Vectored IO for write_all_buf

[conradludgate]

Is your feature request related to a problem? Please describe.

The AsyncWriteExt trait provides the write_all_buf function to write the entire contents of a Buf type to the underlying writer. However, if the buf is fragmented (eg a VecDeque or Chain), then it can have potentially bad performance with the current implementation, writing many small buffers at a time. This is because the current impl only uses chunk() to get the first chunk slice only.

tokio/tokio/src/io/util/write_all_buf.rs

Line 47 in a024071

let n = ready!(Pin::new(&mut *me.writer).poll_write(cx, me.buf.chunk())?);

Describe the solution you'd like

If the underlying writer is_write_vectored(), write_all_buf could make use of Buf::chunks_vectored to fill an IO slice to use with poll_write_vectored.

The vectored io-slice can use a fixed size array, eg 4 or 8. When advancing the io-slice, should a chunk be removed, it could call chunks_vectored again to fill the io-slice, considering that chunks_vectored should be a fairly cheap operation.

Describe alternatives you've considered

Similar implementation discussions have occurred in #3679.
Performance testing is needed, and real-world use cases of write_all_buf should be examined

[Darksonn]

This seems reasonable to me.