🐛 fix(vm): memory leak in _stackFrameFreeList

packages/vm/src/engine/runtime.ts

@@ -10,7 +10,7 @@ import Sequencer from './sequencer';
 import execute from './execute';
 import ScratchBlocksConstants from './scratch-blocks-constants';
 import TargetType from '../extension-support/target-type';
-import Thread from './thread';
+import Thread, {clearStackFrameFreeList} from './thread';
 import log from '../util/log';
 import maybeFormatMessage from '../util/maybe-format-message';
 import StageLayering from './stage-layering';
@@ -2059,6 +2059,7 @@ class Runtime extends EventEmitter<RuntimeEvents> {
         });
 
         this.targets.map(this.disposeTarget, this);
+        clearStackFrameFreeList();
         this._monitorState = OrderedMap({});
         this.emit(Runtime.RUNTIME_DISPOSED);
         this.ioDevices.clock.resetProjectTimer();
@@ -2163,6 +2164,7 @@ class Runtime extends EventEmitter<RuntimeEvents> {
             if (disposingTarget !== target) return true;
             // Allow target to do dispose actions.
             target.dispose();
+            clearStackFrameFreeList(disposingTarget);
             // Remove from list of targets.
             return false;
         });

packages/vm/src/engine/thread.ts

@@ -41,10 +41,25 @@ class _StackFrame {
      * A context passed to block implementations.
      */
     executionContext: unknown = null;
+    /**
+     * The target of blocks that this stack frame will execute.
+     * Stored as a weak reference to make GC happy.
+     */
+    #target: WeakRef<RenderedTarget> | null = null;
+
     /**
      * The target of blocks that this thread will execute.
+     *
+     * Note that the stack frame only holds the weak reference to the target,
+     * so it may be null if the target has been garbage collected.
      */
-    target: RenderedTarget | null = null;
+    get target (): RenderedTarget | null {
+        return this.#target?.deref() ?? null;
+    }
+
+    set target (value: RenderedTarget | null) {
+        this.#target = value ? new WeakRef(value) : null;
+    }
 
     /**
      * @param warpMode Whether this level is in warp mode.  Is set by some legacy blocks and
@@ -412,6 +427,25 @@ class Thread {
     }
 }
 
+/**
+ * Clear the stack frame recycle bin.
+ * @param target If provided, only remove frames referencing this target or whose target is no longer alive.
+ * If falsy, clear the entire free list.
+ */
+function clearStackFrameFreeList (target?: RenderedTarget) {
+    if (!target) {
+        _stackFrameFreeList.length = 0;
+        return;
+    }
+    for (let i = _stackFrameFreeList.length - 1; i >= 0; --i) {
+        const frameTarget = _stackFrameFreeList[i].target;
+        if (!frameTarget || frameTarget === target) {
+            _stackFrameFreeList.splice(i, 1);
+        }
+    }
+}
+
 export type {Thread, _StackFrame as ThreadStackFrame};
+export {clearStackFrameFreeList};
 
 export default Thread;