breaker,
292	# admission does not read the clock.
293	def attempt(~judge: Result<&1, &1, Http.Err, Http.Res> -> Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, String>,
294	    +o: Opts, +enc: String, +idem: Bool, +method: String, +u: Maybe<&2, Url.Abs>,
295	    +headers: Map<&2, List<&2, String>>, +n: Nat, s: Step) -> IO(Step):
296	  Step{pool, jar, +br, +b, body, last, plan} = s
297	  do IO<Step>:
298	    now : Nat <- clock(Bool.or(breaker.on(br), deadline.some(b)))
299	    attempt.gate(~judge, Retry.admit(br, b, now), o, enc, idem, method, u, headers, n, pool, jar, body, last)
300	
301	def pause(ms: Nat) -> IO(Unit):
302	  match ms:
303	    case 0n:
304	      IO.pure(Unit, Unit{})
305	    case 1n+k:
306	      IO.sleep(U32.from_nat(1n+k))
307	
308	# left: attempts this layer may still make. n: attempts made so far. Each turn follows the plan of the last step.
309	def go(~judge: Result<&1, &1, Http.Err, Http.Res> -> Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, String>,
310	    left: Nat, +n: Nat, +o: Opts, +enc: String, +idem: Bool, +method: String, +u: Maybe<&2, Url.Abs>,
311	    +headers: Map<&2, List<&2, String>>, s: Step) -> IO(Step):
312	  match left:
313	    case 0n:
314	      IO.pure(Step, s)
315	    case 1n+rest:
316	      Step{pool, jar, br, b, body, last, plan} = s
317	      match plan:
318	        case Retry.PlanStop{}:
319	          IO.pure(Step, Step{pool, jar, br, b, body, last, Retry.PlanStop{}})
320	        case Retry.PlanWait{ms}:
321	          do IO<Step>:
322	            pause(ms)
323	            next : Step <- attempt(~judge, o, enc, idem, method, u, headers, n, Step{pool, jar, br, b, body, last, Retry.PlanStop{}})
324	            go(~judge, rest, 1n+n, o, enc, idem, method, u, headers, next)
325	
326	def result.of(m: Maybe<&1, Result<&1, &1, Err, Http.Res>>) -> Result<&1, &1, Err, Http.Res>:
327	  match m:
328	    case None{}:
329	      Fail{ErrDenied{Retry.DenySpent{}}}
330	    case Some{r}:
331	      r
332	
333	def request.done(+o: Opts, s: Step) -> Client & Retry.Budget & Result<&1, &1, Err, Http.Res>:
334	  Step{pool, jar, br, b, body, last, plan} = s
335	  (Client{o, pool, jar, br}, b, result.of(last))
336	
337	def attempts(p: Retry.Policy) -> Nat:
338	  Retry.Policy{a, base, cap, d} = p
339	  a
340	
341	# The budget of one plain request: the policy's attempts, and its deadline from now. The clock is read only for a deadline.
342	def budget(c: Client) -> IO(Client & Retry.Budget):
343	  Client{+o, pool, jar, br} = c
344	  Opts{base, h, ms, mode, Retry.Policy{+a, bs, cap, +d}, cert} = o
345	  do IO<Client & Retry.Budget>:
346	    now : Nat <- clock(some(d))
347	    return (Client{o, pool, jar, br}, Retry.Budget.new(a, now, d))
348	
349	# One request inside a caller's retry layer, with every choice explicit. Every attempt takes from b, the
350	# caller's shared budget, and obeys its deadline; the client's own policy caps the attempts of this call.
351	# The budget left comes back beside the result: pass it to the next call, so nested layers never reset or
352	# multiply it. The client's own deadline does not apply here; b's does.
353	# judge: Some{Retry-After} when another attempt may help. idem: the request is safe to repeat, so a judged
354	# failure is retried; else the breaker counts it and it never repeats. enc: the Accept-Encoding codings.
355	def request.as(~judge: Result<&1, &1, Http.Err, Http.Res> -> Result<&1, &1, Http.Err, Http.Res> & Maybe<&2, String>,
356	    c: Client, b: Retry.Budget, +idem: Bool, +enc: String, +method: String, url: String,
357	    headers: Map<&2, List<&2, String>>, body: Bytes.Bytes) -> IO(Client & Retry.Budget & Result<&1, &1, Err, Http.Res>):
358	  Client{+o, pool, jar, br} = c
359	  Opts{+base, +h, ms, mode, +p, cert} = o
360	  +u = target.abs(target(base, url))
361	  +hs = merge(h, headers)
362	  do IO<Client & Retry.Budget & Result<&1, &1, Err, Http.Res>>:
363	    s : Step <- go(~judge, attempts(p), 0n, o, enc, idem, method, u, hs, Step{pool, jar, br, b, body, None{}, Retry.PlanWait{0n}})
364	    return request.done(o, s)
365	
366	# request.as with judge, the idempotency of method, and every coding Http decodes.
367	def request.in(c: Client, b: Retry.Budget, +method: String, url: String, headers: Map<&2, List<&2, String>>,
368	    body: Bytes.Bytes) -> IO(Client & Retry.Budget & Result<&1, &1, Err, Http.Res>):
369	  do IO<Client & Retry.Budget & Result<&1, &1, Err, Http.Res>>:
370	    enc : String <- Http.codings()
371	    request.as(~judge, c, b, Http.pool.idempotent(method), enc, method, url, headers, body)
372	
373	def request.drop(x: Client & Retry.Budget & Result<&1, &1, Err, Http.Res>) -> Client & Result<&1, &1, Err, Http.Res>:
374	  (c, b, r) = x
375	  (c, r)
376	
377	def request.budget(+method: String, url: String, headers: Map<&2, List<&2, String>>, body: Bytes.Bytes,
378	    x: Client & Retry.Budget) -> IO(Client & Retry.Budget & Result<&1, &1, Err, Http.Res>):
379	  (c, b) = x
380	  request.in(c, b, method, url, headers, body)
381	
382	# url resolves against the base; headers go over the defaults. Redirects follow the client's mode, at most 20.
383	# Each request gets a fresh budget from budget(c). Before each attempt the deadline, the
384	# budget, and the breaker must admit it. GET, HEAD, OPTIONS, TRACE, PUT, and DELETE try again after a connect
385	# error, a timeout, or 408, 429, 500, 502, 503, or 504, as Http.retry.judge says, on the same pool.
386	# When the attempts run out, the last result comes back; when none started, ErrDenied says why.
387	def request(c: Client, +method: String, url: String, headers: Map<&2, List<&2, String>>, body: Bytes.Bytes) ->
388	    IO(Client & Result<&1, &1, Err, Http.Res>):
389	  do IO<Client & Result<&1, &1, Err, Http.Res>>:
390	    y : Client & Retry.Budget <- budget(c)
391	    x : Client & Retry.Budget & Result<&1, &1, Err, Http.Res> <- request.budget(method, url, headers, body, y)
392	    return request.drop(x)
393	
394	def get(c: Client, url: String) -> IO(Client & Result<&1, &1, Err, Http.Res>):
395	  request(c, "GET", url, Http.empty(), Bytes.new(0))
396	
397	# POST v as compact JSON, with content-type: application/json.
398	def post.json(c: Client, url: String, v: Json.Val) -> IO(Client & Result<&1, &1, Err, Http.Res>):
399	  request(c, "POST", url, Http.set(Http.empty(), "content-type", "application/json"), Json.encode.bytes(v))