Limit number of parallel image conversions

Tokio will happy start hundreds of parallel image conversions, which
overwhelms seaf_fuse, so limit the parallel conversion to 4 using a
semaphore.

src/main.rs

@@ -10,7 +10,7 @@ use error::AppError;
 use exif::Exif;
 use image::{imageops::FilterType, DynamicImage};
 use serde::{Serialize, Deserialize};
-use tokio::task;
+use tokio::{task, sync::Semaphore};
 use tower_http::{trace::{self, TraceLayer}, compression::CompressionLayer};
 use tracing::Level;
 use tracing_subscriber::{EnvFilter, layer::SubscriberExt, util::SubscriberInitExt};
@@ -34,6 +34,7 @@ type SecretKey = [u8; 64];
 type ImageListCache = Arc<RwLock<Vec<ImageInfo>>>;
 type ImageCache = Arc<RwLock<HashMap<OsString, (DateTime<Utc>, Vec<u8>)>>>;
 type Config = Arc<RwLock<config::Config>>;
+type CpuTaskLimit = Arc<Semaphore>;
 
 #[derive(Clone, extract::FromRef)]
 struct ApplicationState {
@@ -43,6 +44,7 @@ struct ApplicationState {
     image_dir: ImageDir,
     image_list_cache: ImageListCache,
     secret_key: SecretKey,
+    cpu_task_limit: CpuTaskLimit,
 }
 
 #[tokio::main]
@@ -102,6 +104,7 @@ async fn main() {
             image_dir,
             image_list_cache,
             secret_key,
+            cpu_task_limit: Arc::new(Semaphore::new(4)),
         });
 
     let addr = SocketAddr::from(([0, 0, 0, 0], 3000));
@@ -265,6 +268,7 @@ async fn converted_image(
     State(image_dir): State<ImageDir>,
     State(secret_key): State<SecretKey>,
     session: ReadableSession,
+    State(cpu_task_limit): State<CpuTaskLimit>,
 ) -> Result<impl IntoResponse, AppError> {
     session.get::<()>("logged_in")
         .ok_or(anyhow!("Trying to load image while not logged in!"))
@@ -308,6 +312,7 @@ async fn converted_image(
             image_buffer
         }
         None => {
+            let _cpu_task_permit = cpu_task_limit.clone().acquire_owned().await?;
             let image_buffer = task::spawn_blocking(move || {
                 convert_image(&image_path)
                     .with_context(|| format!("Could not convert image {:?}", image_path))