Optimize TrustDnsResolver (#1967)

* Optimize initialization of `TrustDnsResolver`

Replace use of `tokio::sync::Mutex` with `once_cell::sync::OnceCell`
since there's no need for it to be `async` and thus it can just use
`OnceCell` instead of `Mutex`.

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

* Optimize `TrustDnsResolver`: Rm use of `Lazy`

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

* Optimize `TrustDnsResolver`: Rm unnecessary `Arc`

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

* Derive `Default` on `TrustDnsResolver`

instead of implementing `TrustDnsResolver::new()`

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

---------

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

src/async_impl/client.rs

@@ -266,7 +266,7 @@ impl ClientBuilder {
             let mut resolver: Arc<dyn Resolve> = match config.trust_dns {
                 false => Arc::new(GaiResolver::new()),
                 #[cfg(feature = "trust-dns")]
-                true => Arc::new(TrustDnsResolver::new().map_err(crate::error::builder)?),
+                true => Arc::new(TrustDnsResolver::default()),
                 #[cfg(not(feature = "trust-dns"))]
                 true => unreachable!("trust-dns shouldn't be enabled unless the feature is"),
             };

src/dns/trust_dns.rs

@@ -1,8 +1,7 @@
 //! DNS resolution via the [trust_dns_resolver](https://github.com/bluejekyll/trust-dns) crate
 
 use hyper::client::connect::dns::Name;
-use once_cell::sync::Lazy;
-use tokio::sync::Mutex;
+use once_cell::sync::OnceCell;
 pub use trust_dns_resolver::config::{ResolverConfig, ResolverOpts};
 use trust_dns_resolver::{lookup_ip::LookupIpIntoIter, system_conf, TokioAsyncResolver};
 
@@ -12,62 +11,24 @@ use std::sync::Arc;
 
 use super::{Addrs, Resolve, Resolving};
 
-type SharedResolver = Arc<TokioAsyncResolver>;
-
-static SYSTEM_CONF: Lazy<io::Result<(ResolverConfig, ResolverOpts)>> =
-    Lazy::new(|| system_conf::read_system_conf().map_err(io::Error::from));
-
 /// Wrapper around an `AsyncResolver`, which implements the `Resolve` trait.
-#[derive(Debug, Clone)]
+#[derive(Debug, Default, Clone)]
 pub(crate) struct TrustDnsResolver {
-    state: Arc<Mutex<State>>,
+    /// 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>>,
 }
 
 struct SocketAddrs {
     iter: LookupIpIntoIter,
 }
 
-#[derive(Debug)]
-enum State {
-    Init,
-    Ready(SharedResolver),
-}
-
-impl TrustDnsResolver {
-    /// Create a new resolver with the default configuration,
-    /// which reads from `/etc/resolve.conf`.
-    pub fn new() -> io::Result<Self> {
-        SYSTEM_CONF.as_ref().map_err(|e| {
-            io::Error::new(e.kind(), format!("error reading DNS system conf: {}", e))
-        })?;
-
-        // At this stage, we might not have been called in the context of a
-        // Tokio Runtime, so we must delay the actual construction of the
-        // resolver.
-        Ok(TrustDnsResolver {
-            state: Arc::new(Mutex::new(State::Init)),
-        })
-    }
-}
-
 impl Resolve for TrustDnsResolver {
     fn resolve(&self, name: Name) -> Resolving {
         let resolver = self.clone();
         Box::pin(async move {
-            let mut lock = resolver.state.lock().await;
-
-            let resolver = match &*lock {
-                State::Init => {
-                    let resolver = new_resolver().await;
-                    *lock = State::Ready(resolver.clone());
-                    resolver
-                }
-                State::Ready(resolver) => resolver.clone(),
-            };
-
-            // Don't keep lock once the resolver is constructed, otherwise
-            // only one lookup could be done at a time.
-            drop(lock);
+            let resolver = resolver.state.get_or_try_init(new_resolver)?;
 
             let lookup = resolver.lookup_ip(name.as_str()).await?;
             let addrs: Addrs = Box::new(SocketAddrs {
@@ -86,14 +47,10 @@ impl Iterator for SocketAddrs {
     }
 }
 
-async fn new_resolver() -> SharedResolver {
-    let (config, opts) = SYSTEM_CONF
-        .as_ref()
-        .expect("can't construct TrustDnsResolver if SYSTEM_CONF is error")
-        .clone();
-    new_resolver_with_config(config, opts)
-}
-
-fn new_resolver_with_config(config: ResolverConfig, opts: ResolverOpts) -> SharedResolver {
-    Arc::new(TokioAsyncResolver::tokio(config, opts))
+/// Create a new resolver with the default configuration,
+/// which reads from `/etc/resolve.conf`.
+fn new_resolver() -> io::Result<TokioAsyncResolver> {
+    let (config, opts) = system_conf::read_system_conf()
+        .map_err(|e| io::Error::new(e.kind(), format!("error reading DNS system conf: {}", e)))?;
+    Ok(TokioAsyncResolver::tokio(config, opts))
 }