It turns out that interval parameter is mandatory

[b-a-t]

I was trying to use a sample configuration from the README on a newer fluentd v5 aka fluentd-1.16.2 with the config:

  <worker 0>
    <source>
      @type utmpx
      tag "wtmp"
      pos_file "/var/lib/fluent/wtmp.pos"
      path "/var/log/wtmp"
    </source>
  </worker>

and got a stack trace in the log:

2023-10-29 01:21:07 +0000 [info]: #0 adding source type="utmpx"
2023-10-29 01:21:17 +0000 [warn]: #0 emit transaction failed: error_class=ArgumentError error="time must be a Fluent::EventTime (or Integer): Time" location="/opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin/output.rb:914:in `metadata'" tag="wtmp"
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin/output.rb:914:in `metadata'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin/output.rb:1042:in `block in handle_stream_with_custom_format'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/event.rb:196:in `block in each'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/event.rb:195:in `each'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/event.rb:195:in `each'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin/output.rb:1041:in `handle_stream_with_custom_format'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin/output.rb:969:in `execute_chunking'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin/output.rb:897:in `emit_buffered'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/event_router.rb:115:in `emit_stream'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluent-plugin-utmpx-0.5.0/lib/fluent/plugin/in_utmpx.rb:100:in `block in refresh_watchers'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluent-plugin-utmpx-0.5.0/lib/fluent/plugin/in_utmpx.rb:93:in `open'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluent-plugin-utmpx-0.5.0/lib/fluent/plugin/in_utmpx.rb:93:in `refresh_watchers'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin_helper/timer.rb:80:in `on_timer'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/cool.io-1.7.1/lib/cool.io/loop.rb:88:in `run_once'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/cool.io-1.7.1/lib/cool.io/loop.rb:88:in `run'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin_helper/event_loop.rb:93:in `block in start'
  2023-10-29 01:21:17 +0000 [warn]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin_helper/thread.rb:78:in `block in thread_create'
2023-10-29 01:21:17 +0000 [error]: #0 Unexpected error raised. Stopping the timer. title=:execute_utmpx error_class=ArgumentError error="time must be a Fluent::EventTime (or Integer): Time"
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin/output.rb:914:in `metadata'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin/output.rb:1042:in `block in handle_stream_with_custom_format'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/event.rb:196:in `block in each'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/event.rb:195:in `each'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/event.rb:195:in `each'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin/output.rb:1041:in `handle_stream_with_custom_format'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin/output.rb:969:in `execute_chunking'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin/output.rb:897:in `emit_buffered'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/event_router.rb:115:in `emit_stream'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluent-plugin-utmpx-0.5.0/lib/fluent/plugin/in_utmpx.rb:100:in `block in refresh_watchers'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluent-plugin-utmpx-0.5.0/lib/fluent/plugin/in_utmpx.rb:93:in `open'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluent-plugin-utmpx-0.5.0/lib/fluent/plugin/in_utmpx.rb:93:in `refresh_watchers'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin_helper/timer.rb:80:in `on_timer'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/cool.io-1.7.1/lib/cool.io/loop.rb:88:in `run_once'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/cool.io-1.7.1/lib/cool.io/loop.rb:88:in `run'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin_helper/event_loop.rb:93:in `block in start'
  2023-10-29 01:21:17 +0000 [error]: #0 /opt/fluent/lib/ruby/gems/3.2.0/gems/fluentd-1.16.2/lib/fluent/plugin_helper/thread.rb:78:in `block in thread_create'
2023-10-29 01:21:17 +0000 [error]: #0 Timer detached. title=:execute_utmpx

After adding option interval 10 to the source configuration and restarting the fluentd, the error has gone.

Interestingly enough, removing the option and restarting the service didn't bring the stack trace back.

It turned out that the issue is also connected with the presence of the file from the pos_file - if it already exists and contains valid information the error doesn't pop up. But if that file doesn't (yet) exist - I get the stack trace from the above.