Add sync to file_helper

[espkk]

Since file_helper already works well for managing file sinks and generally can be reused, it seems reasonable to me to implement fsync there which is required in some cases.
This adds sync() method to file_helper. It doesn't affect existing sinks and doesn't touch public interfaces.

[gabime]

Thanks. However since it is not used by any of spdlog sinks, I prefer not to add unused code,

[gabime]

Thanks. However since it is not used by any of spdlog sinks, I prefer not to add unused code,

[gabime]

@espkk Reopened due to popular demand for fsync. Could you please fix the code so it passes the tests?

[espkk]

Should be good now. I could add a sink for it but the use-cases can be different. I need it for termination, but someone might need it after each flush

[gabime]

Thanks @espkk . I am wondering myself whether to

1. add fsync() to the logger api.
2. or add fsync()just to file sinks.
3. or perhaps provide auto sync by using logger->set_auto_fsync(true/false)

[espkk]

Thanks @espkk . I am wondering myself whether to

1. add fsync() to the logger api.
2. or add fsync()just to file sinks.
3. or perhaps provide auto sync by using logger->set_auto_fsync(true/false)

IMO, fsync on file sink should be flexible enough, while I would still prefer implementing custom sink if the desired behavior is sync-after-write

[cpp977]

For my use case (#2605), the third option would be the most convenient.
logger->set_auto_sync() would then lead to a call to fsync after each flush. One could then just use the existing spdlog::flush_every() to also have a regular fsync call.