feat(trace): filter action events in trace cat

bin/trace.ml

@@ -236,6 +236,17 @@ let cat =
         value
         & flag
         & info [ "follow"; "f" ] ~doc:(Some "follow the trace file until the exit event"))
+    and+ action_events =
+      Arg.(
+        value
+        & vflag
+            `All
+            [ ( `Exclude
+              , info [ "no-actions" ] ~doc:(Some "exclude events emitted by actions") )
+            ; ( `Only
+              , info [ "only-actions" ] ~doc:(Some "print only events emitted by actions")
+              )
+            ])
     in
     Common.No_build.set_debug_backtraces debug_backtraces;
     let mode =
@@ -246,38 +257,50 @@ let cat =
       | true, false -> `Chrome
       | false, false -> `Json
     in
+    let first_chrome_event = ref true in
     let print =
       match mode with
       | `Sexp -> fun sexp -> print_endline (Sexp.to_string sexp)
       | `Json ->
         fun sexp -> print_endline (Json.to_string (json_of_event ~chrome:false sexp))
       | `Chrome ->
-        let first = ref true in
         fun sexp ->
           let char =
-            if !first
+            if !first_chrome_event
             then (
-              let () = first := false in
+              let () = first_chrome_event := false in
               '[')
             else ','
           in
           print_char char;
           print_endline (Json.to_string (json_of_event ~chrome:true sexp))
     in
-    let print_with_flush sexp =
-      print sexp;
-      if follow then flush stdout
+    let print_if_selected sexp =
+      let selected =
+        match action_events with
+        | `All -> true
+        | (`Exclude | `Only) as action_events ->
+          let _, _, _, _, digest = base_of_sexp sexp in
+          let is_action = Option.is_some digest in
+          (match action_events with
+           | `Exclude -> not is_action
+           | `Only -> is_action)
+      in
+      if selected
+      then (
+        print sexp;
+        if follow then flush stdout)
     in
     let trace_file =
       match trace_file with
       | Some s -> s
       | None -> Common.find_default_trace_file ()
     in
     if follow
-    then iter_sexps_follow trace_file ~f:print_with_flush
-    else iter_sexps trace_file ~f:print;
+    then iter_sexps_follow trace_file ~f:print_if_selected
+    else iter_sexps trace_file ~f:print_if_selected;
     match mode with
-    | `Chrome -> print_endline "]"
+    | `Chrome -> print_endline (if !first_chrome_event then "[]" else "]")
     | `Json | `Sexp -> ()
   in
   Cmd.v info term

doc/changes/added/16245.md

@@ -0,0 +1,2 @@
+- Add `--no-actions` and `--only-actions` to `dune trace cat` to filter
+  events collected from actions. (#16245, @rgrinberg)

test/blackbox-tests/test-cases/trace/action-traces/basic.t

@@ -20,3 +20,76 @@ Dune's actions may produce trace events
       "digest": "REDACTED"
     }
   }
+
+Action events may be selected explicitly.
+
+  $ dune trace cat --only-actions | jq_dune -s 'redactedActionTraces'
+  {
+    "cat": "bar",
+    "name": "foo",
+    "ts": 0,
+    "args": {
+      "arg": "baz",
+      "digest": "REDACTED"
+    }
+  }
+
+Chrome output applies the same selection.
+
+  $ dune trace cat --only-actions --chrome-trace | jq '
+  >   .[] | .ts = 0 | .pid = 0 | .args.digest = "REDACTED"
+  > '
+  {
+    "cat": "bar",
+    "name": "foo",
+    "ts": 0,
+    "args": {
+      "arg": "baz",
+      "digest": "REDACTED"
+    },
+    "ph": "i",
+    "pid": 0
+  }
+
+Excluding action events retains Dune's own trace events.
+
+  $ dune trace cat --no-actions | jq -s '
+  >   first
+  > | .ts = 0
+  > | .args.argv = ["REDACTED"]
+  > | .args.env = ["REDACTED"]
+  > | .args.root = "REDACTED"
+  > | .args.pid = 0
+  > | .args.initial_cwd = "REDACTED"
+  > | .args.start = 0
+  > '
+  {
+    "cat": "config",
+    "name": "init",
+    "ts": 0,
+    "args": {
+      "build_dir": "_build",
+      "argv": [
+        "REDACTED"
+      ],
+      "env": [
+        "REDACTED"
+      ],
+      "root": "REDACTED",
+      "pid": 0,
+      "initial_cwd": "REDACTED",
+      "start": 0
+    }
+  }
+
+There are no action events in the filtered output.
+
+  $ dune trace cat --no-actions | jq_dune -s 'redactedActionTraces'
+
+The selection flags are mutually exclusive.
+
+  $ dune trace cat --no-actions --only-actions
+  Usage: dune trace cat [--help] [OPTION]…
+  dune: options '--no-actions' and '--only-actions' cannot be present at the
+        same time
+  [1]