From 7e48c7521e9c07ec8c0d5139a7f16beb074a652b Mon Sep 17 00:00:00 2001
From: Matt Bray <mattjbray@gmail.com>
Date: Wed, 23 Mar 2022 18:47:14 +0000
Subject: [PATCH] feat(client): read container id from /proc/self/cgroup

---
 dune-project                             |  3 +-
 opentelemetry-client-ocurl.opam          |  1 +
 src/client/dune                          |  2 +-
 src/client/opentelemetry_client_ocurl.ml | 43 +++++++++++++++++++++++-
 4 files changed, 46 insertions(+), 3 deletions(-)

diff --git a/dune-project b/dune-project
index 21e3f5a2..f133b415 100644
--- a/dune-project
+++ b/dune-project
@@ -43,5 +43,6 @@
    (opentelemetry (= :version))
    (ocaml-protoc (>= 2.1))
    (odoc :with-doc)
-   ocurl)
+   ocurl
+   re)
   (synopsis "Collector client for opentelemetry, using http + ocurl"))
diff --git a/opentelemetry-client-ocurl.opam b/opentelemetry-client-ocurl.opam
index 7133660b..e7a2e672 100644
--- a/opentelemetry-client-ocurl.opam
+++ b/opentelemetry-client-ocurl.opam
@@ -15,6 +15,7 @@ depends: [
   "ocaml-protoc" {>= "2.1"}
   "odoc" {with-doc}
   "ocurl"
+  "re"
 ]
 build: [
   ["dune" "subst"] {pinned}
diff --git a/src/client/dune b/src/client/dune
index 11696169..8ba08cb9 100644
--- a/src/client/dune
+++ b/src/client/dune
@@ -3,5 +3,5 @@
   (name opentelemetry_client_ocurl)
   (public_name opentelemetry-client-ocurl)
   (libraries opentelemetry curl ocaml-protoc threads
-             atomic mtime mtime.clock.os))
+             atomic mtime mtime.clock.os re))
 
diff --git a/src/client/opentelemetry_client_ocurl.ml b/src/client/opentelemetry_client_ocurl.ml
index d0ea0d31..ebe93185 100644
--- a/src/client/opentelemetry_client_ocurl.ml
+++ b/src/client/opentelemetry_client_ocurl.ml
@@ -24,6 +24,40 @@ let set_mutex ~lock ~unlock : unit =
   lock_ := lock;
   unlock_ := unlock
 
+let container_id_ = ref None
+
+(** Read the container ID from [/proc/self/cgroup], if it exists, and send it in the [Datadog-Container-ID] HTTP header.
+
+    Datadog uses this add container information to traces.
+
+    See https://github.com/DataDog/dd-trace-js/blob/253fce6fceaf776b14b10f19be586b961cbb66ec/packages/dd-trace/src/exporters/agent/docker.js#L5-L8
+ *)
+let read_container_id_ =
+  let proc_self_cgroup = "/proc/self/cgroup" in
+  let uuid_source = "[0-9a-f]{8}[-_][0-9a-f]{4}[-_][0-9a-f]{4}[-_][0-9a-f]{4}[-_][0-9a-f]{12}" in
+  let container_source = "[0-9a-f]{64}" in
+  let task_source = "[0-9a-f]{32}-\\d+" in
+  let re =
+    Re.Pcre.re ~flags:[`MULTILINE]
+      (Printf.sprintf {|.*(%s|%s|%s)(?:\.scope)?$|} uuid_source container_source task_source)
+    |> Re.compile in
+  fun () ->
+  match open_in proc_self_cgroup  with
+  | ic ->
+     Fun.protect ~finally:(fun () -> close_in_noerr ic) (fun () ->
+         match input_line ic with
+         | exception End_of_file -> None
+         | line ->
+            match Re.exec_opt re line with
+            | None -> None
+            | Some groups ->
+               let c_id = Re.Group.get groups 1 in
+               if !debug_ then Printf.eprintf "read container id %S from %s\n%!" c_id proc_self_cgroup;
+               Some c_id
+       )
+  | exception _ -> None
+
+
 module Config = struct
   type t = {
     debug: bool;
@@ -112,7 +146,13 @@ module Curl() : CURL = struct
     if !debug_ then Curl.set_verbose curl true;
     Curl.set_url curl (!url ^ path);
     Curl.set_httppost curl [];
-    Curl.set_httpheader curl ["Content-Type: application/x-protobuf"];
+    let header = ["Content-Type: application/x-protobuf"] in
+    let header =
+      match !container_id_ with
+      | None -> header
+      | Some cid -> Printf.sprintf "Datadog-Container-ID: %s" cid :: header
+    in
+    Curl.set_httpheader curl header;
     (* write body *)
     Curl.set_post curl true;
     Curl.set_postfieldsize curl (String.length bod);
@@ -528,6 +568,7 @@ end
 
 let setup_ ~(config:Config.t) () =
   debug_ := config.debug;
+  container_id_ := read_container_id_ ();
   let module B = Backend(struct let config=config end)() in
   Opentelemetry.Collector.backend := Some (module B);
   B.cleanup