From 9aaffafe826d390169be60dbd95f4769c28a3420 Mon Sep 17 00:00:00 2001
From: Victor Shyba <victor.shyba@gmail.com>
Date: Fri, 10 Apr 2026 15:47:16 -0300
Subject: [PATCH] fix: stack trace formatting and proper ensure_started call

---
 src/hb_format.erl         | 38 ++++++++++++++++++++++++++++++++++
 src/hb_system_monitor.erl | 43 +++------------------------------------
 2 files changed, 41 insertions(+), 40 deletions(-)

diff --git a/src/hb_format.erl b/src/hb_format.erl
index d9ea1ca6d..a256b2fc9 100644
--- a/src/hb_format.erl
+++ b/src/hb_format.erl
@@ -21,6 +21,7 @@
 -export([escape_format/1, short_id/1, trace_to_list/1]).
 -export([get_trace/1, print_trace/4, trace_macro_helper/5, print_trace_short/4]).
 -export([process_from_trace/1]).
+-export([format_mfa/1, stack_location/1]).
 -include("include/hb.hrl").
 -include_lib("eunit/include/eunit.hrl").
 
@@ -658,6 +659,43 @@ process_from_trace([TraceElement | Rest], Spawner) ->
             >>
         end.
 
+%% @doc Format an MFA tuple or stack frame as `mod:func/arity`.
+format_mfa({Mod, Func, Arity, _}) when is_integer(Arity) ->
+    format_mfa({Mod, Func, Arity});
+format_mfa({Mod, Func, Args, _}) when is_list(Args) ->
+    format_mfa({Mod, Func, length(Args)});
+format_mfa({Mod, Func, Arity}) when is_integer(Arity) ->
+    <<
+        (atom_to_binary(Mod))/binary, ":",
+        (atom_to_binary(Func))/binary, "/",
+        (integer_to_binary(Arity))/binary
+    >>;
+format_mfa(_) ->
+    <<"unknown">>.
+
+%% @doc Build a compact location label from a stack: `mid/current`.
+%% Current is the innermost frame (head of stacktrace), mid is
+%% roughly 1/3 from the bottom — a frame that gives codebase
+%% context without being the generic entry or the leaf.
+stack_location([]) ->
+    <<"unknown">>;
+stack_location([Only]) ->
+    format_mfa(Only);
+stack_location(Stack) ->
+    Current = hd(Stack),
+    Len = length(Stack),
+    MidIdx = max(1, Len - (Len div 3)),
+    Mid = lists:nth(MidIdx, Stack),
+    case Mid =:= Current of
+        true ->
+            format_mfa(Current);
+        false ->
+            <<
+                (format_mfa(Mid))/binary, "/",
+                (format_mfa(Current))/binary
+            >>
+    end.
+
 trace_element_is_glue({proc_lib, init_p_do_apply, _, _}) ->
     true;
 trace_element_is_glue({hb_pmap, F, _, _}) ->
diff --git a/src/hb_system_monitor.erl b/src/hb_system_monitor.erl
index baadb1182..b3eaa40fd 100644
--- a/src/hb_system_monitor.erl
+++ b/src/hb_system_monitor.erl
@@ -154,14 +154,8 @@ init_prometheus() ->
 %% @doc Format a schedule location for use as a prometheus label.
 format_location(undefined) ->
     <<"undefined">>;
-format_location({Mod, Func, Arity}) ->
-    <<
-        (atom_to_binary(Mod))/binary, ":",
-        (atom_to_binary(Func))/binary, "/",
-        (integer_to_binary(Arity))/binary
-    >>;
-format_location(_) ->
-    <<"unknown">>.
+format_location(MFA) ->
+    hb_format:format_mfa(MFA).
 
 %% @doc If the timeout exceeds the deep inspection threshold and
 %% enough time has passed since the last inspection, grab detailed
@@ -215,7 +209,7 @@ deep_inspect(Pid, ScheduleInfo) ->
     ?event(system_monitor, {deep_inspect, Pid, ScheduleInfo, ProcInfo}),
     Stack = proplists:get_value(current_stacktrace, ProcInfo, []),
     Entry = stack_entry(Stack),
-    Location = stack_location(Stack),
+    Location = hb_format:stack_location(Stack),
     hb_prometheus:inc(counter, system_monitor_deep_inspect_total,
         [Entry, Location]).
 
@@ -225,37 +219,6 @@ stack_entry([]) ->
 stack_entry(Stack) ->
     hb_format:process_from_trace(Stack).
 
-%% @doc Build a compact location label from the stack: `mid/current`.
-%% Current is the innermost frame (head of stacktrace), mid is
-%% roughly 1/3 from the bottom — a frame that gives codebase context
-%% without being the generic entry or the leaf.
-stack_location([]) ->
-    <<"unknown">>;
-stack_location([Only]) ->
-    format_frame(Only);
-stack_location(Stack) ->
-    Current = hd(Stack),
-    Len = length(Stack),
-    MidIdx = max(1, Len - (Len div 3)),
-    Mid = lists:nth(MidIdx, Stack),
-    case Mid =:= Current of
-        true ->
-            format_frame(Current);
-        false ->
-            <<
-                (format_frame(Mid))/binary, "/",
-                (format_frame(Current))/binary
-            >>
-    end.
-
-%% @doc Format a single stack frame as `mod:func/arity`.
-format_frame({Mod, Func, Arity, _}) ->
-    format_location({Mod, Func, Arity});
-format_frame({Mod, Func, Arity}) ->
-    format_location({Mod, Func, Arity});
-format_frame(_) ->
-    <<"unknown">>.
-
 %% @doc Safely retrieve process info. The process may have died
 %% between the monitor event and our inspection.
 safe_process_info(Pid, Items) ->