feature: io-uring zerocopy send support

[AsakuraMizu]

References

• io_uring_prep_send_zc(3)
• io_uring_prep_sendmsg_zc(3)
• io_uring::opcode::SendZc
• io_uring::opcode::SendMsgZc

Notes

1. This may also be related to Multishot opcode support #104, because these opcodes "might result in two completion queue entries", which may also require changes to the semantics of the current OpCode and related stuffs.
2. There is also zero copy rx but its support appears to be very limited.

[Berrysoft]

I think that it meets the same problem as #104 . It's easier to design the high-level APIs, but hard to write the driver.

For send_zc, I think it's necessary to wait for the second cqe before return.

[George-Miao]

We need to change how the driver work. It need to be able to take n CQE per SQE instead of only 1. This will also solve multishot.

[AsakuraMizu]

It's easier to design the high-level APIs

Well, not actually; I'm thinking of making something that returns a BufResult<usize, impl Future<Output = T>> 🤣

[Berrysoft]

I have a similar idea.

But eventually you'll find that such a safe API is not that convenient as the unsafe one. If you use SendZc in C, you can carefully deal with part of the buffer that is not consumed by the kernel. However, you cannot do so in safe Rust.