Add option to allow specifying address to bind to

app/prometheus_unix.ml

@@ -2,7 +2,6 @@ open Prometheus
 
 module Metrics = struct
   let namespace = "prometheus"
-
   let subsystem = "logs"
 
   let inc_messages =
@@ -20,54 +19,85 @@ module Unix_runtime = struct
   let start_time = Unix.gettimeofday ()
 
   let simple_metric ~metric_type ~help name fn =
-    let info = {
-      MetricInfo.
-      name = MetricName.v name;
-      help;
-      metric_type;
-      label_names = [];
-    }
-    in
-    let collect () =
-      LabelSetMap.singleton [] [Sample_set.sample (fn ())]
+    let info =
+      {
+        MetricInfo.name = MetricName.v name;
+        help;
+        metric_type;
+        label_names = [];
+      }
     in
-    info, collect
+    let collect () = LabelSetMap.singleton [] [ Sample_set.sample (fn ()) ] in
+    (info, collect)
 
   let process_start_time_seconds =
-    simple_metric ~metric_type:Counter "process_start_time_seconds" (fun () -> start_time)
+    simple_metric ~metric_type:Counter "process_start_time_seconds"
+      (fun () -> start_time)
       ~help:"Start time of the process since unix epoch in seconds."
 
-  let metrics = [
-    process_start_time_seconds;
-  ]
+  let metrics = [ process_start_time_seconds ]
 end
 
-type config = int option
+type config = string option
 
-module Server = Prometheus_app.Cohttp(Cohttp_lwt_unix.Server)
+module Server = Prometheus_app.Cohttp (Cohttp_lwt_unix.Server)
 
-let serve = function
+let bind addr port =
+  let open! Unix in
+  let[@ocaml.warning "-partial-match"] (addrinfo :: _) =
+    getaddrinfo addr port [ AI_SOCKTYPE SOCK_STREAM ]
+  in
+  let socket =
+    socket ~cloexec:true addrinfo.ai_family addrinfo.ai_socktype
+      addrinfo.ai_protocol
+  in
+  let () = setsockopt socket SO_REUSEADDR true in
+  let callback = Server.callback in
+  let () = bind socket addrinfo.ai_addr in
+  let () = listen socket 20 in
+  let mode = `TCP (`Socket (Lwt_unix.of_unix_file_descr socket)) in
+  let thread =
+    Cohttp_lwt_unix.Server.create ~mode
+      (Cohttp_lwt_unix.Server.make ~callback ())
+  in
+  [ thread ]
+
+let serve config =
+  let addr = "0.0.0.0" in
+  let port = "9090" in
+  match config with
   | None -> []
-  | Some port ->
-    let mode = `TCP (`Port port) in
-    let callback = Server.callback in
-    let thread = Cohttp_lwt_unix.Server.create ~mode (Cohttp_lwt_unix.Server.make ~callback ()) in
-    [thread]
+  | Some config_s -> (
+      try
+        match String.split_on_char ':' config_s with
+        | [] -> bind addr port
+        | port :: [] -> bind addr port
+        | [ addr; port ] -> bind addr port
+      with Match_failure _ ->
+        Printf.printf
+          "ERROR: Incorrect addr:port pair specified, prometheus listener not \
+           starting.\n";
+        flush_all ();
+        [])
+[@@ocaml.warning "-partial-match"]
 
 let listen_prometheus =
   let open! Cmdliner in
   let doc =
-    Arg.info ~docs:"MONITORING OPTIONS" ~docv:"PORT" ~doc:
-      "Port on which to provide Prometheus metrics over HTTP."
-      ["listen-prometheus"]
+    Arg.info ~docs:"MONITORING OPTIONS" ~docv:"ADDR_PORT"
+      ~doc:
+        "Port or address and port on which to provide Prometheus metrics over \
+         HTTP."
+      [ "listen-prometheus" ]
   in
-  Arg.(value @@ opt (some int) None doc)
+  Arg.(value @@ opt (some string) None doc)
 
 let opts = listen_prometheus
 
 let () =
   let add (info, collector) =
-    CollectorRegistry.(register default) info collector in
+    CollectorRegistry.(register default) info collector
+  in
   List.iter add Unix_runtime.metrics
 
 module Logging = struct
@@ -81,26 +111,34 @@ module Logging = struct
 
   let reporter formatter =
     let report src level ~over k msgf =
-      let k _ = over (); k () in
+      let k _ =
+        over ();
+        k ()
+      in
       let src = Logs.Src.name src in
       Metrics.inc_messages level src;
       msgf @@ fun ?header ?tags:_ fmt ->
-      Fmt.kpf k formatter ("%a %a %a @[" ^^ fmt ^^ "@]@.")
+      Fmt.kpf k formatter
+        ("%a %a %a @[" ^^ fmt ^^ "@]@.")
         pp_timestamp (Unix.gettimeofday ())
-        Fmt.(styled `Magenta string) (Printf.sprintf "%14s" src)
+        Fmt.(styled `Magenta string)
+        (Printf.sprintf "%14s" src)
         Logs_fmt.pp_header (level, header)
     in
-    { Logs.report = report }
+    { Logs.report }
 
   let set_level (src, level) =
     let rec aux = function
-      | [] -> Logs.warn (fun f -> f "set_level: logger %S not registered; ignoring" src)
+      | [] ->
+          Logs.warn (fun f ->
+              f "set_level: logger %S not registered; ignoring" src)
       | x :: _ when Logs.Src.name x = src -> Logs.Src.set_level x (Some level)
       | _ :: xs -> aux xs
     in
     aux (Logs.Src.list ())
 
-  let init ?(default_level=Logs.Info) ?(levels=[]) ?(formatter=Fmt.stderr) () =
+  let init ?(default_level = Logs.Info) ?(levels = []) ?(formatter = Fmt.stderr)
+      () =
     Fmt_tty.setup_std_outputs ();
     Logs.set_reporter (reporter formatter);
     Logs.set_level (Some default_level);