2 files changed, 47 insertions, 3 deletions
diff --git a/base/Cargo.toml b/base/Cargo.toml
index 86675b9..f49db80 100644
--- a/base/Cargo.toml
+++ b/base/Cargo.toml
@@ -9,3 +9,5 @@ serde_json = "1.0.107"
 log = { workspace = true }
 sha2 = "0.10.8"
 base64 = "0.21.4"
+tokio = {workspace=true}
+anyhow = "1.0.75"
diff --git a/base/src/lib.rs b/base/src/lib.rs
index 46ab576..b97f924 100644
--- a/base/src/lib.rs
+++ b/base/src/lib.rs
@@ -6,7 +6,8 @@
 #![feature(lazy_cell)]
 use base64::Engine;
 use jellycommon::{config::GlobalConfig, AssetLocation};
-use std::{fs::File, path::PathBuf, sync::LazyLock};
+use std::{fs::File, future::Future, path::PathBuf, sync::LazyLock};
+use tokio::sync::Mutex;
 
 pub static CONF: LazyLock<GlobalConfig> = LazyLock::new(|| {
     serde_json::from_reader(
@@ -20,7 +21,7 @@ pub static CONF: LazyLock<GlobalConfig> = LazyLock::new(|| {
     .unwrap()
 });
 
-pub fn cache_file(seed: &[&str]) -> AssetLocation {
+pub fn cache_location(seed: &[&str]) -> (usize, AssetLocation) {
     use sha2::Digest;
     let mut d = sha2::Sha512::new();
     for s in seed {
@@ -28,9 +29,50 @@ pub fn cache_file(seed: &[&str]) -> AssetLocation {
         d.update(b"\0");
     }
     let d = d.finalize();
+    let n = d[0] as usize | (d[1] as usize) << 8 | (d[2] as usize) << 16 | (d[3] as usize) << 24;
     let fname = base64::engine::general_purpose::URL_SAFE.encode(d);
     let fname = &fname[..22]; // about 128 bits
-    AssetLocation::Cache(fname.into())
+    (n, AssetLocation::Cache(fname.into()))
+}
+
+const CACHE_GENERATION_BUCKET_COUNT: usize = 1024;
+pub static CACHE_GENERATION_LOCKS: LazyLock<[Mutex<()>; CACHE_GENERATION_BUCKET_COUNT]> =
+    LazyLock::new(|| [(); CACHE_GENERATION_BUCKET_COUNT].map(|_| Mutex::new(())));
+
+pub async fn async_cache_file<Fun, Fut>(
+    seed: &[&str],
+    generate: Fun,
+) -> Result<AssetLocation, anyhow::Error>
+where
+    Fun: FnOnce(tokio::fs::File) -> Fut,
+    Fut: Future<Output = Result<(), anyhow::Error>>,
+{
+    let (bucket, location) = cache_location(seed);
+    // we need a lock even if it exists since somebody might be still in the process of writing.
+    let _guard = CACHE_GENERATION_LOCKS[bucket % CACHE_GENERATION_BUCKET_COUNT].lock();
+    let exists = tokio::fs::try_exists(location.path()).await?;
+    if !exists {
+        let f = tokio::fs::File::create(location.path()).await?;
+        generate(f).await?;
+    }
+    drop(_guard);
+    Ok(location)
+}
+
+pub fn cache_file<Fun>(seed: &[&str], mut generate: Fun) -> Result<AssetLocation, anyhow::Error>
+where
+    Fun: FnMut(std::fs::File) -> Result<(), anyhow::Error>,
+{
+    let (bucket, location) = cache_location(seed);
+    // we need a lock even if it exists since somebody might be still in the process of writing.
+    let _guard = CACHE_GENERATION_LOCKS[bucket % CACHE_GENERATION_BUCKET_COUNT].lock();
+    let exists = location.path().exists();
+    if !exists {
+        let f = std::fs::File::create(location.path())?;
+        generate(f)?;
+    }
+    drop(_guard);
+    Ok(location)
 }
 
 pub trait AssetLocationExt {