eio(client): add ohost.exe sample app

This commit adds `ohost.exe` sample app in the same spirit as
`dns-client.unix` package.

eio/client/dns_client_eio.ml

@@ -4,16 +4,16 @@ type env = <
   clock         : E.Time.clock ;
   net           : E.Net.t;
   fs            : E.Dir.t;
-  secure_random : Eio.Flow.source;
+  secure_random : E.Flow.source;
 >
 
 type io_addr = Ipaddr.t * int
 type stack = env * E.Switch.t 
 
 module Transport : Dns_client.S 
   with type io_addr = io_addr
-   and type stack = stack
-   and type +'a io = 'a  
+   and type stack   = stack
+   and type +'a io  = 'a  
 = struct 
   type nonrec io_addr = io_addr
   type nonrec stack = stack
@@ -36,7 +36,7 @@ module Transport : Dns_client.S
   }
 
   let read_file env file =
-    match E.Dir.load (E.Stdenv.fs env) file with
+    match E.Dir.load env#fs file with
     | content -> Ok content
     | exception e -> 
       let err = "Error while reading file: " ^ file ^ ". " ^ (Printexc.to_string e) in  
@@ -159,8 +159,8 @@ module Transport : Dns_client.S
       let stream = `Tcp (ip, port) in
       try 
         let timeout = Duration.to_f t.timeout_ns in
-        E.Time.with_timeout_exn (E.Stdenv.clock t.env) timeout @@ fun () ->
-        let flow = E.Net.connect ~sw:t.sw (E.Stdenv.net t.env) stream in 
+        E.Time.with_timeout_exn t.env#clock timeout @@ fun () ->
+        let flow = E.Net.connect ~sw:t.sw t.env#net stream in 
         Ok flow
       with E.Time.Timeout ->
         (* Push the non responsive nameserver to the back of the queue. *)

eio/client/dns_client_eio.mli

@@ -2,7 +2,7 @@ type env = <
   clock         : Eio.Time.clock ;
   net           : Eio.Net.t ;
   fs            : Eio.Dir.t ;
-  secure_random : Eio.Flow.source;
+  secure_random : Eio.Flow.source ;
 >
 
 module Transport : Dns_client.S 

eio/client/dune

@@ -1,14 +1,20 @@
 (library
- (name dns_client_eio)
+ (name        dns_client_eio)
+ (modules     dns_client_eio)
  (public_name dns-client-eio)
  (libraries
   cstruct
   duration
-  logs
   ipaddr
   dns-client
   dns-client.resolvconf
   mtime.clock.os
   mirage-crypto-rng
   mirage-crypto-rng-eio))
 
+(executable 
+ (name        ohost)
+ (modules     ohost)
+ (public_name dns-client-eio.ohost)
+ (package     dns-client-eio)
+ (libraries   dns-client-eio mtime.clock.os eio_main domain-name fmt))

eio/client/ohost.ml

@@ -0,0 +1,48 @@
+(**
+   A Simple command line app to demonstrate usage of dns-client-eio package. 
+   Usage: ohost.exe [HOSTNAME]
+   e.g: ohost.exe google.com 
+*)
+
+let (let+) r f = Result.map f r
+
+let display_host_ips h_name = 
+  Eio_main.run @@ fun env -> 
+  Eio.Switch.run @@ fun sw -> 
+  Dns_client_eio.run env @@ fun (module Client) ->   
+  let env = (env :> Dns_client_eio.env) in
+  let c = Client.create (env, sw) in
+  let domain = Domain_name.(host_exn (of_string_exn h_name)) in
+  let ipv4 = 
+    let+ addr = Client.gethostbyname c domain in 
+    Fmt.pr "%a has IPv4 address %a\n" Domain_name.pp domain Ipaddr.V4.pp addr
+  in 
+  let ipv6 = 
+    let+ addr = Client.gethostbyname6 c domain in
+    Fmt.pr "%a has IPv6 address %a\n" Domain_name.pp domain Ipaddr.V6.pp addr
+  in 
+  let mx = 
+    let+ (_ttl,resp) = Client.getaddrinfo c Mx domain in 
+    Fmt.pr "%a\n" 
+      (Fmt.list (fun ppf -> 
+           Fmt.pf ppf "%a mail is handled by %a" 
+             Domain_name.pp domain
+             Dns.Mx.pp))
+      (Dns.Rr_map.Mx_set.elements resp);
+  in 
+  let results = [ ipv4 ; ipv6 ; mx ] in
+  let is_error = (function Error _ -> true | Ok _ -> false) in 
+  match List.find_opt is_error results with
+  | None | Some Ok _ -> () (* no errors *)
+  | Some (Error `Msg msg) -> (* at least one error *)
+    if List.for_all is_error results then begin
+      (Fmt.epr "Host %a not found: @[<v>%s@]\n") Domain_name.pp domain msg;
+      exit 1
+    end
+
+let () = 
+  if Array.length Sys.argv <= 1 then
+    Printf.printf "Usage: ohost.exe [HOSTNAME]\n\nFor example:\nohost.exe google.com\nohost.exe firefox.com"
+  else 
+    let h_name = Sys.argv.(1) in
+    display_host_ips h_name