feat(client): add Resolve, used by HttpConnector

This introduces a `Resolve` trait to describe asynchronous DNS
resolution. The `HttpConnector` can be configured with a resolver,
allowing a user to still use all the functionality of the
`HttpConnector`, while customizing the DNS resolution.

To prevent a breaking change, the `HttpConnector` has its `Resolve`
generic set by default to `GaiResolver`. This is same as the existing
resolver, which uses `getaddrinfo` inside a thread pool.

Closes #1517

Author: seanmonstar

Diff for: src/client/connect/dns.rs

@@ -1,45 +1,183 @@
-use std::io;
+use std::{fmt, io, vec};
 use std::net::{
-    Ipv4Addr, Ipv6Addr,
+    IpAddr, Ipv4Addr, Ipv6Addr,
     SocketAddr, ToSocketAddrs,
     SocketAddrV4, SocketAddrV6,
 };
-use std::vec;
+use std::sync::Arc;
 
-use ::futures::{Async, Future, Poll};
+use futures::{Async, Future, Poll};
+use futures::future::{Executor, ExecuteError};
+use futures::sync::oneshot;
+use futures_cpupool::{Builder as CpuPoolBuilder};
 
-pub struct Work {
+use self::sealed::GaiTask;
+
+/// Resolve a hostname to a set of IP addresses.
+pub trait Resolve {
+    /// The set of IP addresses to try to connect to.
+    type Addrs: Iterator<Item=IpAddr>;
+    /// A Future of the resolved set of addresses.
+    type Future: Future<Item=Self::Addrs, Error=io::Error>;
+    /// Resolve a hostname.
+    fn resolve(&self, name: Name) -> Self::Future;
+}
+
+/// A domain name to resolve into IP addresses.
+pub struct Name {
+    host: String,
+}
+
+/// A resolver using blocking `getaddrinfo` calls in a threadpool.
+#[derive(Clone)]
+pub struct GaiResolver {
+    executor: GaiExecutor,
+}
+
+pub struct GaiAddrs {
+    inner: IpAddrs,
+}
+
+pub struct GaiFuture {
+    rx: oneshot::SpawnHandle<IpAddrs, io::Error>,
+}
+
+impl Name {
+    pub(super) fn new(host: String) -> Name {
+        Name {
+            host,
+        }
+    }
+
+    /// View the hostname as a string slice.
+    pub fn as_str(&self) -> &str {
+        &self.host
+    }
+}
+
+impl fmt::Debug for Name {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Debug::fmt(&self.host, f)
+    }
+}
+
+impl GaiResolver {
+    /// Construct a new `GaiResolver`.
+    ///
+    /// Takes number of DNS worker threads.
+    pub fn new(threads: usize) -> Self {
+        let pool = CpuPoolBuilder::new()
+            .name_prefix("hyper-dns")
+            .pool_size(threads)
+            .create();
+        GaiResolver::new_with_executor(pool)
+    }
+
+    /// Construct a new `GaiResolver` with a shared thread pool executor.
+    ///
+    /// Takes an executor to run blocking `getaddrinfo` tasks on.
+    pub fn new_with_executor<E: 'static>(executor: E) -> Self
+    where
+        E: Executor<GaiTask> + Send + Sync,
+    {
+        GaiResolver {
+            executor: GaiExecutor(Arc::new(executor)),
+        }
+    }
+}
+
+impl Resolve for GaiResolver {
+    type Addrs = GaiAddrs;
+    type Future = GaiFuture;
+
+    fn resolve(&self, name: Name) -> Self::Future {
+        let blocking = GaiBlocking::new(name.host);
+        let rx = oneshot::spawn(blocking, &self.executor);
+        GaiFuture {
+            rx,
+        }
+    }
+}
+
+impl fmt::Debug for GaiResolver {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("GaiResolver")
+    }
+}
+
+impl Future for GaiFuture {
+    type Item = GaiAddrs;
+    type Error = io::Error;
+
+    fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
+        let addrs = try_ready!(self.rx.poll());
+        Ok(Async::Ready(GaiAddrs {
+            inner: addrs,
+        }))
+    }
+}
+
+impl fmt::Debug for GaiFuture {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("GaiFuture")
+    }
+}
+
+impl Iterator for GaiAddrs {
+    type Item = IpAddr;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.inner.next().map(|sa| sa.ip())
+    }
+}
+
+impl fmt::Debug for GaiAddrs {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("GaiAddrs")
+    }
+}
+
+#[derive(Clone)]
+struct GaiExecutor(Arc<Executor<GaiTask> + Send + Sync>);
+
+impl Executor<oneshot::Execute<GaiBlocking>> for GaiExecutor {
+    fn execute(&self, future: oneshot::Execute<GaiBlocking>) -> Result<(), ExecuteError<oneshot::Execute<GaiBlocking>>> {
+        self.0.execute(GaiTask { work: future })
+            .map_err(|err| ExecuteError::new(err.kind(), err.into_future().work))
+    }
+}
+
+pub(super) struct GaiBlocking {
     host: String,
-    port: u16
 }
 
-impl Work {
-    pub fn new(host: String, port: u16) -> Work {
-        Work { host: host, port: port }
+impl GaiBlocking {
+    pub(super) fn new(host: String) -> GaiBlocking {
+        GaiBlocking { host }
     }
 }
 
-impl Future for Work {
+impl Future for GaiBlocking {
     type Item = IpAddrs;
     type Error = io::Error;
 
     fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
-        debug!("resolving host={:?}, port={:?}", self.host, self.port);
-        (&*self.host, self.port).to_socket_addrs()
+        debug!("resolving host={:?}", self.host);
+        (&*self.host, 0).to_socket_addrs()
             .map(|i| Async::Ready(IpAddrs { iter: i }))
     }
 }
 
-pub struct IpAddrs {
+pub(super) struct IpAddrs {
     iter: vec::IntoIter<SocketAddr>,
 }
 
 impl IpAddrs {
-    pub fn new(addrs: Vec<SocketAddr>) -> Self {
+    pub(super) fn new(addrs: Vec<SocketAddr>) -> Self {
         IpAddrs { iter: addrs.into_iter() }
     }
 
-    pub fn try_parse(host: &str, port: u16) -> Option<IpAddrs> {
+    pub(super) fn try_parse(host: &str, port: u16) -> Option<IpAddrs> {
         if let Ok(addr) = host.parse::<Ipv4Addr>() {
             let addr = SocketAddrV4::new(addr, port);
             return Some(IpAddrs { iter: vec![SocketAddr::V4(addr)].into_iter() })
@@ -51,7 +189,7 @@ impl IpAddrs {
         None
     }
 
-    pub fn split_by_preference(self) -> (IpAddrs, IpAddrs) {
+    pub(super) fn split_by_preference(self) -> (IpAddrs, IpAddrs) {
         let preferring_v6 = self.iter
             .as_slice()
             .first()
@@ -64,7 +202,7 @@ impl IpAddrs {
         (IpAddrs::new(preferred), IpAddrs::new(fallback))
     }
 
-    pub fn is_empty(&self) -> bool {
+    pub(super) fn is_empty(&self) -> bool {
         self.iter.as_slice().is_empty()
     }
 }
@@ -77,6 +215,30 @@ impl Iterator for IpAddrs {
     }
 }
 
+// Make this Future unnameable outside of this crate.
+pub(super) mod sealed {
+    use super::*;
+    // Blocking task to be executed on a thread pool.
+    pub struct GaiTask {
+        pub(super) work: oneshot::Execute<GaiBlocking>
+    }
+
+    impl fmt::Debug for GaiTask {
+        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+            f.pad("GaiTask")
+        }
+    }
+
+    impl Future for GaiTask {
+        type Item = ();
+        type Error = ();
+
+        fn poll(&mut self) -> Poll<(), ()> {
+            self.work.poll()
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::net::{Ipv4Addr, Ipv6Addr};