fix memory leak when panicking with detached ComponentInstance

In `ComponentInstance::from_vmctx` and
`StoreContextMut::with_detached_instance[_async]` we were leaking memory due to
having taken `InstanceData` out of the `Store` and making it unreachable except
via a raw pointer, meaning there was nothing responsible for dropping it on
panic.  This commit adds an RAII wrapper to take care of that.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Author: dicej

crates/wasmtime/src/runtime/component/concurrent.rs

@@ -685,6 +685,14 @@ impl fmt::Debug for WorkItem {
     }
 }
 
+struct DataDropper<'a>(&'a mut ComponentInstance);
+
+impl Drop for DataDropper<'_> {
+    fn drop(&mut self) {
+        drop(self.0.data.take());
+    }
+}
+
 impl<T> StoreContextMut<'_, T> {
     /// Temporarily split the specified store and instance into a "detached"
     /// state (i.e. clearing the pointers they have to each other) so that they
@@ -721,11 +729,12 @@ impl<T> StoreContextMut<'_, T> {
             // it and can take an exclusive reference to it.
             let instance = unsafe { &mut *data.instance_ptr() };
             assert!(instance.data.is_none());
-            instance.data = Some(data);
-            instance.set_store(None);
-            let result = fun(self.as_context_mut(), instance);
-            instance.set_store(Some(VMStoreRawPtr(self.traitobj())));
-            (result, instance.data.take())
+            let instance = DataDropper(instance);
+            instance.0.data = Some(data);
+            instance.0.set_store(None);
+            let result = fun(self.as_context_mut(), instance.0);
+            instance.0.set_store(Some(VMStoreRawPtr(self.traitobj())));
+            (result, instance.0.data.take())
         };
         if self.0[instance.0].is_none() {
             self.0[instance.0] = data;
@@ -753,10 +762,11 @@ impl<T> StoreContextMut<'_, T> {
             let instance = unsafe { &mut *data.instance_ptr() };
             assert!(instance.data.is_none());
             instance.data = Some(data);
-            instance.set_store(None);
-            let result = fun(self.as_context_mut(), instance).await;
-            instance.set_store(Some(VMStoreRawPtr(self.traitobj())));
-            (result, instance.data.take())
+            let instance = DataDropper(instance);
+            instance.0.set_store(None);
+            let result = fun(self.as_context_mut(), instance.0).await;
+            instance.0.set_store(Some(VMStoreRawPtr(self.traitobj())));
+            (result, instance.0.data.take())
         };
         if self.0[instance.0].is_none() {
             self.0[instance.0] = data;

crates/wasmtime/src/runtime/vm/component.rs

@@ -183,21 +183,29 @@ impl ComponentInstance {
         vmctx: NonNull<VMComponentContext>,
         f: impl FnOnce(&mut dyn VMStore, &mut ComponentInstance) -> R,
     ) -> R {
+        struct DataDropper<'a>(&'a mut ComponentInstance);
+
+        impl Drop for DataDropper<'_> {
+            fn drop(&mut self) {
+                drop(self.0.data.take());
+            }
+        }
+
         let mut ptr = vmctx
             .byte_sub(mem::size_of::<ComponentInstance>())
             .cast::<ComponentInstance>();
-        let reference = ptr.as_mut();
-        let store = &mut *reference.store();
-        if let Some(instance) = reference.instance {
-            assert!(reference.data.is_none());
-            reference.data = Some(store[instance.0].take().unwrap());
+        let reference = DataDropper(ptr.as_mut());
+        let store = &mut *reference.0.store();
+        if let Some(instance) = reference.0.instance {
+            assert!(reference.0.data.is_none());
+            reference.0.data = Some(store[instance.0].take().unwrap());
         }
-        reference.set_store(None);
-        let result = f(store, reference);
-        reference.set_store(Some(VMStoreRawPtr(store.traitobj())));
-        if let Some(instance) = reference.instance {
+        reference.0.set_store(None);
+        let result = f(store, reference.0);
+        reference.0.set_store(Some(VMStoreRawPtr(store.traitobj())));
+        if let Some(instance) = reference.0.instance {
             if store[instance.0].is_none() {
-                store[instance.0] = reference.data.take();
+                store[instance.0] = reference.0.data.take();
             }
         }
         result