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/agent.rs

@@ -6,6 +6,7 @@ use std::sync::Mutex;
 use crate::header::{self, Header};
 use crate::pool::ConnectionPool;
 use crate::request::Request;
+use crate::resolve::ArcResolver;
 
 /// Agents keep state between requests.
 ///
@@ -53,15 +54,12 @@ pub(crate) struct AgentState {
     /// Cookies saved between requests.
     #[cfg(feature = "cookie")]
     pub(crate) jar: CookieJar,
+    pub(crate) resolver: ArcResolver,
 }
 
 impl AgentState {
     fn new() -> Self {
-        AgentState {
-            pool: ConnectionPool::new(),
-            #[cfg(feature = "cookie")]
-            jar: CookieJar::new(),
-        }
+        Self::default()
     }
     pub fn pool(&mut self) -> &mut ConnectionPool {
         &mut self.pool
@@ -194,6 +192,29 @@ impl Agent {
             .set_max_idle_connections_per_host(max_connections);
     }
 
+    /// Configures a custom resolver to be used by this agent. By default,
+    /// address-resolution is done by std::net::ToSocketAddrs. This allows you
+    /// to override that resolution with your own alternative. Useful for
+    /// testing and special-cases like DNS-based load balancing.
+    ///
+    /// A `Fn(&str) -> io::Result<Vec<SocketAddr>>` is a valid resolver,
+    /// passing a closure is a simple way to override. Note that you might need
+    /// explicit type `&str` on the closure argument for type inference to
+    /// succeed.
+    /// ```
+    /// use std::net::ToSocketAddrs;
+    ///
+    /// let mut agent = ureq::agent();
+    /// agent.set_resolver(|addr: &str| match addr {
+    ///    "example.com" => Ok(vec![([127,0,0,1], 8096).into()]),
+    ///    addr => addr.to_socket_addrs().map(Iterator::collect),
+    /// });
+    /// ```
+    pub fn set_resolver(&mut self, resolver: impl crate::Resolver + 'static) -> &mut Self {
+        self.state.lock().unwrap().resolver = resolver.into();
+        self
+    }
+
     /// Gets a cookie in this agent by name. Cookies are available
     /// either by setting it in the agent, or by making requests
     /// that `Set-Cookie` in the agent.

src/lib.rs

@@ -125,6 +125,7 @@ mod header;
 mod pool;
 mod proxy;
 mod request;
+mod resolve;
 mod response;
 mod stream;
 mod unit;
@@ -140,6 +141,7 @@ pub use crate::error::Error;
 pub use crate::header::Header;
 pub use crate::proxy::Proxy;
 pub use crate::request::Request;
+pub use crate::resolve::Resolver;
 pub use crate::response::Response;
 
 // re-export

src/pool.rs

@@ -74,10 +74,6 @@ impl Default for ConnectionPool {
 }
 
 impl ConnectionPool {
-    pub fn new() -> Self {
-        Self::default()
-    }
-
     pub fn set_max_idle_connections(&mut self, max_connections: usize) {
         if self.max_idle_connections == max_connections {
             return;
@@ -251,7 +247,7 @@ fn pool_connections_limit() {
     // Test inserting connections with different keys into the pool,
     // filling and draining it. The pool should evict earlier connections
     // when the connection limit is reached.
-    let mut pool = ConnectionPool::new();
+    let mut pool = ConnectionPool::default();
     let hostnames = (0..DEFAULT_MAX_IDLE_CONNECTIONS * 2).map(|i| format!("{}.example", i));
     let poolkeys = hostnames.map(|hostname| PoolKey {
         scheme: "https".to_string(),
@@ -276,7 +272,7 @@ fn pool_per_host_connections_limit() {
     // Test inserting connections with the same key into the pool,
     // filling and draining it. The pool should evict earlier connections
     // when the per-host connection limit is reached.
-    let mut pool = ConnectionPool::new();
+    let mut pool = ConnectionPool::default();
     let poolkey = PoolKey {
         scheme: "https".to_string(),
         hostname: "example.com".to_string(),
@@ -301,7 +297,7 @@ fn pool_per_host_connections_limit() {
 
 #[test]
 fn pool_update_connection_limit() {
-    let mut pool = ConnectionPool::new();
+    let mut pool = ConnectionPool::default();
     pool.set_max_idle_connections(50);
 
     let hostnames = (0..pool.max_idle_connections).map(|i| format!("{}.example", i));
@@ -321,7 +317,7 @@ fn pool_update_connection_limit() {
 
 #[test]
 fn pool_update_per_host_connection_limit() {
-    let mut pool = ConnectionPool::new();
+    let mut pool = ConnectionPool::default();
     pool.set_max_idle_connections(50);
     pool.set_max_idle_connections_per_host(50);
 
@@ -347,7 +343,7 @@ fn pool_update_per_host_connection_limit() {
 fn pool_checks_proxy() {
     // Test inserting different poolkeys with same address but different proxies.
     // Each insertion should result in an additional entry in the pool.
-    let mut pool = ConnectionPool::new();
+    let mut pool = ConnectionPool::default();
     let url = Url::parse("zzz:///example.com").unwrap();
 
     pool.add(

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()
+    }
+}

src/stream.rs

@@ -4,7 +4,6 @@ use std::io::{
 };
 use std::net::SocketAddr;
 use std::net::TcpStream;
-use std::net::ToSocketAddrs;
 use std::time::Duration;
 use std::time::Instant;
 
@@ -387,14 +386,16 @@ pub(crate) fn connect_host(unit: &Unit, hostname: &str, port: u16) -> Result<Tcp
         unit.deadline
     };
   
-    // TODO: Find a way to apply deadline to DNS lookup.
-    let sock_addrs: Vec<SocketAddr> = match unit.req.proxy {
+    let netloc = match unit.req.proxy {
         Some(ref proxy) => format!("{}:{}", proxy.server, proxy.port),
         None => format!("{}:{}", hostname, port),
-    }
-    .to_socket_addrs()
-    .map_err(|e| Error::DnsFailed(format!("{}", e)))?
-    .collect();
+    };
+
+    // TODO: Find a way to apply deadline to DNS lookup.
+    let sock_addrs = unit
+        .resolver()
+        .resolve(&netloc)
+        .map_err(|e| Error::DnsFailed(format!("{}", e)))?;
 
     if sock_addrs.is_empty() {
         return Err(Error::DnsFailed(format!("No ip address for {}", hostname)));
@@ -419,6 +420,7 @@ pub(crate) fn connect_host(unit: &Unit, hostname: &str, port: u16) -> Result<Tcp
         // connect with a configured timeout.
         let stream = if Some(Proto::SOCKS5) == proto {
             connect_socks5(
+                &unit,
                 unit.req.proxy.to_owned().unwrap(),
                 deadline,
                 sock_addr,
@@ -496,11 +498,15 @@ pub(crate) fn connect_host(unit: &Unit, hostname: &str, port: u16) -> Result<Tcp
 }
 
 #[cfg(feature = "socks-proxy")]
-fn socks5_local_nslookup(hostname: &str, port: u16) -> Result<TargetAddr, std::io::Error> {
-    let addrs: Vec<SocketAddr> = format!("{}:{}", hostname, port)
-        .to_socket_addrs()
-        .map_err(|e| std::io::Error::new(ErrorKind::NotFound, format!("DNS failure: {}.", e)))?
-        .collect();
+fn socks5_local_nslookup(
+    unit: &Unit,
+    hostname: &str,
+    port: u16,
+) -> Result<TargetAddr, std::io::Error> {
+    let addrs: Vec<SocketAddr> = unit
+        .resolver()
+        .resolve(&format!("{}:{}", hostname, port))
+        .map_err(|e| std::io::Error::new(ErrorKind::NotFound, format!("DNS failure: {}.", e)))?;
 
     if addrs.is_empty() {
         return Err(std::io::Error::new(
@@ -522,6 +528,7 @@ fn socks5_local_nslookup(hostname: &str, port: u16) -> Result<TargetAddr, std::i
 
 #[cfg(feature = "socks-proxy")]
 fn connect_socks5(
+    unit: &Unit,
     proxy: Proxy,
     deadline: Option<Instant>,
     proxy_addr: SocketAddr,
@@ -533,7 +540,7 @@ fn connect_socks5(
     use std::str::FromStr;
 
     let host_addr = if Ipv4Addr::from_str(host).is_ok() || Ipv6Addr::from_str(host).is_ok() {
-        match socks5_local_nslookup(host, port) {
+        match socks5_local_nslookup(unit, host, port) {
             Ok(addr) => addr,
             Err(err) => return Err(err),
         }
@@ -625,6 +632,7 @@ fn get_socks5_stream(
 
 #[cfg(not(feature = "socks-proxy"))]
 fn connect_socks5(
+    _unit: &Unit,
     _proxy: Proxy,
     _deadline: Option<Instant>,
     _proxy_addr: SocketAddr,

src/test/agent_test.rs

@@ -101,6 +101,31 @@ fn connection_reuse() {
     assert_eq!(resp.status(), 200);
 }
 
+#[test]
+fn custom_resolver() {
+    use std::io::Read;
+    use std::net::TcpListener;
+
+    let listener = TcpListener::bind("127.0.0.1:0").unwrap();
+
+    let local_addr = listener.local_addr().unwrap();
+
+    let server = std::thread::spawn(move || {
+        let (mut client, _) = listener.accept().unwrap();
+        let mut buf = vec![0u8; 16];
+        let read = client.read(&mut buf).unwrap();
+        buf.truncate(read);
+        buf
+    });
+
+    crate::agent()
+        .set_resolver(move |_: &str| Ok(vec![local_addr]))
+        .get("http://cool.server/")
+        .call();
+
+    assert_eq!(&server.join().unwrap(), b"GET / HTTP/1.1\r\n");
+}
+
 #[cfg(feature = "cookie")]
 #[cfg(test)]
 fn cookie_and_redirect(mut stream: TcpStream) -> io::Result<()> {

src/unit.rs

@@ -10,6 +10,7 @@ use cookie::{Cookie, CookieJar};
 use crate::agent::AgentState;
 use crate::body::{self, Payload, SizedReader};
 use crate::header;
+use crate::resolve::ArcResolver;
 use crate::stream::{self, connect_test, Stream};
 use crate::{Error, Header, Request, Response};
 
@@ -95,6 +96,10 @@ impl Unit {
         self.req.method.eq_ignore_ascii_case("head")
     }
 
+    pub fn resolver(&self) -> ArcResolver {
+        self.req.agent.lock().unwrap().resolver.clone()
+    }
+
     #[cfg(test)]
     pub fn header(&self, name: &str) -> Option<&str> {
         header::get_header(&self.headers, name)