Add cache for image info list

Preparing the images list involves reading a directory (which might be
slow on non-local directories) and opening every file to read the exif
data. To avoid having to do this on every request, we do it every 60s in
a background job and only read from the cache on requests.

src/main.rs

@@ -24,11 +24,12 @@ static GLOBAL: jemallocator::Jemalloc = jemallocator::Jemalloc;
 // * Polish the css
 // * Add configuration file
 // * Support for headlines
-// * Cache generated images (+ cache cleanup)
+// * Image cache cleanup
 
 type TemplateEngine = Engine<minijinja::Environment<'static>>;
 type ImageDir = PathBuf;
 type SecretKey = [u8; 64];
+type ImageListCache = Arc<RwLock<Vec<ImageInfo>>>;
 type ImageCache = Arc<RwLock<HashMap<OsString, (DateTime<Utc>, Vec<u8>)>>>;
 
 #[derive(Clone, extract::FromRef)]
@@ -36,6 +37,7 @@ struct ApplicationState {
     engine: TemplateEngine,
     image_cache: ImageCache,
     image_dir: ImageDir,
+    image_list_cache: ImageListCache,
     secret_key: SecretKey,
 }
 
@@ -56,6 +58,15 @@ async fn main() {
         .with(tracing_formatter)
         .init();
 
+    let image_dir = PathBuf::from(image_path);
+    let image_cache = ImageCache::default();
+    let image_list_cache = ImageListCache::default();
+
+    tokio::spawn(update_image_list_cache_job(
+        image_dir.clone(),
+        image_list_cache.clone()
+    ));
+
     let mut jinja = minijinja::Environment::new();
     jinja.add_template("base", include_str!("../templates/base.html")).unwrap();
     jinja.add_template("index", include_str!("../templates/index.html")).unwrap();
@@ -76,8 +87,9 @@ async fn main() {
         .layer(session_layer)
         .with_state(ApplicationState {
             engine: Engine::from(jinja),
-            image_cache: Default::default(),
-            image_dir: PathBuf::from(image_path),
+            image_cache,
+            image_dir,
+            image_list_cache,
             secret_key,
         });
 
@@ -89,7 +101,33 @@ async fn main() {
         .unwrap();
 }
 
-#[derive(Debug, Serialize, Default)]
+async fn update_image_list_cache_job(image_dir: ImageDir, image_cache: ImageListCache) {
+    let mut interval = tokio::time::interval(tokio::time::Duration::from_secs(60));
+    interval.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Skip);
+
+    loop {
+        interval.tick().await;
+
+        let image_dir = image_dir.clone();
+        let images = task::spawn_blocking(move || {
+            // TODO: Only update images with changed modification times
+            read_images(&image_dir)
+        }).await
+            .unwrap_or_else(|error| {
+                tracing::error!("Could not read images due to panic: {:#}", error);
+                Ok(Vec::new())
+            })
+            .unwrap_or_else(|error| {
+                tracing::error!("Could not read images: {:#}", error);
+                Vec::new()
+            });
+
+        tracing::debug!("{} images in the image list cache", images.len());
+        *image_cache.write().unwrap() = images;
+    }
+}
+
+#[derive(Debug, Serialize, Default, Clone)]
 pub struct ImageInfo {
     width: u32,
     height: u32,
@@ -108,19 +146,19 @@ pub struct IndexTempalte {
 
 async fn index(
     engine: TemplateEngine,
-    State(image_dir): State<ImageDir>,
+    State(image_list_cache): State<ImageListCache>,
     State(secret_key): State<SecretKey>,
     session: ReadableSession,
 ) -> Result<impl IntoResponse, AppError> {
     let logged_in = session.get::<()>("logged_in").is_some();
 
     if logged_in {
-        let images = read_images(&image_dir)?;
+        let images = image_list_cache.read().unwrap();
 
         // Encrypt image names
         let chacha_key = chacha20poly1305::Key::from_slice(&secret_key[0..32]);
         let chacha = XChaCha20Poly1305::new(chacha_key);
-        let mut images = images.into_iter().map(|mut image_info| {
+        let mut images = images.iter().cloned().map(|mut image_info| {
             let nonce = XChaCha20Poly1305::generate_nonce(&mut OsRng);
             let mut ciphertext = chacha.encrypt(&nonce, image_info.name.as_bytes())?;