//! Resolver configuration
//!
//! There are two parts to this module: Query options that allow you to
//! modify the behaviour of the resolver on a query by query basis and
//! the global resolver configuration (normally read from the system’s
//! `/etc/resolv.conf`) that contains things like the name servers to query
//! and a set of default options.
//!
//! Both parts are modeled along the lines of glibc’s resolver.

use crate::base::name::{self, Name};
use smallvec::SmallVec;
use std::cmp::Ordering;
use std::default::Default;
use std::io::Read;
use std::net::{IpAddr, Ipv4Addr, SocketAddr};
use std::path::Path;
use std::slice::SliceIndex;
use std::str::{self, FromStr, SplitWhitespace};
use std::time::Duration;
use std::vec::Vec;
use std::{convert, error, fmt, fs, io, ops};

//------------ ResolvOptions ------------------------------------------------

/// Options for the resolver configuration.
///
/// This type contains a lot of options that influence the resolver
/// configuration. It collects all server-indpendent options that glibc’s
/// resolver supports. Not all of them are currently supported by this
/// implementation.
#[derive(Clone, Debug)]
pub struct ResolvOptions {
    /// Search list for host-name lookup.
    pub search: SearchList,

    /// TODO Sortlist
    /// sortlist: ??

    /// Number of dots before an initial absolute query is made.
    pub ndots: usize,

    /// Timeout to wait for a response.
    pub timeout: Duration,

    /// Number of retries before giving up.
    pub attempts: usize,

    /// Accept authoritative answers only.
    ///
    /// Only responses with the AA bit set will be considered. If there
    /// aren’t any, the query will fail.
    ///
    /// This option is not currently implemented. It is likely to be
    /// eventually implemented by the query.
    pub aa_only: bool,

    /// Always use TCP.
    ///
    /// This option is implemented by the query.
    pub use_vc: bool,

    /// Query primary name servers only.
    ///
    /// This option is not currently implemented. It is unclear what exactly
    /// it is supposed to mean.
    pub primary: bool,

    /// Set the recursion desired bit in queries.
    ///
    /// Enabled by default.
    ///
    /// Implemented by the query request.
    pub recurse: bool,

    /// Append the default domain name to single component names.
    ///
    /// Enabled by default.
    ///
    /// This is not currently implemented. Instead, the resolver config’s
    /// `search` and `ndots` fields govern resolution of relative names of
    /// all kinds.
    pub default_names: bool,

    /// Keep TCP connections open between queries.
    ///
    /// This is not currently implemented.
    pub stay_open: bool,

    /// Search hostnames in the current domain and parent domains.
    ///
    /// Enabled by default.
    ///
    /// This options is not currently implemented. Instead, the resolver
    /// config’s `search` and `ndots` fields govern resolution of relative
    /// names.
    pub dn_search: bool,

    /// Try AAAA query before A query and map IPv4 responses to tunnel form.
    ///
    /// This option is not currently implemented. It is only relevant for
    /// `lookup_host`.
    pub use_inet6: bool,

    /// Use round-robin selection of name servers.
    ///
    /// This option is implemented by the query.
    pub rotate: bool,

    /// Disable checking of incoming hostname and mail names.
    ///
    /// This is not currently implemented. Or rather, this is currently
    /// always on—there is no name checking as yet.
    pub no_check_name: bool,

    /// Do not strip TSIG records.
    ///
    /// This is not currently implemented. Or rather, no records are stripped
    /// at all.
    pub keep_tsig: bool,

    /// Send each query simultaneously to all name servers.
    ///
    /// This is not currently implemented. It would be a query option.
    pub blast: bool,

    /// Use bit-label format for IPv6 reverse lookups.
    ///
    /// Bit labels have been deprecated and consequently, this option is not
    /// implemented.
    pub use_bstring: bool,

    /// Use ip6.int instead of the recommended ip6.arpa.
    ///
    /// (This option is the reverse of glibc’s `RES_NOIP6DOTINT` option).
    ///
    /// This option is only relevant for `lookup_addr()` and is implemented
    /// there already.
    pub use_ip6dotint: bool,

    /// Use EDNS0.
    ///
    /// EDNS is not yet supported.
    pub use_edns0: bool,

    /// Perform IPv4 and IPv6 lookups sequentially instead of in parallel.
    ///
    /// This is not yet implemented but would be an option for
    /// `lookup_host()`.
    pub single_request: bool,

    /// Open a new socket for each request.
    ///
    /// This is not currently implemented.
    pub single_request_reopen: bool,

    /// Don’t look up unqualified names as top-level-domain.
    ///
    /// This is not currently implemented. Instead, the resolver config’s
    /// `search` and `ndots` fields govern resolution of relative names of
    /// all kinds.
    pub no_tld_query: bool,
}

impl Default for ResolvOptions {
    fn default() -> Self {
        ResolvOptions {
            // non-flags:
            search: SearchList::new(),
            //sortlist,
            ndots: 1,
            timeout: Duration::new(5, 0),
            attempts: 2,

            // enabled by default:
            recurse: true,
            default_names: true,
            dn_search: true,

            // everthing else is not:
            aa_only: false,
            use_vc: false,
            primary: false,
            stay_open: false,
            use_inet6: false,
            rotate: false,
            no_check_name: false,
            keep_tsig: false,
            blast: false,
            use_bstring: false,
            use_ip6dotint: false,
            use_edns0: false,
            single_request: false,
            single_request_reopen: false,
            no_tld_query: false,
        }
    }
}

//------------ Transport -----------------------------------------------------

/// The transport protocol to be used for a server.
#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
pub enum Transport {
    /// Unencrypted UDP transport, switch to TCP for truncated responses.
    UdpTcp,

    /// Unencrypted TCP transport.
    Tcp,
}

//------------ ServerConf ----------------------------------------------------

/// Configuration for one upstream DNS server.
///
/// The server is identified by a socket address, ie., an address/port pair.
/// For each server you can set how it should operate on all supported
/// transport protocols, including not at all, and two timeouts for each
/// request and sockets. The timeouts are used for all transports. If you
/// need different timeouts for, say, UDP and TCP, you can always use two
/// server entries with the same address.
#[derive(Clone, Debug)]
pub struct ServerConf {
    /// Server address.
    pub addr: SocketAddr,

    /// Transport protocol.
    pub transport: Transport,

    /// How long to wait for a response before returning a timeout error.
    ///
    /// This field defaults to 2 seconds.
    pub request_timeout: Duration,

    /// Size of the message receive buffer in bytes.
    ///
    /// This is used for datagram transports only. It defaults to 1232 bytes
    /// for both IPv6 and IPv4.
    ///
    /// (Note: While 1372 bytes works for IPv4 in most scenarios, there has
    /// been [research] indicating that sometimes 1232 bytes is the limit here,
    /// sometimes too.)
    ///
    /// [research]: https://rp.delaat.net/2019-2020/p78/report.pdf
    pub recv_size: usize,

    /// Advertised UDP payload size.
    ///
    /// This values will be announced in request if EDNS is supported by the
    /// server. It will be included both for datagram and streaming transport
    /// but really only matters for UDP.
    pub udp_payload_size: u16,
}

impl ServerConf {
    /// Returns a new default server config for a given address and transport.
    ///
    /// The function sets default values as described in the field
    /// descriptions above.
    pub fn new(addr: SocketAddr, transport: Transport) -> Self {
        ServerConf {
            addr,
            transport,
            request_timeout: Duration::from_secs(2),
            recv_size: 1232,
            udp_payload_size: 1232,
        }
    }
}

//------------ ResolvConf ---------------------------------------------------

/// Resolver configuration.
///
/// This type collects all information necessary to configure how a stub
/// resolver talks to its upstream resolvers.
///
/// The type follows the builder pattern. After creating a value with
/// [`ResolvConf::new`] you can manipulate the members. Once you are happy
/// with them, you call [`finalize`] to make sure the configuration is valid.
/// It mostly just fixes the `servers`.
///
/// Additionally, the type can parse a glibc-style configuration file,
/// commonly known as `/etc/resolv.conf` through the [`parse`] and
/// [`parse_file`] methods. You still need to call [`finalize`] after
/// parsing.
///
/// The easiest way, however, to get the system resolver configuration is
/// through [`ResolvConf::default`]. This will parse the configuration
/// file or return a default configuration if that fails.
///
/// [`parse`]: Self::parse
/// [`parse_file`]: Self::parse_file
/// [`finalize`]: Self::finalize
#[derive(Clone, Debug)]
pub struct ResolvConf {
    /// Addresses of servers to query.
    pub servers: Vec<ServerConf>,

    /// Default options.
    pub options: ResolvOptions,
}

/// # Management
///
impl ResolvConf {
    /// Creates a new, empty configuration.
    ///
    /// Using an empty configuration will fail since it does not contain
    /// any name servers. Call [`Self::finalize`] to make it usable.
    pub fn new() -> Self {
        ResolvConf {
            servers: Vec::new(),
            options: ResolvOptions::default(),
        }
    }

    /// Finalizes the configuration for actual use.
    ///
    /// The function does two things. If `servers` is empty, it adds
    /// `127.0.0.1:53`. This is exactly what glibc does. If `search` is
    /// empty, it adds the root domain `"."`. This differs from what
    /// glibc does which considers the machine’s host name.
    pub fn finalize(&mut self) {
        if self.servers.is_empty() {
            // glibc just simply uses 127.0.0.1:53. Let's do that, too,
            // and claim it is for compatibility.
            self.servers.push(ServerConf::new(
                SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)), 53),
                Transport::UdpTcp,
            ));
        }
        if self.options.search.is_empty() {
            self.options.search.push(Name::root())
        }
        for server in &mut self.servers {
            server.request_timeout = self.options.timeout
        }
    }
}

/// # Parsing Configuration File
///
impl ResolvConf {
    /// Parses the configuration from a file.
    pub fn parse_file<P: AsRef<Path>>(
        &mut self,
        path: P,
    ) -> Result<(), Error> {
        let mut file = fs::File::open(path)?;
        self.parse(&mut file)
    }

    /// Parses the configuration from a reader.
    ///
    /// The format is that of the `/etc/resolv.conf`` file.
    pub fn parse<R: Read>(&mut self, reader: &mut R) -> Result<(), Error> {
        use std::io::BufRead;

        for line in io::BufReader::new(reader).lines() {
            let line = line?;
            let line = line.trim_end();

            if line.is_empty()
                || line.starts_with(';')
                || line.starts_with('#')
            {
                continue;
            }

            let mut words = line.split_whitespace();
            let keyword = words.next();
            match keyword {
                Some("nameserver") => self.parse_nameserver(words)?,
                Some("domain") => self.parse_domain(words)?,
                Some("search") => self.parse_search(words)?,
                Some("sortlist") => { /* TODO: self.parse_sortlist(words)? */
                }
                Some("options") => self.parse_options(words)?,
                _ => return Err(Error::ParseError),
            }
        }
        Ok(())
    }

    fn parse_nameserver(
        &mut self,
        mut words: SplitWhitespace,
    ) -> Result<(), Error> {
        use std::net::ToSocketAddrs;

        for addr in (next_word(&mut words)?, 53).to_socket_addrs()? {
            self.servers.push(ServerConf::new(addr, Transport::UdpTcp));
        }
        no_more_words(words)
    }

    fn parse_domain(
        &mut self,
        mut words: SplitWhitespace,
    ) -> Result<(), Error> {
        let domain = SearchSuffix::from_str(next_word(&mut words)?)?;
        self.options.search = domain.into();
        no_more_words(words)
    }

    fn parse_search(&mut self, words: SplitWhitespace) -> Result<(), Error> {
        let mut search = SearchList::new();
        let mut root = false;
        for word in words {
            let name = SearchSuffix::from_str(word)?;
            if name.is_root() {
                root = true
            }
            search.push(name);
        }
        if !root {
            search.push(SearchSuffix::root());
        }
        self.options.search = search;
        Ok(())
    }

    /*
    fn parse_sortlist(
        &mut self,
        _words: SplitWhitespace
    ) -> Result<(), Error> {
        // XXX TODO
    }
    */

    fn parse_options(&mut self, words: SplitWhitespace) -> Result<(), Error> {
        for word in words {
            match split_arg(word)? {
                ("debug", None) => {}
                ("ndots", Some(n)) => self.options.ndots = n,
                ("timeout", Some(n)) => {
                    self.options.timeout = Duration::new(n as u64, 0)
                }
                ("attempts", Some(n)) => self.options.attempts = n,
                ("rotate", None) => self.options.rotate = true,
                ("no-check-names", None) => self.options.no_check_name = true,
                ("inet6", None) => self.options.use_inet6 = true,
                ("ip6-bytestring", None) => self.options.use_bstring = true,
                ("ip6-dotint", None) => self.options.use_ip6dotint = true,
                ("no-ip6-dotint", None) => self.options.use_ip6dotint = false,
                ("edns0", None) => self.options.use_edns0 = true,
                ("single-request", None) => {
                    self.options.single_request = true
                }
                ("single-request-reopen", None) => {
                    self.options.single_request_reopen = true
                }
                ("no-tld-query", None) => self.options.no_tld_query = true,
                ("use-vc", None) => self.options.use_vc = true,
                // Ignore unknown or misformated options.
                _ => {}
            }
        }
        Ok(())
    }
}

//--- Default

impl Default for ResolvConf {
    /// Creates a default configuration for this system.
    ///
    /// XXX This currently only works for Unix-y systems.
    fn default() -> Self {
        let mut res = ResolvConf::new();
        let _ = res.parse_file("/etc/resolv.conf");
        res.finalize();
        res
    }
}

//--- Display

impl fmt::Display for ResolvConf {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        for server in &self.servers {
            let server = server.addr;
            f.write_str("nameserver ")?;
            if server.port() == 53 {
                server.ip().fmt(f)?;
            } else {
                server.fmt(f)?;
            }
            "\n".fmt(f)?;
        }
        match self.options.search.len().cmp(&1) {
            Ordering::Equal => {
                writeln!(f, "domain {}", self.options.search[0])?;
            }
            Ordering::Greater => {
                "search".fmt(f)?;
                for name in self.options.search.as_slice() {
                    write!(f, " {}", name)?;
                }
                "\n".fmt(f)?;
            }
            Ordering::Less => {}
        }

        // Collect options so we only print them if there are any non-default
        // ones.
        let mut options = Vec::new();

        if self.options.ndots != 1 {
            options.push(format!("ndots:{}", self.options.ndots));
        }
        if self.options.timeout != Duration::new(5, 0) {
            // XXX This ignores fractional seconds.
            options
                .push(format!("timeout:{}", self.options.timeout.as_secs()));
        }
        if self.options.attempts != 2 {
            options.push(format!("attempts:{}", self.options.attempts));
        }
        if self.options.aa_only {
            options.push("aa-only".into())
        }
        if self.options.use_vc {
            options.push("use-vc".into())
        }
        if self.options.primary {
            options.push("primary".into())
        }
        if !self.options.recurse {
            options.push("no-recurse".into())
        }
        if !self.options.default_names {
            options.push("no-default-names".into())
        }
        if self.options.stay_open {
            options.push("stay-open".into())
        }
        if !self.options.dn_search {
            options.push("no-dn-search".into())
        }
        if self.options.use_inet6 {
            options.push("use-inet6".into())
        }
        if self.options.rotate {
            options.push("rotate".into())
        }
        if self.options.no_check_name {
            options.push("no-check-name".into())
        }
        if self.options.keep_tsig {
            options.push("keep-tsig".into())
        }
        if self.options.blast {
            options.push("blast".into())
        }
        if self.options.use_bstring {
            options.push("use-bstring".into())
        }
        if self.options.use_ip6dotint {
            options.push("ip6dotint".into())
        }
        if self.options.use_edns0 {
            options.push("use-edns0".into())
        }
        if self.options.single_request {
            options.push("single-request".into())
        }
        if self.options.single_request_reopen {
            options.push("single-request-reopen".into())
        }
        if self.options.no_tld_query {
            options.push("no-tld-query".into())
        }

        if !options.is_empty() {
            "options".fmt(f)?;
            for option in options {
                write!(f, " {}", option)?;
            }
            "\n".fmt(f)?;
        }

        Ok(())
    }
}

//------------ SearchSuffix --------------------------------------------------

pub type SearchSuffix = Name<SmallVec<[u8; 24]>>;

//------------ SearchList ----------------------------------------------------

#[derive(Clone, Debug, Default)]
pub struct SearchList {
    search: Vec<SearchSuffix>,
}

impl SearchList {
    pub fn new() -> Self {
        Self::default()
    }

    pub fn push(&mut self, name: SearchSuffix) {
        self.search.push(name)
    }

    pub fn push_root(&mut self) {
        self.search.push(Name::root())
    }

    pub fn len(&self) -> usize {
        self.search.len()
    }

    pub fn is_empty(&self) -> bool {
        self.search.is_empty()
    }

    pub fn get(&self, pos: usize) -> Option<&SearchSuffix> {
        self.search.get(pos)
    }

    pub fn as_slice(&self) -> &[SearchSuffix] {
        self.as_ref()
    }
}

impl From<SearchSuffix> for SearchList {
    fn from(name: SearchSuffix) -> Self {
        let mut res = Self::new();
        res.push(name);
        res
    }
}

impl<I: SliceIndex<[SearchSuffix]>> ops::Index<I> for SearchList {
    type Output = <I as SliceIndex<[SearchSuffix]>>::Output;

    fn index(&self, index: I) -> &<I as SliceIndex<[SearchSuffix]>>::Output {
        self.search.index(index)
    }
}

//--- AsRef

impl AsRef<[SearchSuffix]> for SearchList {
    fn as_ref(&self) -> &[SearchSuffix] {
        self.search.as_ref()
    }
}

//------------ Private Helpers -----------------------------------------------
//
// These are here to wrap stuff into Results.

/// Returns a reference to the next word or an error.
fn next_word<'a>(
    words: &'a mut str::SplitWhitespace,
) -> Result<&'a str, Error> {
    match words.next() {
        Some(word) => Ok(word),
        None => Err(Error::ParseError),
    }
}

/// Returns nothing but errors out if there are words left.
fn no_more_words(mut words: str::SplitWhitespace) -> Result<(), Error> {
    match words.next() {
        Some(..) => Err(Error::ParseError),
        None => Ok(()),
    }
}

/// Splits the name and argument from an option with arguments.
///
/// These options consist of a name followed by a colon followed by a
/// value, which so far is only `usize`, so we do that.
fn split_arg(s: &str) -> Result<(&str, Option<usize>), Error> {
    match s.find(':') {
        Some(idx) => {
            let (left, right) = s.split_at(idx);
            Ok((left, Some(right[1..].parse()?)))
        }
        None => Ok((s, None)),
    }
}

//------------ Error --------------------------------------------------------

/// The error that can happen when parsing `resolv.conf`.
#[derive(Debug)]
pub enum Error {
    /// The file is not a proper file.
    ParseError,

    /// Something happend while reading.
    Io(io::Error),
}

impl error::Error for Error {}

impl convert::From<io::Error> for Error {
    fn from(error: io::Error) -> Error {
        Error::Io(error)
    }
}

impl convert::From<name::FromStrError> for Error {
    fn from(_: name::FromStrError) -> Error {
        Error::ParseError
    }
}

impl convert::From<::std::num::ParseIntError> for Error {
    fn from(_: ::std::num::ParseIntError) -> Error {
        Error::ParseError
    }
}

impl fmt::Display for Error {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            Error::ParseError => write!(f, "error parsing configuration"),
            Error::Io(ref e) => e.fmt(f),
        }
    }
}

//============ Testing ======================================================

#[cfg(test)]
mod test {
    use super::*;
    use std::string::ToString;

    #[test]
    fn parse_resolv_conf() {
        let mut conf = ResolvConf::new();
        let data = "nameserver 192.0.2.0\n\
                    nameserver 192.0.2.1\n\
                    options use-vc ndots:122\n"
            .to_string();
        assert!(conf.parse(&mut io::Cursor::new(data)).is_ok());
        assert!(conf.options.use_vc);
        assert_eq!(conf.options.ndots, 122);
    }
}