preadv

fn preadv(
    ev: *Evented,
    cancel_region: *CancelRegion,
    fd: fd_t,
    iov: []const iovec,
    offset: ?u64,
) File.Reader.Error!usize {
    if (iov.len == 0) return 0;
    const gather = iov.len > 1 or iov[0].len > 0xfffff000;
    while (true) {
        const thread = try cancel_region.awaitIoUring();
        thread.enqueue().* = .{
            .opcode = if (gather) .READV else .READ,
            .flags = 0,
            .ioprio = 0,
            .fd = fd,
            .off = offset orelse std.math.maxInt(u64),
            .addr = if (gather) @intFromPtr(iov.ptr) else @intFromPtr(iov[0].base),
            .len = @intCast(if (gather) iov.len else iov[0].len),
            .rw_flags = 0,
            .user_data = @intFromPtr(cancel_region.fiber),
            .buf_index = 0,
            .personality = 0,
            .splice_fd_in = 0,
            .addr3 = 0,
            .resv = 0,
        };
        ev.yield(null, .nothing);
        const completion = cancel_region.completion();
        switch (completion.errno()) {
            .SUCCESS => return @as(u32, @bitCast(completion.result)),
            .INTR, .CANCELED => {},
            .INVAL => |err| return errnoBug(err),
            .FAULT => |err| return errnoBug(err),
            .AGAIN => return error.WouldBlock,
            .BADF => |err| return errnoBug(err), // File descriptor used after closed
            .IO => return error.InputOutput,
            .ISDIR => return error.IsDir,
            .NOBUFS => return error.SystemResources,
            .NOMEM => return error.SystemResources,
            .NOTCONN => return error.SocketUnconnected,
            .CONNRESET => return error.ConnectionResetByPeer,
            else => |err| return unexpectedErrno(err),
        }
    }
}