Don't block the reactor on DNS

postgres-native-tls/src/lib.rs

@@ -49,13 +49,13 @@
 #![warn(rust_2018_idioms, clippy::all, missing_docs)]
 #![feature(async_await)]
 
+use std::future::Future;
+use std::pin::Pin;
 use tokio_io::{AsyncRead, AsyncWrite};
 #[cfg(feature = "runtime")]
 use tokio_postgres::tls::MakeTlsConnect;
 use tokio_postgres::tls::{ChannelBinding, TlsConnect};
 use tokio_tls::TlsStream;
-use std::pin::Pin;
-use std::future::Future;
 
 #[cfg(test)]
 mod test;
@@ -111,7 +111,9 @@ where
 {
     type Stream = TlsStream<S>;
     type Error = native_tls::Error;
-    type Future = Pin<Box<dyn Future<Output = Result<(TlsStream<S>, ChannelBinding), native_tls::Error>> + Send>>;
+    type Future = Pin<
+        Box<dyn Future<Output = Result<(TlsStream<S>, ChannelBinding), native_tls::Error>> + Send>,
+    >;
 
     fn connect(self, stream: S) -> Self::Future {
         let future = async move {

postgres-native-tls/src/test.rs

@@ -1,7 +1,7 @@
+use futures::{FutureExt, TryStreamExt};
 use native_tls::{self, Certificate};
 use tokio::net::TcpStream;
 use tokio_postgres::tls::TlsConnect;
-use futures::{FutureExt, TryStreamExt};
 
 #[cfg(feature = "runtime")]
 use crate::MakeTlsConnector;
@@ -44,7 +44,8 @@ async fn require() {
     smoke_test(
         "user=ssl_user dbname=postgres sslmode=require",
         TlsConnector::new(connector, "localhost"),
-    ).await;
+    )
+    .await;
 }
 
 #[tokio::test]
@@ -58,7 +59,8 @@ async fn prefer() {
     smoke_test(
         "user=ssl_user dbname=postgres",
         TlsConnector::new(connector, "localhost"),
-    ).await;
+    )
+    .await;
 }
 
 #[tokio::test]
@@ -72,7 +74,8 @@ async fn scram_user() {
     smoke_test(
         "user=scram_user password=password dbname=postgres sslmode=require",
         TlsConnector::new(connector, "localhost"),
-    ).await;
+    )
+    .await;
 }
 
 #[tokio::test]

postgres/src/config.rs

@@ -277,8 +277,12 @@ impl Config {
         });
         match &self.executor {
             Some(executor) => {
-                executor.lock().unwrap().spawn(Box::pin(connection)).unwrap();
-            },
+                executor
+                    .lock()
+                    .unwrap()
+                    .spawn(Box::pin(connection))
+                    .unwrap();
+            }
             None => {
                 RUNTIME.spawn(connection);
             }

tokio-postgres/src/connect_socket.rs

@@ -1,9 +1,12 @@
 use crate::config::Host;
 use crate::{Error, Socket};
+use std::vec;
+use futures::channel::oneshot;
+use futures::future;
 use std::future::Future;
-use std::io;
 use std::net::{IpAddr, SocketAddr, ToSocketAddrs};
 use std::time::Duration;
+use std::{io, thread};
 use tokio::net::TcpStream;
 #[cfg(unix)]
 use tokio::net::UnixStream;
@@ -23,10 +26,7 @@ pub(crate) async fn connect_socket(
                     // avoid dealing with blocking DNS entirely if possible
                     vec![SocketAddr::new(ip, port)].into_iter()
                 }
-                Err(_) => {
-                    // FIXME what do?
-                    (&**host, port).to_socket_addrs().map_err(Error::connect)?
-                }
+                Err(_) => dns(host, port).await.map_err(Error::connect)?,
             };
 
             let mut error = None;
@@ -64,6 +64,25 @@ pub(crate) async fn connect_socket(
     }
 }
 
+async fn dns(host: &str, port: u16) -> io::Result<vec::IntoIter<SocketAddr>> {
+    // if we're running on a threadpool, use its blocking support
+    if let Ok(r) =
+        future::poll_fn(|_| tokio_threadpool::blocking(|| (host, port).to_socket_addrs())).await
+    {
+        return r;
+    }
+
+    // FIXME what should we do here?
+    let (tx, rx) = oneshot::channel();
+    let host = host.to_string();
+    thread::spawn(move || {
+        let addrs = (&*host, port).to_socket_addrs();
+        let _ = tx.send(addrs);
+    });
+
+    rx.await.unwrap()
+}
+
 async fn connect_with_timeout<F, T>(connect: F, timeout: Option<Duration>) -> Result<T, Error>
 where
     F: Future<Output = io::Result<T>>,