const std = @import("std");

const ControlMessage = extern struct {
    const Level = enum(u32) {
        socket = std.posix.SOL.SOCKET,
    };

    // WARNING: this kind enum might be wrong for non-linux posix systems.
    const Kind = enum(u32) {
        rights = 1,
        credentials = 2,
    };

    const Header = extern struct {
        length: usize,
        level: Level,
        kind: Kind,
    };

    header: Header,
    payload: union {
        fd: [255]std.posix.fd_t,
    },

    pub fn alignment(length: usize) usize {
        return (length + @sizeOf(usize) - 1) & ~(@sizeOf(usize) - 1);
    }

    pub fn space(self: @This()) usize {
        return self.alignment(self.length) + self.alignment(@sizeOf(Header));
    }

    pub fn from_fds(fds: []std.posix.fd_t) @This() {
        var self: @This() = .{
        };

        self.level = .socket;
        self.kind = .rights;
        self.length = self.alignment(@sizeOf(Header)) + (fds.len * @sizeOf(std.posix.fd_t));

        @memcpy(self.payload, fds);
    }
};

pub fn send_fds(socket: std.posix.socket_t, fds: []std.posix.fd_t) !void {
    var iobuf: [1]u8 = std.mem.zeroes([1]u8);

    const io = std.posix.iovec {
        .base = &iobuf,
        .len = iobuf.len,
    };

    var cmsg: ControlMessage = .from_fds(fds);

    const msghdr = std.posix.msghdr_const {
        .name = null,
        .namelen = 0,
        .iov = &io,
        .iolen = 1,
        .control = &cmsg,
        .controllen = cmsg.control_length(),
    };

    _ = try std.posix.sendmsg(socket, &msghdr, 0);
}