feat(server): use an environment variable for configuring the thread

pool size while falling back to num_cpus if it is not present

Cargo.toml

@@ -15,6 +15,7 @@ travis-ci = { repository = "steveklabnik/simple-server" }
 http = "0.1.0"
 httparse = "1.2.3"
 log = "0.3"
+num_cpus = "1"
 scoped_threadpool = "0.1.7"
 
 [dev-dependencies]

src/lib.rs

@@ -28,6 +28,7 @@ extern crate log;
 
 extern crate http;
 extern crate httparse;
+extern crate num_cpus;
 extern crate scoped_threadpool;
 
 pub use http::Request;
@@ -38,6 +39,7 @@ pub use http::response::Builder as ResponseBuilder;
 
 use scoped_threadpool::Pool;
 
+use std::env;
 use std::fs::File;
 use std::io::prelude::*;
 use std::net::{TcpListener, TcpStream};
@@ -117,7 +119,8 @@ impl Server {
     /// }
     /// ```
     pub fn listen(&self, host: &str, port: &str) {
-        let mut pool = Pool::new(4);
+        let num_threads = self.pool_size();
+        let mut pool = Pool::new(num_threads);
         let listener =
             TcpListener::bind(format!("{}:{}", host, port)).expect("Error starting the server.");
 
@@ -136,6 +139,22 @@ impl Server {
         }
     }
 
+    // Try and fetch the environment variable NUM_THREADS and parse it as a u32. If this fails
+    // we fall back to using the num_cpus crate.
+    fn pool_size(&self) -> u32 {
+        const NUM_THREADS: &str = "SIMPLESERVER_THREADS";
+        let logical_cores = num_cpus::get() as u32;
+
+        match env::var(NUM_THREADS) {
+            Ok(v) => {
+                v.parse::<u32>().unwrap_or(logical_cores)
+            }
+            Err(_) => {
+                logical_cores
+            }
+        }
+    }
+
     fn handle_connection(&self, mut stream: TcpStream) -> Result<(), Error> {
         let mut buffer = [0; 512];