perf(server): generate chunk baselines outside the cache lock

crates/server/src/chunk_cache.rs

@@ -26,18 +26,30 @@ impl ChunkCache {
     }
 
     /// Returns the baseline for `pos`, generating and caching it on a miss.
+    ///
+    /// Generation runs outside the lock, so concurrent callers do not serialise on a cache miss. Two callers racing on the same position may each generate a baseline; generation is deterministic and side-effect-free, so the duplicated work is redundant rather than incorrect, and is far cheaper than serialising every miss behind the store.
     #[must_use]
     pub fn get_or_generate(&self, pos: ChunkPos, generator: &VoxelGenerator) -> Chunk {
-        // A poisoned lock cannot yield a usable cache; recovering the guard lets generation proceed rather than propagating a panic across every worker that shares this cache.
-        let mut guard = self
-            .inner
-            .lock()
-            .unwrap_or_else(std::sync::PoisonError::into_inner);
-        if let Some(hit) = guard.get(&pos) {
-            return hit.clone();
+        // Probe under the lock, then release it before generating.
+        {
+            // A poisoned lock cannot yield a usable cache; recovering the guard lets generation proceed rather than propagating a panic across every worker that shares this cache.
+            let mut guard = self
+                .inner
+                .lock()
+                .unwrap_or_else(std::sync::PoisonError::into_inner);
+            if let Some(hit) = guard.get(&pos) {
+                return hit.clone();
+            }
         }
+
         let chunk = generator.generate_chunk(pos);
-        guard.put(pos, chunk.clone());
+
+        // Retake the lock only to publish. A concurrent caller may have inserted the same position in the meantime; overwriting is harmless because both baselines are identical.
+        self.inner
+            .lock()
+            .unwrap_or_else(std::sync::PoisonError::into_inner)
+            .put(pos, chunk.clone());
+
         chunk
     }
 }