in_tail: extract line parsing and emitting into LineFeeder - #5520

Thanks for the refactoring.
I'd like to backport this PR to v1.19.
Then future in_tail fixes can be backported without conflicts. For that, this change needs to keep the existing behavior.

Right now it breaks plugins that inherit Fluent::Plugin::TailInput and override receive_lines. IOHandler now calls @line_feeder.method(:feed_lines) directly, so the override is never called.

Could you keep TailInput#receive_lines and #flush_buffer, and make them delegate to @line_feeder? If IOHandler keeps calling method(:receive_lines), overrides in subclasses keep working.

Here are the plugins that inherit Fluent::Plugin::TailInput and override its methods.

• fluent-plugin-mysqlslowquery (overrides receive_lines)
  • https://github.com/yuku/fluent-plugin-mysqlslowquery
  • The repository is archived.
• fluent-plugin-bin (overrides convert_line_to_event and setup_watcher)
  • The source code repository is not public.
• fluent-plugin-multiline-extended (overrides parse_multilines)
  • https://github.com/bodhi-space/fluent-plugin-multiline-extended
  • The last release was in 2018.
  • https://bestgems.org/gems/fluent-plugin-multiline-extended
    • Almost no recent downloads.
• fluent-plugin-nata2 (overrides receive_lines)
  • https://github.com/studio3104/fluent-plugin-nata2
  • The last release was in 2018.
  • https://bestgems.org/gems/fluent-plugin-nata2
    • Almost no recent downloads.

There may also be private plugins that do the same....

Oops, there are more plugins that depend on in_tail than I had anticipated...
I'll consider the matter of compatibility, but implementing multi-threading while maintaining it will likely be difficult...

I've revived compatibility endpoints for third party plugins.