Add resolver that returns a single host (#55)

Author: bnaecker

src/resolvers/mod.rs

@@ -1,3 +1,4 @@
 //! Default implementations of [crate::resolver::Resolver]
 
 pub mod dns;
+pub mod single_host;

src/resolvers/single_host.rs

@@ -0,0 +1,63 @@
+//! Implementation of [Resolver] that always returns an explicit address.
+
+use tokio::sync::watch;
+
+use crate::backend;
+use crate::resolver::{AllBackends, Resolver};
+
+use std::collections::BTreeMap;
+use std::net::SocketAddr;
+use std::sync::Arc;
+
+/// A [`Resolver`] that always returns a single address.
+#[derive(Clone, Debug)]
+pub struct SingleHostResolver {
+    tx: watch::Sender<AllBackends>,
+}
+
+const SINGLE_HOST_BACKEND_NAME: &str = "singleton";
+
+impl SingleHostResolver {
+    /// Construct a resolver to always return the provided address.
+    pub fn new(address: SocketAddr) -> Self {
+        let backends = Arc::new(BTreeMap::from([(
+            backend::Name::new(SINGLE_HOST_BACKEND_NAME),
+            backend::Backend { address },
+        )]));
+        let (tx, _rx) = watch::channel(backends.clone());
+        Self { tx }
+    }
+}
+
+impl Resolver for SingleHostResolver {
+    fn monitor(&mut self) -> watch::Receiver<AllBackends> {
+        self.tx.subscribe()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::net::{IpAddr, Ipv4Addr, SocketAddr};
+
+    use crate::{
+        backend::Backend, resolver::Resolver as _, resolvers::single_host::SINGLE_HOST_BACKEND_NAME,
+    };
+
+    use super::SingleHostResolver;
+
+    #[test]
+    fn single_host_resolver_returns_address() {
+        let addr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 4444);
+        let mut res = SingleHostResolver::new(addr);
+        let rx = res.monitor();
+        let backends = rx.borrow();
+        assert_eq!(backends.len(), 1);
+        let Backend { address } = backends
+            .get(SINGLE_HOST_BACKEND_NAME)
+            .expect("Expected the single host backend name");
+        assert_eq!(
+            address, &addr,
+            "Single host resolver returned wrong address"
+        );
+    }
+}