feat(dns): tokio_threadpool::blocking resolver

Unlike the default resolver, this avoids spawning extra dedicated
threads but only works on the multi-threaded Tokio runtime.

Closes #1676

Cargo.toml

@@ -35,6 +35,7 @@ tokio-io = "0.1"
 tokio-reactor = { version = "0.1", optional = true }
 tokio-tcp = { version = "0.1", optional = true }
 tokio-timer = { version = "0.2", optional = true }
+tokio-threadpool = { version = "0.1", optional = true }
 want = "0.0.6"
 
 [dev-dependencies]
@@ -62,6 +63,7 @@ runtime = [
     "tokio-reactor",
     "tokio-tcp",
     "tokio-timer",
+    "tokio-threadpool",
 ]
 nightly = []
 __internal_flaky_tests = []

src/client/connect/dns.rs

@@ -13,6 +13,9 @@ use futures_cpupool::{Builder as CpuPoolBuilder};
 
 use self::sealed::GaiTask;
 
+#[cfg(feature = "runtime")]
+pub use self::blocking::{TokioThreadpoolGaiFuture, TokioThreadpoolGaiResolver};
+
 /// Resolve a hostname to a set of IP addresses.
 pub trait Resolve {
     /// The set of IP addresses to try to connect to.
@@ -239,6 +242,50 @@ pub(super) mod sealed {
     }
 }
 
+#[cfg(feature = "runtime")]
+mod blocking {
+    use futures::{Async, Future, Poll};
+    use std::io;
+    use std::net::ToSocketAddrs;
+    use tokio_threadpool;
+
+    use super::{Name, IpAddrs, GaiAddrs, Resolve};
+
+    /// A resolver using `getaddrinfo` calls via the `tokio_threadpool::blocking` API.
+    /// 
+    /// Unlike the `GaiResolver` this will not spawn dedicated threads, but only works when running on the
+    /// multi-threaded Tokio runtime.
+    #[derive(Clone)]
+    pub struct TokioThreadpoolGaiResolver(());
+
+    pub struct TokioThreadpoolGaiFuture {
+        name: Name,
+    }
+
+    impl Resolve for TokioThreadpoolGaiResolver {
+        type Addrs = GaiAddrs;
+        type Future = TokioThreadpoolGaiFuture;
+
+        fn resolve(&self, name: Name) -> TokioThreadpoolGaiFuture {
+            TokioThreadpoolGaiFuture { name }
+        }
+    }
+
+    impl Future for TokioThreadpoolGaiFuture {
+        type Item = GaiAddrs;
+        type Error = io::Error;
+
+        fn poll(&mut self) -> Poll<GaiAddrs, io::Error> {
+            match tokio_threadpool::blocking(|| (self.name.as_str(), 0).to_socket_addrs()) {
+                Ok(Async::Ready(Ok(iter))) => Ok(Async::Ready(GaiAddrs { inner: IpAddrs { iter } })),
+                Ok(Async::Ready(Err(e))) => Err(e),
+                Ok(Async::NotReady) => Ok(Async::NotReady),
+                Err(e) => Err(io::Error::new(io::ErrorKind::Other, e)),
+            }
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::net::{Ipv4Addr, Ipv6Addr};

src/lib.rs

@@ -31,6 +31,7 @@ extern crate time;
 #[cfg(feature = "runtime")] extern crate tokio_reactor;
 #[cfg(feature = "runtime")] extern crate tokio_tcp;
 #[cfg(feature = "runtime")] extern crate tokio_timer;
+#[cfg(feature = "runtime")] extern crate tokio_threadpool;
 extern crate want;
 
 #[cfg(all(test, feature = "nightly"))]