feat: implement & test open_at

host/src/component.rs

@@ -298,7 +298,7 @@ impl WasmComponentInstance {
         let component = component.hydrate(&engine)?;
 
         // resource/mem limiter
-        let mut limiter = Limiter::new(permissions.resource_limits.clone(), memory_pool);
+        let limiter = Limiter::new(permissions.resource_limits.clone(), memory_pool);
 
         // Create in-memory VFS
         let vfs_state = VfsState::new(permissions.vfs.clone(), limiter.clone());

host/src/limiter.rs

@@ -78,7 +78,7 @@ impl Limiter {
     }
 
     /// Grow memory usage.
-    pub(crate) fn grow(&mut self, bytes: usize) -> Result<(), GrowthError> {
+    pub(crate) fn grow(&self, bytes: usize) -> Result<(), GrowthError> {
         let mut self_guard = self
             .memory_reservation
             .lock()
@@ -89,6 +89,18 @@ impl Limiter {
         })
     }
 
+    /// Shrink memory usage.
+    pub(crate) fn shrink(&self, bytes: usize) -> Result<usize, GrowthError> {
+        let mut self_guard = self
+            .memory_reservation
+            .lock()
+            .expect("memory reservation lock poisoned");
+        self_guard.try_shrink(bytes).map_err(|e| {
+            log::debug!("failed to shrink memory: {e}");
+            GrowthError(e)
+        })
+    }
+
     /// Get current allocation size.
     pub(crate) fn size(&self) -> usize {
         self.memory_reservation