[Feature Request] Event handlers for `FileSink` and `RotatingFileSink`

[SpriteOvO]

Implement event handlers (reference from C++ spdlog) for FileSink and RotatingFileSink by adding new methods on the builders of these file sinks.

The names of these methods are tentatively designated as before_file_open, after_file_open, before_file_close and after_file_close.

Unsolved questions:

• Should we pass a &mut PathBuf to before_file_open handler to allow users to change the file path?

[Lancern]

Should we pass a &mut PathBuf to before_file_open handler to allow users to change the file path?

I don't think this is necessary since you can change the path via FileSinkBuilder::path. A &Path shoud suffice.

[SpriteOvO]

I don't think this is necessary since you can change the path via FileSinkBuilder::path. A &Path shoud suffice.

But for RotatingFileSink, a &mut PathBuf allows the user to customize the rotated file name before creating the file, rather than renaming it after the file is closed. (context #49)

I'm thinking if there are any cons to doing it this way.

[Lancern]

The reason I feel a bit strange is that if you give your user a &mut PathBuf in the hook, it strongly expresses the intent that the hook exists solely for a very specific purpose: changing the filename. To implement this very specific requirement, a better API might be:

pub trait RotateFileOpener {
    fn open(&self /*, other params */) -> PathBuf;
}

impl<ArgBP, ArgRP> RotateFileSinkBuilder<ArgBP, ArgRP> {
    pub fn opener(self, opener: Box<dyn RotateFileOpener>) -> Self {
        todo!()
    }
}

The hook can be kept (and it should be kept). It's purpose is to give the user a change to do monitoring and bookkeeping jobs. But I still don't think changing the filename from the hook is a good idea. We have other approaches to express the indent more clearly.

[SpriteOvO]

The reason I feel a bit strange is that if you give your user a &mut PathBuf in the hook, it strongly expresses the intent that the hook exists solely for a very specific purpose: changing the filename.

Makes sense. So let's keep &Path for before_file_open of RotatingFileSink.

The new trait RotateFileOpener is a bit verbose if it's just for custom paths, and it should be designed to be a bit more powerful if it goes this way. Another option would be to just add something like the filename_calculator in C++ spdlog. But for now let's table this idea, it seems a bit out of the scope of this issue. If anyone is interested in this in the future we will discuss it further in a new issue. For users who currently need to customize file names, they can workaround renaming files in after_file_close.

So the current signatures of the event handlers will be
(reference from C++ spdlog)

before_file_open  : impl Fn(path: &Path)
after_file_open   : impl Fn(path: &Path, file: &mut File)
before_file_close : impl Fn(path: &Path, file: &mut File)
after_file_close  : impl Fn(path: &Path)

Is it right?

[Lancern]

So the current signatures of the event handlers will be
[...]
Is it right?

What's the purpose of the &mut File parameter? This would allow the user to replace the File object completely through the mutable reference.