Prefetch nearby scene assets in the background

crates/libremetaverse/CHANGES.md

@@ -37,3 +37,11 @@ Files: `src/grid_manager.rs`, `src/client_core.rs`.
   the existing UDP asset transfer path instead of reporting a missing asset immediately.
 
 File: `src/asset_manager.rs`.
+
+## Deterministic persistent-asset LRU
+
+- Successful immutable asset-cache reads explicitly refresh the file modification timestamp.
+  Pruning uses that timestamp as its cross-platform LRU order instead of filesystem access time,
+  which can be disabled or coarsened by `noatime` and `relatime` mounts.
+
+Files: `src/asset_cache.rs`, `src/asset_pipeline_semantics.rs`.

crates/libremetaverse/src/asset_cache.rs

@@ -189,6 +189,10 @@ impl AssetCache {
         if bytes.len() as u64 != metadata.len() {
             return Err(Error::InvalidOperation);
         }
+        // Use the modification timestamp as a portable, explicit LRU marker.
+        // Filesystems commonly mount with relatime/noatime, so access time is
+        // not reliable enough for cache eviction.
+        let _ = fs::File::open(path).and_then(|file| file.set_modified(SystemTime::now()));
         Ok(Some(bytes))
     }
 
@@ -353,10 +357,7 @@ impl AssetCache {
                     entries.push((
                         entry.path(),
                         metadata.len(),
-                        metadata
-                            .accessed()
-                            .or_else(|_| metadata.modified())
-                            .unwrap_or(UNIX_EPOCH),
+                        metadata.modified().unwrap_or(UNIX_EPOCH),
                     ));
                 }
             }

crates/libremetaverse/src/asset_pipeline_semantics.rs

@@ -137,13 +137,16 @@ fn custom_cache_naming_is_used_but_cannot_escape_the_cache_directory() {
 #[tokio::test]
 async fn cache_pruning_reclaims_old_entries_to_below_the_limit() {
     let (_client, cache, path) = fixture(12);
+    let first = UUID::random().unwrap();
+    let second = UUID::random().unwrap();
     cache
-        .save_asset_to_cache_with_uuid_bytes(UUID::random().unwrap(), vec![1; 8])
+        .save_asset_to_cache_with_uuid_bytes(first, vec![1; 8])
         .unwrap();
-    std::thread::sleep(std::time::Duration::from_millis(5));
+    std::thread::sleep(std::time::Duration::from_millis(20));
     cache
-        .save_asset_to_cache_with_uuid_bytes(UUID::random().unwrap(), vec![2; 8])
+        .save_asset_to_cache_with_uuid_bytes(second, vec![2; 8])
         .unwrap();
+    cache.get_cached_asset_bytes_with_uuid(first).unwrap();
     cache.prune(None).await.unwrap();
     let bytes: u64 = fs::read_dir(&path)
         .unwrap()
@@ -152,5 +155,7 @@ async fn cache_pruning_reclaims_old_entries_to_below_the_limit() {
         .map(|metadata| metadata.len())
         .sum();
     assert!(bytes <= 10);
+    assert!(path.join(first.to_string()).is_file());
+    assert!(!path.join(second.to_string()).exists());
     fs::remove_dir_all(path).unwrap();
 }