Add option to use formatter inside systemd sink

[Delgan]

Hi.

We are migrating from file_sink to systemd_sink and we would like to preserve the spdlog format. It's dropped (by design, I understand) during conversion because journald implements its own structured logging. That makes sense, but we're losing some important fields in the process.

Basically, I'm looking #1289 but using the alternative approach proposed.
That is, adding a boolean to constructor and re-implement #1107 for systemd_sink.

I understand that I can create my own sink to implement it this way, but I having this feature upstream is of course preferable (avoid duplication and guarantees further updates).

I can definitely open a Pull Request if you think this is an acceptable addition.

I have also just one question: sd_journal_send() being thread safe, does that mean I can safely use systemd_sink_st in a multi-threaded environment with multiple loggers? Is there any reason to prefer systemd_sink_mt?

[gabime]

I can definitely open a Pull Request if you think this is an acceptable addition.

Sure, please open a pr.

does that mean I can safely use systemd_sink_st in a multi-threaded environment with multiple loggers? Is there any reason to prefer systemd_sink_mt?

I guess so, unless you will use the future version with the formatter in which case it must be mutex protected since the formatter is not thread safe.

[Delgan]

Sure, please open a pr.

Great, thanks!
PR opened in #2324.

I guess so, unless you will use the future version with the formatter in which case it must be mutex protected since the formatter is not thread safe.

Oh, yeah, I didn't realize the formatters weren't thread-safe for optimization reasons. I was hoping to avoid different threads blocking each other, but it's certainly negligible in view of the overall performance of spdlog anyway. Thanks for the clarification, it's better to use the _mt sink.