Herb crashes puma (/src/hb-buffer.cc:1998)

[drale2k]

I was debugging a nasty issue for quite some time today. Out of nowhere, after multiple successful deploys throughout the day, suddenly any request to a controller using Action View template rendering starts to crash puma:

puma 7.1.0 (tcp://0.0.0.0:3000) [rails]: ../src/hb-buffer.cc:1998: void hb_buffer_append(hb_buffer_t*, const hb_buffer_t*, unsigned int, unsigned int): Assertion `!buffer->have_output && !source->have_output' failed.

This affected only prod, not development. It turned out removing the reactionview gem fixes the issue. I am on Rails 8.1 and deploying to Ubuntu with Kamal. Using Herb v0.8 if that matters.

I have no clue why this happened suddenly. From what i see the latest release is 2 weeks old and i have deployed plenty of times during the past weeks.

[drale2k]

After thinking about it, my only guess is that some linux library reactionview relies on, was updated and is incompatible?

[drjayvee]

I've got the same issue, but in development.

It only happens if config.intercept_erb = true is enabled. Update: this also happens when I rename a template to .html.herb. Meaning Puma always crashes when the Herb Engine is used.

$ ./bin/dev
13:21:55 web.1  | started with pid 797188
13:21:55 css.1  | started with pid 797189
13:21:56 web.1  | => Booting Puma
13:21:56 web.1  | => Rails 8.1.1 application starting in development
13:21:56 web.1  | => Run `bin/rails server --help` for more startup options
13:21:56 web.1  | Puma starting in single mode...
13:21:56 web.1  | * Puma version: 7.1.0 ("Neon Witch")
13:21:56 web.1  | * Ruby version: ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [x86_64-linux]
13:21:56 web.1  | *  Min threads: 3
13:21:56 web.1  | *  Max threads: 3
13:21:56 web.1  | *  Environment: development
13:21:56 web.1  | *          PID: 797188
13:21:56 web.1  | * Listening on http://127.0.0.1:3000
13:21:56 web.1  | * Listening on http://[::1]:3000
13:21:56 web.1  | Use Ctrl-C to stop
13:21:56 css.1  | ≈ tailwindcss v4.1.16
13:21:56 css.1  |
13:21:56 css.1  | sh: 1: watchman: not found
13:21:56 css.1  | Done in 40ms
13:22:02 web.1  | Started GET "/hotwire-spark" for 127.0.0.1 at 2025-11-14 13:22:02 +0100
13:22:02 css.1  | Done in 1ms
13:22:02 web.1  |   ActiveRecord::SchemaMigration Load (0.1ms)  SELECT "schema_migrations"."version" FROM "schema_migrations" ORDER BY "schema_migrations"."version" ASC /*application='Hotboard'*/
13:22:02 web.1  | Started GET "/hotwire-spark" [WebSocket] for 127.0.0.1 at 2025-11-14 13:22:02 +0100
13:22:02 web.1  | Successfully upgraded to WebSocket (REQUEST_METHOD: GET, HTTP_CONNECTION: keep-alive, Upgrade, HTTP_UPGRADE: websocket)
13:22:02 web.1  | Hotwire::Spark::Channel is transmitting the subscription confirmation
13:22:02 web.1  | Hotwire::Spark::Channel is streaming from hotwire_spark
13:22:02 css.1  | Done in 3ms
13:22:11 web.1  | Started GET "/" for 127.0.0.1 at 2025-11-14 13:22:11 +0100
13:22:11 css.1  | Done in 3ms
13:22:11 web.1  | Processing by TicketsController#index as HTML
13:22:11 web.1  |   Rendering layout layouts/application.html.erb
13:22:11 web.1  |   Rendering tickets/index.html.erb within layouts/application
13:22:11 web.1  | puma 7.1.0 (tcp://localhost:3000) [Hotboard]: ../src/hb-buffer.cc:1998: void hb_buffer_append(hb_buffer_t*, const hb_buffer_t*, unsigned int, unsigned int): Assertion `!buffer->have_output && !source->have_output' failed.
13:22:11 css.1  | Done in 2ms
13:22:12 web.1  | terminated by SIGIOT
13:22:12 system | sending SIGTERM to all processes
13:22:12 css.1  | bin/rails aborted!
13:22:12 css.1  | SignalException: SIGTERM (SignalException)
13:22:12 css.1  |
13:22:12 css.1  | Tasks: TOP => tailwindcss:watch
13:22:12 css.1  | (See full trace by running task with --trace)
13:22:12 css.1  | exited with code 1

You can (try to) reproduce in this branch on a very simple Rails project.

Edit: I'm on Ubuntu 25.10, in case that matters.

[cmer]

Having a similar issue on macOS 26:

Assertion failed: (buffer->have_positions == source->have_positions || !buffer->len || !source->len), function hb_buffer_append, file hb-buffer.cc, line 2036.

[marcoroth]

They should be fixed with Herb v0.8.3 via marcoroth/herb#906.

You should be able to update herb using bundle update herb.