//! DNS resolver configuration for internal lookups.
//!
//! The resolver itself is the set combination of the google, cloudflare, and quad9 endpoints
//! supporting DoH and DoT.
//!
//! This resolver implements a fallback mechanism where, should the DNS-over-TLS resolution fail, a
//! followup resolution will be done using the hosts configured default (e.g. `/etc/resolve.conf` on
//! linux).
//!
//! Requires the `dns-over-https-rustls`, `webpki-roots` feature for the
//! `hickory-resolver` crate
#![deny(missing_docs)]

use crate::ClientBuilder;

use std::{
    net::SocketAddr,
    sync::{Arc, LazyLock},
};

use hickory_resolver::{
    config::{LookupIpStrategy, NameServerConfigGroup, ResolverConfig, ResolverOpts},
    error::ResolveError,
    lookup_ip::LookupIpIntoIter,
    TokioAsyncResolver,
};
use hickory_resolver::{error::ResolveErrorKind, lookup_ip::LookupIp};
use once_cell::sync::OnceCell;
use reqwest::dns::{Addrs, Name, Resolve, Resolving};
use tracing::warn;

impl ClientBuilder {
    /// Override the DNS resolver implementation used by the underlying http client.
    pub fn dns_resolver<R: Resolve + 'static>(mut self, resolver: Arc<R>) -> Self {
        self.reqwest_client_builder = self.reqwest_client_builder.dns_resolver(resolver);
        self
    }
}

struct SocketAddrs {
    iter: LookupIpIntoIter,
}

// n.b. static items do not call [`Drop`] on program termination, so this won't be deallocated.
// this is fine, as the OS can deallocate the terminated program faster than we can free memory
// but tools like valgrind might report "memory leaks" as it isn't obvious this is intentional.
static SHARED_RESOLVER: LazyLock<HickoryDnsResolver> = LazyLock::new(|| {
    tracing::debug!("Initializing shared DNS resolver");
    HickoryDnsResolver::default()
});

#[derive(Debug, thiserror::Error)]
#[error("hickory-dns resolver error: {hickory_error}")]
/// Error occurring while resolving a hostname into an IP address.
pub struct HickoryDnsError {
    #[from]
    hickory_error: ResolveError,
}

/// Wrapper around an `AsyncResolver`, which implements the `Resolve` trait.
///
/// Typical use involves instantiating using the `Default` implementation and then resolving using
/// methods or trait implementations.
///
/// The default initialization uses a shared underlying `AsyncResolver`. If a thread local resolver
/// is required use `thread_resolver()` to build a resolver with an independently instantiated
/// internal `AsyncResolver`.
#[derive(Debug, Default, Clone)]
pub struct HickoryDnsResolver {
    // Since we might not have been called in the context of a
    // Tokio Runtime in initialization, so we must delay the actual
    // construction of the resolver.
    state: Arc<OnceCell<TokioAsyncResolver>>,
    fallback: Arc<OnceCell<TokioAsyncResolver>>,
    dont_use_shared: bool,
}

impl Resolve for HickoryDnsResolver {
    fn resolve(&self, name: Name) -> Resolving {
        let resolver = self.state.clone();
        let fallback = self.fallback.clone();
        let independent = self.dont_use_shared;
        Box::pin(async move {
            let resolver = resolver.get_or_try_init(|| {
                // using a closure here is slightly gross, but this makes sure that if the
                // lazy-init returns an error it can be handled by the client
                if independent {
                    new_resolver()
                } else {
                    Ok(SHARED_RESOLVER.state.get_or_try_init(new_resolver)?.clone())
                }
            })?;

            // try the primary DNS resolver that we set up (DoH or DoT or whatever)
            let lookup = match resolver.lookup_ip(name.as_str()).await {
                Ok(res) => res,
                Err(e) => {
                    // on failure use the fall back system configured DNS resolver
                    match e.kind() {
                        ResolveErrorKind::NoRecordsFound { .. } => {}
                        _ => {
                            warn!("primary DNS failed w/ error {e}: using system fallback");
                        }
                    }
                    let resolver = fallback.get_or_try_init(|| {
                        // using a closure here is slightly gross, but this makes sure that if the
                        // lazy-init returns an error it can be handled by the client
                        if independent {
                            new_resolver_system()
                        } else {
                            Ok(SHARED_RESOLVER
                                .fallback
                                .get_or_try_init(new_resolver_system)?
                                .clone())
                        }
                    })?;
                    resolver.lookup_ip(name.as_str()).await?
                }
            };

            let addrs: Addrs = Box::new(SocketAddrs {
                iter: lookup.into_iter(),
            });
            Ok(addrs)
        })
    }
}

impl Iterator for SocketAddrs {
    type Item = SocketAddr;

    fn next(&mut self) -> Option<Self::Item> {
        self.iter.next().map(|ip_addr| SocketAddr::new(ip_addr, 0))
    }
}

impl HickoryDnsResolver {
    /// Attempt to resolve a domain name to a set of ['IpAddr']s
    pub async fn resolve_str(&self, name: &str) -> Result<LookupIp, HickoryDnsError> {
        let resolver = self.state.get_or_try_init(|| self.new_resolver())?;

        // try the primary DNS resolver that we set up (DoH or DoT or whatever)
        let lookup = match resolver.lookup_ip(name).await {
            Ok(res) => res,
            Err(e) => {
                // on failure use the fall back system configured DNS resolver
                match e.kind() {
                    ResolveErrorKind::NoRecordsFound { .. } => {}
                    _ => {
                        warn!("primary DNS failed w/ error {e}: using system fallback");
                    }
                }
                let resolver = self
                    .fallback
                    .get_or_try_init(|| self.new_resolver_system())?;
                resolver.lookup_ip(name).await?
            }
        };

        Ok(lookup)
    }

    /// Create a (lazy-initialized) resolver that is not shared across threads.
    pub fn thread_resolver() -> Self {
        Self {
            dont_use_shared: true,
            ..Default::default()
        }
    }

    fn new_resolver(&self) -> Result<TokioAsyncResolver, HickoryDnsError> {
        if self.dont_use_shared {
            new_resolver()
        } else {
            Ok(SHARED_RESOLVER.state.get_or_try_init(new_resolver)?.clone())
        }
    }

    fn new_resolver_system(&self) -> Result<TokioAsyncResolver, HickoryDnsError> {
        if self.dont_use_shared {
            new_resolver_system()
        } else {
            Ok(SHARED_RESOLVER
                .fallback
                .get_or_try_init(new_resolver_system)?
                .clone())
        }
    }
}

/// Create a new resolver with a custom DoT based configuration. The options are overridden to look
/// up for both IPv4 and IPv6 addresses to work with "happy eyeballs" algorithm.
fn new_resolver() -> Result<TokioAsyncResolver, HickoryDnsError> {
    let mut name_servers = NameServerConfigGroup::google_tls();
    name_servers.merge(NameServerConfigGroup::google_https());
    // name_servers.merge(NameServerConfigGroup::google_h3());
    name_servers.merge(NameServerConfigGroup::quad9_tls());
    name_servers.merge(NameServerConfigGroup::quad9_https());
    name_servers.merge(NameServerConfigGroup::cloudflare_tls());
    name_servers.merge(NameServerConfigGroup::cloudflare_https());

    let config = ResolverConfig::from_parts(None, Vec::new(), name_servers);

    let mut opts = ResolverOpts::default();
    opts.ip_strategy = LookupIpStrategy::Ipv4AndIpv6;
    // Would like to enable this when 0.25 stabilizes
    // opts.server_ordering_strategy = ServerOrderingStrategy::RoundRobin;

    Ok(TokioAsyncResolver::tokio(config, opts))
}

/// Create a new resolver with the default configuration, which reads from the system DNS config
/// (i.e. `/etc/resolve.conf` in unix). The options are overridden to look up for both IPv4 and IPv6
/// addresses to work with "happy eyeballs" algorithm.
fn new_resolver_system() -> Result<TokioAsyncResolver, HickoryDnsError> {
    let (config, mut opts) = hickory_resolver::system_conf::read_system_conf()?;
    opts.ip_strategy = LookupIpStrategy::Ipv4AndIpv6;
    Ok(TokioAsyncResolver::tokio(config, opts))
}

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

    #[tokio::test]
    async fn reqwest_hickory_doh() {
        let resolver = HickoryDnsResolver::default();
        let client = reqwest::ClientBuilder::new()
            .dns_resolver(resolver.into())
            .build()
            .unwrap();

        let resp = client
            .get("http://ifconfig.me:80")
            .send()
            .await
            .unwrap()
            .bytes()
            .await
            .unwrap();

        assert!(!resp.is_empty());
    }

    #[tokio::test]
    async fn dns_lookup() -> Result<(), HickoryDnsError> {
        let resolver = HickoryDnsResolver::default();

        let domain = "ifconfig.me";
        let addrs = resolver.resolve_str(domain).await?;

        assert!(addrs.into_iter().next().is_some());

        Ok(())
    }
}