memory leak with `pattern not matched` error

[Watson1978]

Describe the bug

When many logs do not match the pattern, memory usage increases significantly.

To Reproduce

• conf

<source>
  @type tail
  tag test
  path "#{File.expand_path '~/tmp/fluentd/access-*.log'}"
  refresh_interval 5s
  read_from_head true
  <parse>
    @type regexp
    expression /^(?<log>a+)+$/
    timeout 0.1s
  </parse>
</source>

<match **>
  @type stdout
</match>

• script to generate log data

# frozen_string_literal: true
require "fileutils"

FILE_MAX_SIZE = 100 * 1024 * 1024
FILE_PATH = File.expand_path "~/tmp/fluentd/access-1.log"

dir = File.dirname(FILE_PATH)
FileUtils.mkdir_p(dir)

File.open(FILE_PATH, "w") do |f|

  data = "#{'a' * 1024}X"
  loop do
    f.puts data
    break if File.size(FILE_PATH) > FILE_MAX_SIZE
  end
end

When try to reproduce, fluentd will display the warning log as following and increase memory usage during handling the message.

2025-06-04 18:18:37 +0900 [info]: following tail of /home/watson/tmp/fluentd/access-1.log
2025-06-04 18:18:37 +0900 [warn]: pattern not matched: "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaX"
2025-06-04 18:18:37 +0900 [warn]: pattern not matched: "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaX"
...

Memory usage increases linearly while handling the relevant logs.

Expected behavior

Less memory usaage

Your Environment

- Fluentd version:
- Package version:
- Operating system:
- Kernel version:

Your Configuration

<source>
  @type tail
  tag test
  path "#{File.expand_path '~/tmp/fluentd/access-*.log'}"
  refresh_interval 5s
  read_from_head true
  <parse>
    @type regexp
    expression /^(?<log>a+)+$/
    timeout 0.1s
  </parse>
</source>

<match **>
  @type stdout
</match>

Your Error Log

see above

Additional context

No response

[Watson1978]

workaround 1

As workaround,
looks like this issue can be avoided by specifying tags strictly with match, like

<match test>
  @type stdout
</match>

instead of <match **>.

workaround 2

If we have used label properly, looks like we can avoid this issue.

<source>
  @type tail
  tag test
  @label @sample ## add
  path "#{File.expand_path '~/tmp/fluentd/access-*.log'}"
  refresh_interval 5s
  read_from_head true
  <parse>
    @type regexp
    expression /^(?<log>a+)+$/
    timeout 0.1s
  </parse>
</source>

<label @sample> ## add
  <match **>
    @type stdout
  </match>
</label>

[daipom]

I see...

Looks like FluentLogEventRouter has some memory issues.

[Watson1978]

If we implement #1504, it might improve this issue.

[github-actions]

This issue has been automatically marked as stale because it has been open 30 days with no activity. Remove stale label or comment or this issue will be closed in 7 days

[github-actions]

This issue has been automatically marked as stale because it has been open 30 days with no activity. Remove stale label or comment or this issue will be closed in 7 days