fs: add set_max_buf_size to tokio::fs::File

[mox692]

Motivation

#5397 enhanced performance by updating the tokio::fs::File's MAX_BUF from 16KiB to 2MiB for operations involving AsyncWrite / AsyncRead. Related to this, it would also be useful to provide an API that allows to customize this default.

Solution

This introduce a set_max_buf_size to tokio::fs::File to customize the maximum buffer size.
If this is not set, the current default of 2MiB will be applied. In the current implementation, this max_buf_size will be applicable for both read and write operations.

[mox692]

Another FB from @Darksonn: https://discord.com/channels/500028886025895936/500336346770964480/1219623744901746788

It would be great to be able to provide this in a sync function, but as far as I've seen, it seems to be difficult to implement in a simple way ...

[carllerche]

I think it should be doable as a sync function if you set a value somewhere (max buf size) that is checked when the buffer is written to.

[mox692]

@carllerche
If such an implementation is still acceptable, I think we can do it.
I'll give it a try tomorrow.

[mox692]

Okay, I think this is ready for another look. The implementation is still simpler than I expected.

Also, I noticed that the inner Buf type is also used for io::Stdin, io::Stdout, but I've left these as they are, using 2MiB. (If we wanted to provide the same means of customization for Stdin, Stdout, we could easily do so.)