io_uring: implement automatic buffer selection

[vrischmann]

This implements automatic buffer selection for io_uring.

The current api of read and recv is not great when using automatic buffer selection, we have to write relatively ugly code like this:

var slice: []u8 = "";
slice.len = 128;

var sqe = try ring.read(0xdfdfdfdf, fd, slice[0..], 0);
sqe.flags |= linux.IOSQE_BUFFER_SELECT;
sqe.buf_index = group_id;

because read takes a slice and gives its ptr and len to the kernel via these fields.

This is the reason why I added read_raw and recv_raw which take a ?[*]u8 and a separate length, so we can write code like this instead:

var sqe = try ring.read_raw(0xdfdfdfdf, fd, null, buffer_len, 0);
sqe.flags |= linux.IOSQE_BUFFER_SELECT;
sqe.buf_index = group_id;

which is better imo.

I'm not 100% happy with this API either, because:

• it feels redundant given the super small code difference between the two methods
• future operations which take a buffer as input and support automatic buffer selection will also need to implement two different methods.

There are other solutions but they require breaking existing code using read and recv.

1. define read the same as read_raw

pub fn read(self: *IO_Uring, user_data: u64, fd: os.fd_t, buffer: ?[*]u8, len: usize, offset: u64) !*io_uring_sqe

and let the caller give the correct pointer/len.

2. we could add a buffer type, something like this:

pub const ReadBuffer = union(enum) {
    slice: []u8,
    buffer_select: usize,
};

pub fn read(self: *IO_Uring, user_data: u64, fd: os.fd_t, buffer: ReadBuffer, offset: u64) !*io_uring_sqe

var buffer: [128]u8 = undefined;
var sqe = try ring.read(0, fd, .{ .slice = &buffer }, 0);

var sqe = try ring.read(0, fd, .{ .buffer_select = buffer_len }, 0);

Any input on this is appreciated, I might have missed something obvious.

[andrewrk]

This PR had an Azure CI failure and the log was deleted since it is so old, so I rebased this against master.

[vrischmann]

I changed the PR, now IO_Uring.read() takes a ReadBuffer which can be used for simple reads or reads using buffer selection.

Both IO_Uring.readv() and IO_Uring.read_fixed() could be handled in the same way but it should be maybe go in another PR ?

[jorangreef]

Perhaps it makes sense to do the breaking changes in one go? Would be really nice to get recv() into this PR since it's so similar to read().

[vrischmann]

Perhaps it makes sense to do the breaking changes in one go? Would be really nice to get recv() into this PR since it's so similar to read().

Ok, I can do that.

[vrischmann]

I've added support for buffer selection for recv.
I've also removed the readv method; read can now call io_uring_prep_readv for ReadBuffer.iovecs.

This means that I have to handle the iovecs case in recv and there's no API to use a iovec for recv so I simply make a slice from the first buffer in the iovec.
Another option might be to just return a "not supported" error ?

[jorangreef]

I've added support for buffer selection for recv.
I've also removed the readv method; read can now call io_uring_prep_readv for ReadBuffer.iovecs.

Thanks for doing this!

This means that I have to handle the iovecs case in recv and there's no API to use a iovec for recv
so I simply make a slice from the first buffer in the iovec.
Another option might be to just return a "not supported" error ?

Rather than using ReadBuffer also for recv(), I think it would be better to use a separate, more restricted RecvBuffer type to eliminate this case completely.

[vrischmann]

Rather than using ReadBuffer also for recv(), I think it would be better to use a separate, more restricted RecvBuffer type to eliminate this case completely.

Ok makes sense, I'll change it

[vrischmann]

I've pushed an update which uses the RecvBuffer type for recv().

[andrewrk]

Thanks @vrischmann and reviewers!