Normalize anonymous function stack frames

Author: noozo

lib/posthog/handler.ex

@@ -215,7 +215,7 @@ defmodule PostHog.Handler do
         %{
           platform: "custom",
           lang: "elixir",
-          function: Exception.format_mfa(module, function, arity_or_args),
+          function: format_function(module, function, arity_or_args),
           filename: filename,
           lineno: lineno,
           module: inspect(module),
@@ -231,6 +231,16 @@ defmodule PostHog.Handler do
     }
   end
 
+  defp format_function(module, function, arity_or_args) do
+    module
+    |> Exception.format_mfa(function, arity_or_args)
+    |> normalize_anonymous_function()
+  end
+
+  defp normalize_anonymous_function(function) do
+    String.replace(function, ~r/anonymous fn\(\d+\) in /, "anonymous fn in ")
+  end
+
   defp maybe_add_source_context(frame, filename, lineno, config) do
     with true <- config.enable_source_code_context,
          %{} = source_map <- Sources.get_source_map_for_file(config.supervisor_name, filename) do

test/posthog/handler_test.exs

@@ -119,6 +119,31 @@ defmodule PostHog.HandlerTest do
     end
   end
 
+  test "normalizes volatile anonymous function indexes in stack frames", %{
+    handler_ref: ref,
+    config: %{supervisor_name: supervisor_name}
+  } do
+    stacktrace = [
+      {__MODULE__, :"anonymous fn(250) in PostHog.HandlerTest.example/1", 1,
+       [file: ~c"test/posthog/handler_test.exs", line: 1]}
+    ]
+
+    Logger.error("Hello World", crash_reason: {%RuntimeError{message: "boom"}, stacktrace})
+    LoggerHandlerKit.Assert.assert_logged(ref)
+
+    assert [event] = all_captured(supervisor_name)
+    assert %{properties: %{"$exception_list": exception_list}} = event
+
+    assert exception_list
+           |> Enum.flat_map(fn exception ->
+             get_in(exception, [:stacktrace, :frames]) || []
+           end)
+           |> Enum.any?(fn frame ->
+             frame.function ==
+               ~s(PostHog.HandlerTest."anonymous fn in PostHog.HandlerTest.example/1"/1)
+           end)
+  end
+
   test "string message", %{handler_ref: ref, config: %{supervisor_name: supervisor_name}} do
     LoggerHandlerKit.Act.string_message()
     LoggerHandlerKit.Assert.assert_logged(ref)