fix(metrics): output correct process metrics

Before this commit we would output an "async" start event when a process
awould start nd then a "complete" event when it would be finished.

The "async" start event is unnecessary and this commit removes it. All
the information recorded in the "async" start event is therefore moved
to the complete event.

The new output is now properly displayed by the various visualization
tools (perfetto, chrome)

Signed-off-by: Rudi Grinberg <me@rgrinberg.com>

<!-- ps-id: d5f448b8-a896-4612-bc4e-e8d16c41fc51 -->

CHANGES.md

@@ -1,6 +1,10 @@
 Unreleased
 ----------
 
+- Fix `--trace-file` output. Dune now emits a single *complete* event for every
+  executed process. Unterminated *async* events are no longer written. (#6892,
+  @rgrinberg)
+
 - Fix preprocessing with `staged_pps` (#6748, fixes #6644, @rgrinberg)
 
 - Make `dune describe workspace` return consistent dependencies for

src/dune_engine/process.ml

@@ -577,29 +577,22 @@ end = struct
       fail ~loc ~annots paragraphs
 end
 
-let report_process_start stats ~metadata ~id ~pid ~prog ~args ~now =
+let report_process_finished stats ~metadata ~prog ~pid ~args ~started_at
+    (times : Proc.Times.t) =
   let common =
     let name =
       match metadata.name with
       | Some n -> n
       | None -> Filename.basename prog
     in
-    let ts = Timestamp.of_float_seconds now in
+    let ts = Timestamp.of_float_seconds started_at in
     Event.common_fields ~cat:("process" :: metadata.categories) ~name ~ts ()
   in
   let args =
     [ ("process_args", `List (List.map args ~f:(fun arg -> `String arg)))
     ; ("pid", `Int (Pid.to_int pid))
     ]
   in
-  let event =
-    Event.async (Chrome_trace.Id.create (`Int id)) ~args Start common
-  in
-  Dune_stats.emit stats event;
-  (common, args)
-
-let report_process_end stats (common, args) ~now (times : Proc.Times.t) =
-  let common = Event.set_ts common (Timestamp.of_float_seconds now) in
   let dur = Event.Timestamp.of_float_seconds times.elapsed_time in
   let event = Event.complete ~args ~dur common in
   Dune_stats.emit stats event
@@ -692,7 +685,7 @@ let run_internal ?dir ?(stdout_to = Io.stdout) ?(stderr_to = Io.stderr)
           (stdout, stderr)
         | _ -> ((`No_capture, stdout_to), (`No_capture, stderr_to))
       in
-      let event_common, started_at, pid =
+      let started_at, pid =
         (* Output.fd might create the file with Unix.openfile. We need to make
            sure to call it before doing the chdir as the path might be
            relative. *)
@@ -705,26 +698,21 @@ let run_internal ?dir ?(stdout_to = Io.stdout) ?(stderr_to = Io.stderr)
           | false -> env
         in
         let env = Env.to_unix env |> Spawn.Env.of_list in
-        let started_at, pid =
+        let started_at =
           (* jeremiedimino: I think we should do this just before the [execve]
              in the stub for [Spawn.spawn] to be as precise as possible *)
-          let now = Unix.gettimeofday () in
-          ( now
-          , Spawn.spawn () ~prog:prog_str ~argv ~env ~stdout ~stderr ~stdin
-              ~setpgid:Spawn.Pgid.new_process_group
-              ~cwd:
-                (match dir with
-                | None -> Inherit
-                | Some dir -> Path (Path.to_string dir))
-            |> Pid.of_int )
+          Unix.gettimeofday ()
         in
-        let event_common =
-          Option.map config.stats ~f:(fun stats ->
-              ( stats
-              , report_process_start stats ~metadata ~id ~pid ~prog:prog_str
-                  ~args ~now:started_at ))
+        let pid =
+          Spawn.spawn () ~prog:prog_str ~argv ~env ~stdout ~stderr ~stdin
+            ~setpgid:Spawn.Pgid.new_process_group
+            ~cwd:
+              (match dir with
+              | None -> Inherit
+              | Some dir -> Path (Path.to_string dir))
+          |> Pid.of_int
         in
-        (event_common, started_at, pid)
+        (started_at, pid)
       in
       Io.release stdout_to;
       Io.release stderr_to;
@@ -736,8 +724,9 @@ let run_internal ?dir ?(stdout_to = Io.stdout) ?(stderr_to = Io.stderr)
         ; resource_usage = process_info.resource_usage
         }
       in
-      Option.iter event_common ~f:(fun (stats, common) ->
-          report_process_end stats common ~now:process_info.end_time times);
+      Option.iter config.stats ~f:(fun stats ->
+          report_process_finished stats ~metadata ~prog:prog_str ~pid ~args
+            ~started_at times);
       Option.iter response_file ~f:Path.unlink;
       let actual_stdout =
         match stdout_capture with