Implement Pluggable Name-resolution (#148)

This defines a new trait `Resolver`, which turns an address into a
Vec<SocketAddr>. It also provides an implementation of Resolver for
`Fn(&str)` so it's easy to define simple resolvers with a closure.


Fixes #82

Co-authored-by: Ulrik <ulrikm@spotify.com>

src/resolve.rs

@@ -0,0 +1,59 @@
+use std::fmt;
+use std::io::Result as IoResult;
+use std::net::{SocketAddr, ToSocketAddrs};
+use std::sync::Arc;
+
+pub trait Resolver: Send + Sync {
+    fn resolve(&self, netloc: &str) -> IoResult<Vec<SocketAddr>>;
+}
+
+#[derive(Debug)]
+pub(crate) struct StdResolver;
+
+impl Resolver for StdResolver {
+    fn resolve(&self, netloc: &str) -> IoResult<Vec<SocketAddr>> {
+        ToSocketAddrs::to_socket_addrs(netloc).map(|iter| iter.collect())
+    }
+}
+
+impl<F> Resolver for F
+where
+    F: Fn(&str) -> IoResult<Vec<SocketAddr>>,
+    F: Send + Sync,
+{
+    fn resolve(&self, netloc: &str) -> IoResult<Vec<SocketAddr>> {
+        self(netloc)
+    }
+}
+
+#[derive(Clone)]
+pub(crate) struct ArcResolver(Arc<dyn Resolver>);
+
+impl<R> From<R> for ArcResolver
+where
+    R: Resolver + 'static,
+{
+    fn from(r: R) -> Self {
+        Self(Arc::new(r))
+    }
+}
+
+impl fmt::Debug for ArcResolver {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "ArcResolver(...)")
+    }
+}
+
+impl std::ops::Deref for ArcResolver {
+    type Target = dyn Resolver;
+
+    fn deref(&self) -> &Self::Target {
+        self.0.as_ref()
+    }
+}
+
+impl Default for ArcResolver {
+    fn default() -> Self {
+        StdResolver.into()
+    }
+}