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DNS: start task reading Lwt_mvar and distributing DNS replies to clients

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Before, a DNS request was sent and the first thing appearing in the Lwt_mvar
was taken as reply. The issue with this was two-fold:
- it could be a reply for a different request
- there could be DNS replies being sent to the uplink stack leading to
  Lwt_mvar.put being called, which blocks if there is already a value in the
  mvar.

No, the separate task is a loop reading the mvar, using a Lwt_condition to
signal the receive of that ID (potentially discarding if there's no client
waiting). The DNS query registers itself (using the ID) in the map with a
Lwt_condition, and waits to be notified (or a timeout occurs).
This commit is contained in:
Hannes Mehnert 2022-11-11 12:07:06 +01:00
parent ddfb17c0b2
commit 0e0917f4fe
1 changed files with 26 additions and 5 deletions
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31
my_dns.ml
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@ -5,11 +5,14 @@ module Transport (R : Mirage_random.S) (C : Mirage_clock.MCLOCK) (Time : Mirage_
type io_addr = Ipaddr.V4.t * int
type stack = Router.t * (src_port:int -> dst:Ipaddr.V4.t -> dst_port:int -> Cstruct.t -> (unit, [ `Msg of string ]) result Lwt.t) * (Udp_packet.t * Cstruct.t) Lwt_mvar.t
module IM = Map.Make(Int)
type t = {
protocol : Dns.proto ;
nameserver : io_addr ;
stack : stack ;
timeout_ns : int64 ;
mutable requests : Cstruct.t Lwt_condition.t IM.t ;
}
type context = t
@ -17,12 +20,26 @@ module Transport (R : Mirage_random.S) (C : Mirage_clock.MCLOCK) (Time : Mirage_
let rng = R.generate ?g:None
let clock = C.elapsed_ns
let rec read t =
let _, _, answer = t.stack in
Lwt_mvar.take answer >>= fun (_, data) ->
if Cstruct.length data > 2 then begin
match IM.find_opt (Cstruct.BE.get_uint16 data 0) t.requests with
| Some cond -> Lwt_condition.broadcast cond data
| None -> ()
end;
read t
let create ?nameservers ~timeout stack =
let protocol, nameserver = match nameservers with
| None | Some (_, []) -> invalid_arg "no nameserver found"
| Some (proto, ns :: _) -> proto, ns
in
{ protocol ; nameserver ; stack ; timeout_ns = timeout }
let t =
{ protocol ; nameserver ; stack ; timeout_ns = timeout ; requests = IM.empty }
in
Lwt.async (fun () -> read t);
t
let with_timeout timeout_ns f =
let timeout = Time.sleep_ns timeout_ns >|= fun () -> Error (`Msg "DNS request timeout") in
@ -33,14 +50,18 @@ module Transport (R : Mirage_random.S) (C : Mirage_clock.MCLOCK) (Time : Mirage_
let send_recv (ctx : context) buf : (Cstruct.t, [> `Msg of string ]) result Lwt.t =
let open Router in
let dst, dst_port = ctx.nameserver in
let router, send_udp, answer = ctx.stack in
let router, send_udp, _ = ctx.stack in
let src_port, evict =
My_nat.free_udp_port router.nat ~src:router.uplink#my_ip ~dst ~dst_port:53
in
let id = Cstruct.BE.get_uint16 buf 0 in
with_timeout ctx.timeout_ns
((send_udp ~src_port ~dst ~dst_port buf >|= Rresult.R.open_error_msg) >>= function
| Ok () -> (Lwt_mvar.take answer >|= fun (_, dns_response) -> Ok dns_response)
| Error _ as e -> Lwt.return e) >|= fun result ->
(let cond = Lwt_condition.create () in
ctx.requests <- IM.add id cond ctx.requests;
(send_udp ~src_port ~dst ~dst_port buf >|= Rresult.R.open_error_msg) >>= function
| Ok () -> Lwt_condition.wait cond >|= fun dns_response -> Ok dns_response
| Error _ as e -> Lwt.return e) >|= fun result ->
ctx.requests <- IM.remove id ctx.requests;
evict ();
result