Cache the current Rust version.

Cargo.toml

@@ -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"

src/main.rs

@@ -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/rust_version.rs

@@ -1,12 +1,39 @@
 use reqwest;
+use std::sync::RwLock;
+use std::thread;
+use std::time::Instant;
 use toml;
 
+lazy_static! {
+    static ref CACHE: RwLock<Option<(String, Instant)>> = RwLock::new(None);
+}
+
+const CACHE_TTL_SECS: u64 = 120;
+
 pub fn rust_version() -> Option<String> {
+    cached_rust_version().or_else(fetch_rust_version)
+}
+
+fn cached_rust_version() -> Option<String> {
+    let cached = CACHE.read().unwrap();
+    let (version, timestamp) = cached.as_ref()?;
+    if timestamp.elapsed().as_secs() > CACHE_TTL_SECS {
+        // Update the cache in the background.
+        thread::spawn(fetch_rust_version);
+    }
+    Some(version.clone())
+}
+
+fn fetch_rust_version() -> Option<String> {
     let manifest = reqwest::get("https://static.rust-lang.org/dist/channel-rust-stable.toml")
         .ok()?
         .text()
         .ok()?;
     let manifest = manifest.parse::<toml::Value>().ok()?;
-    let rust_version = manifest["pkg"]["rust"]["version"].as_str()?.to_string();
-    Some(rust_version[..rust_version.find(' ')?].to_string())
+    let rust_version = manifest["pkg"]["rust"]["version"].as_str()?;
+    let version = rust_version[..rust_version.find(' ')?].to_string();
+
+    // Update the cache.
+    *CACHE.write().unwrap() = Some((version.clone(), Instant::now()));
+    Some(version)
 }