Cache the current Rust version.

Currently, every time the home page is rendered, the web server
downloads a TOML file from static.rust-lang.org and parses it to find
the current Rust version.  This adds a significant amount of latency to
the home page.

This patch caches the version in memory for up to 10 minutes.  This
greatly speeds up home page rendering, at the cost of a 10-minute delay
before a new version is reflected on the home page.

Author: mbrubeck

Cargo.lock

@@ -4,6 +4,7 @@ version = "0.1.0"
 authors = ["Ashley Williams <[email protected]>"]
 
 [dependencies]
+lazy_static = "1.2.0"
 rand = "0.6"
 rocket = "0.4"
 serde = "1.0"

Cargo.toml

@@ -1,5 +1,7 @@
 #![feature(proc_macro_hygiene, decl_macro)]
 
+#[macro_use]
+extern crate lazy_static;
 extern crate rand;
 extern crate reqwest;
 #[macro_use]

src/main.rs

@@ -1,7 +1,31 @@
+use std::{
+    sync::RwLock,
+    time::Instant,
+};
 use reqwest;
 use toml;
 
+lazy_static! {
+    static ref CACHE: RwLock<Option<(String, Instant)>> = RwLock::new(None);
+}
+
+const CACHE_TTL_SECS: u64 = 600;
+
 pub fn rust_version() -> Option<String> {
+    // Use cached version if available.
+    if let Some((version, timestamp)) = &*CACHE.read().unwrap() {
+        if timestamp.elapsed().as_secs() < CACHE_TTL_SECS {
+            return Some(version.clone())
+        }
+    }
+
+    // Update the cache if it's missing or expired.
+    let version = fetch_rust_version()?;
+    *CACHE.write().unwrap() = Some((version.clone(), Instant::now()));
+    Some(version)
+}
+
+fn fetch_rust_version() -> Option<String> {
     let manifest = reqwest::get("https://static.rust-lang.org/dist/channel-rust-stable.toml")
         .ok()?
         .text()