Error tracking source code context in stack frames by cat-ph · Pull Request #87 · PostHog/posthog-elixir

cat-ph · 2026-03-18T15:17:39Z

Adds source code context to error tracking stack frames, allowing us to display lines of code around the error location.

Changes

Source context: Stack frames now include pre_context, context_line, and post_context when enable_source_code_context: true is configured
Source loading: Files are loaded from disk in dev or from a packaged map (mix posthog.package_source_code) in releases
Exception value fields now drop the redundant ** (Type) prefix - Exception.message/1 for exceptions, inspect/1 for throws, Exception.format_exit/1 for exits
Tests: Unit tests for PostHog.Sources (context extraction, encode/decode roundtrip, file loading)

…r format change

github-actions · 2026-03-18T15:20:46Z

posthog-elixir Compliance Report

Date: 2026-03-19 12:20:20 UTC
Duration: 105872ms

✅ All Tests Passed!

29/29 tests passed

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	610ms
Format Validation.Event Has Uuid	✅	609ms
Format Validation.Event Has Lib Properties	✅	609ms
Format Validation.Distinct Id Is String	✅	610ms
Format Validation.Token Is Present	✅	610ms
Format Validation.Custom Properties Preserved	✅	610ms
Format Validation.Event Has Timestamp	✅	610ms
Retry Behavior.Retries On 503	✅	5615ms
Retry Behavior.Does Not Retry On 400	✅	2612ms
Retry Behavior.Does Not Retry On 401	✅	2612ms
Retry Behavior.Respects Retry After Header	✅	5616ms
Retry Behavior.Implements Backoff	✅	15625ms
Retry Behavior.Retries On 500	✅	5616ms
Retry Behavior.Retries On 502	✅	5616ms
Retry Behavior.Retries On 504	✅	5611ms
Retry Behavior.Max Retries Respected	✅	15625ms
Deduplication.Generates Unique Uuids	✅	620ms
Deduplication.Preserves Uuid On Retry	✅	5611ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10621ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5617ms
Deduplication.No Duplicate Events In Batch	✅	616ms
Deduplication.Different Events Have Different Uuids	✅	612ms
Compression.Sends Gzip When Enabled	✅	610ms
Batch Format.Uses Proper Batch Structure	✅	610ms
Batch Format.Flush With No Events Sends Nothing	✅	607ms
Batch Format.Multiple Events Batched Together	✅	615ms
Error Handling.Does Not Retry On 403	✅	2612ms
Error Handling.Does Not Retry On 413	✅	2612ms
Error Handling.Retries On 408	✅	5616ms

rafaeelaudibert

I'm not familiar with maps but this seems to be working well! Make sure we've done extensive testing before merging this :)

@martosaur I know you're using Error Tracking in production, wondering if you're missing what's being implemented here or does this feel like extra work for no reason?

rafaeelaudibert · 2026-03-18T19:38:36Z

lib/posthog/handler.ex

+  defp value(%{meta: %{crash_reason: {reason, _stacktrace}}}) when is_exception(reason),
+    do: %{value: Exception.message(reason)}

-  defp value(%{meta: %{crash_reason: {{:nocatch, throw}, stacktrace}}}),
-    do: %{value: Exception.format_banner(:throw, throw, stacktrace)}
+  defp value(%{meta: %{crash_reason: {{:nocatch, throw}, _stacktrace}}}),
+    do: %{value: inspect(throw)}

-  defp value(%{meta: %{crash_reason: {reason, stacktrace}}}),
-    do: %{value: Exception.format_banner(:exit, reason, stacktrace)}
+  defp value(%{meta: %{crash_reason: {reason, _stacktrace}}}),
+    do: %{value: Exception.format_exit(reason)}


This will change the message we have in Prod which will create new errors, right? Or am I misunderstanding this @cat-ph ?

we don't use the values for the fingerprint linking to issues, so it keeps them and doesn't create new ones! (just verified this too below, 1. before, 2. after; thanks for checking! 😆)

I think we could consider actually maybe separating the type like ** (exit) as the type and the rest as the value, but that would create new issues, and is right now pretty standard elixir format (I think) so that's why I didn't change that

rafaeelaudibert · 2026-03-18T19:39:20Z

lib/posthog/handler.ex

+    if config.enable_source_code_context do
+      case PostHog.Sources.get_source_map_for_file(filename) do
+        nil ->
+          frame
+
+        source_map_for_file ->
+          {pre_context, context_line, post_context} =
+            PostHog.Sources.get_source_context(
+              source_map_for_file,
+              lineno,
+              config.context_lines
+            )
+
+          frame
+          |> Map.put(:context_line, context_line)
+          |> Map.put(:pre_context, pre_context)
+          |> Map.put(:post_context, post_context)
+      end
+    else
+      frame
+    end


Can probably do this with a with block to make it looks slightly cleaner?

yep, slightly better, thanks!

rafaeelaudibert · 2026-03-18T19:39:45Z

lib/posthog/sources.ex

@@ -0,0 +1,198 @@
+defmodule PostHog.Sources do


Sources is a very generic name. How about PostHog.ErrorTracking.Sources?

good point, I was thinking we might use them later for other stuff, but not sure what exactly for so YAGNI, updated!

rafaeelaudibert · 2026-03-18T19:40:09Z

lib/posthog/sources.ex

+  # Part of this approach is inspired by getsentry/sentry-elixir by Software, Inc. dba Sentry
+  # Licensed under the MIT License
+  # - sentry-elixir/lib/sentry/sources.ex
+  # - sentry-elixir/lib/mix/tasks/sentry.package_source_code.ex
+
+  # 💖 open source (under MIT License)


I assume this should be a trailer comment at the top of the file instead?

Also, confirm the base LICENSE files mention this or else we're not compliant.

updated, ty! trailer comment in both + LICENSE mention

martosaur · 2026-03-18T19:59:32Z

I know you're using Error Tracking in production, wondering if you're missing what's being implemented here or does this feel like extra work for no reason?

code source is a nice QoL addition for sure. I'm just not very familiar with how it works exactly in any other lib, so can't really tell good from bad without doing some research

…ate LICENSE - Move PostHog.Sources → PostHog.ErrorTracking.Sources - Add Sentry-elixir attribution headers in sources.ex and mix task - Update LICENSE to note inspired portions and credit Sentry - Refactor handler.maybe_add_source_context to use with block - Fix nested module alias (credo strict) - Move test file to match new module path

cat-ph · 2026-03-19T11:22:19Z

thanks @martosaur and @rafaeelaudibert - I think there are some potential approaches, this seems like probably the most straightforward, also has its cons (since technically source code will live with the release files, and hot code upgrades could get messy / cause drift).

Others we were thinking could be 1. using something like :beam_lib.chunks(module, [:debug_info]) but we'd get Erlang format code not the original one + some may strip those; or 2. trying to do a compiler tracer and store the AST (but similarly, might not look like the original and more complicated to get right)

We could also in the future support uploading these custom maps with a predefined format & version and resolving them on our side instead

hpouillot and others added 6 commits March 17, 2026 15:34

feat(error-tracking): add source code context and fix exception values

8cf1088

fix(error-tracking): fix pre_context line order to be most-recent first

3ce42fe

test(error-tracking): add sources tests and fix value assertions afte…

891af40

…r format change

refactor(error-tracking): move Sources child into PostHog.Supervisor

3cbe112

chore: add references to sentry-elixir

44d9fe9

chore: changeset

4addaed

cat-ph requested review from a team, ablaszkiewicz and hpouillot March 18, 2026 15:17

cat-ph requested a review from rafaeelaudibert as a code owner March 18, 2026 15:17

rafaeelaudibert reviewed Mar 18, 2026

View reviewed changes

cat-ph requested a review from rafaeelaudibert March 19, 2026 11:22

chore: revert handler bare process change for non-pre_19

ae980b6

Conversation

cat-ph commented Mar 18, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

github-actions bot commented Mar 18, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

posthog-elixir Compliance Report

✅ All Tests Passed!

Capture Tests

Uh oh!

rafaeelaudibert left a comment

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

martosaur commented Mar 18, 2026

Uh oh!

cat-ph commented Mar 19, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

cat-ph commented Mar 18, 2026 •

edited

Loading

github-actions bot commented Mar 18, 2026 •

edited

Loading