Make pool idle timeout configurable (#866)

hyper’s ClientBuilder has an option to define the idle timeout of the
connection pool. As it’s quite useful to be able to modify this value,
this patch extends reqwest’s ClientBuilder to expose that option as
well. The default value of 90 seconds is taken from hyper.

src/async_impl/client.rs

@@ -76,6 +76,7 @@ struct Config {
     certs_verification: bool,
     connect_timeout: Option<Duration>,
     connection_verbose: bool,
+    pool_idle_timeout: Option<Duration>,
     max_idle_per_host: usize,
     #[cfg(feature = "__tls")]
     identity: Option<Identity>,
@@ -125,6 +126,7 @@ impl ClientBuilder {
                 certs_verification: true,
                 connect_timeout: None,
                 connection_verbose: false,
+                pool_idle_timeout: Some(Duration::from_secs(90)),
                 max_idle_per_host: std::usize::MAX,
                 proxies: Vec::new(),
                 auto_sys_proxy: true,
@@ -304,6 +306,7 @@ impl ClientBuilder {
             builder.http2_initial_connection_window_size(http2_initial_connection_window_size);
         }
 
+        builder.pool_idle_timeout(config.pool_idle_timeout);
         builder.pool_max_idle_per_host(config.max_idle_per_host);
 
         if config.http1_title_case_headers {
@@ -596,6 +599,19 @@ impl ClientBuilder {
 
     // HTTP options
 
+    /// Set an optional timeout for idle sockets being kept-alive.
+    ///
+    /// Pass `None` to disable timeout.
+    ///
+    /// Default is 90 seconds.
+    pub fn pool_idle_timeout<D>(mut self, val: D) -> ClientBuilder
+    where
+        D: Into<Option<Duration>>,
+    {
+        self.config.pool_idle_timeout = val.into();
+        self
+    }
+
     /// Sets the maximum idle connection per host allowed in the pool.
     pub fn max_idle_per_host(mut self, max: usize) -> ClientBuilder {
         self.config.max_idle_per_host = max;

src/blocking/client.rs

@@ -296,6 +296,18 @@ impl ClientBuilder {
 
     // HTTP options
 
+    /// Set an optional timeout for idle sockets being kept-alive.
+    ///
+    /// Pass `None` to disable timeout.
+    ///
+    /// Default is 90 seconds.
+    pub fn pool_idle_timeout<D>(self, val: D) -> ClientBuilder
+    where
+        D: Into<Option<Duration>>,
+    {
+        self.with_inner(|inner| inner.pool_idle_timeout(val))
+    }
+
     /// Sets the maximum idle connection per host allowed in the pool.
     pub fn max_idle_per_host(self, max: usize) -> ClientBuilder {
         self.with_inner(move |inner| inner.max_idle_per_host(max))