From 3abc45cb867ba85400e324e09c32b2c805905daa Mon Sep 17 00:00:00 2001
From: Andras Schmelczer <andras@schmelczer.dev>
Date: Tue, 10 Mar 2026 20:38:07 +0000
Subject: [PATCH] Add wasm leak detector & fix leak

---
 reconcile-js/src/index.test.ts         | 12 +++++
 reconcile-js/src/index.ts              | 18 ++++++--
 reconcile-js/src/wasm-leak-detector.ts | 63 ++++++++++++++++++++++++++
 3 files changed, 88 insertions(+), 5 deletions(-)
 create mode 100644 reconcile-js/src/wasm-leak-detector.ts

diff --git a/reconcile-js/src/index.test.ts b/reconcile-js/src/index.test.ts
index 1a4394f..0de924c 100644
--- a/reconcile-js/src/index.test.ts
+++ b/reconcile-js/src/index.test.ts
@@ -1,10 +1,22 @@
 import { reconcile, reconcileWithHistory, diff, undiff } from './index';
+import { installWasmLeakDetector, checkForWasmLeaks } from './wasm-leak-detector';
 import * as fs from 'fs';
 import * as path from 'path';
 import { fileURLToPath } from 'url';
 
 const __dirname = path.dirname(fileURLToPath(import.meta.url));
 
+installWasmLeakDetector();
+
+afterEach(() => {
+  const leaks = checkForWasmLeaks();
+  if (leaks.length > 0) {
+    throw new Error(
+      `WASM memory leak: ${leaks.length} object(s) not freed:\n  ${leaks.join('\n  ')}`
+    );
+  }
+});
+
 describe('reconcile', () => {
   it('call reconcile without cursors', () => {
     expect(reconcile('Hello', 'Hello world', 'Hi world').text).toEqual('Hi world');
diff --git a/reconcile-js/src/index.ts b/reconcile-js/src/index.ts
index 3fa998c..006e5d1 100644
--- a/reconcile-js/src/index.ts
+++ b/reconcile-js/src/index.ts
@@ -325,9 +325,15 @@ function toWasmCursorPosition({ id, position }: CursorPosition): wasmCursorPosit
 }
 
 function toTextWithCursors(textWithCursor: wasmTextWithCursors): TextWithCursors {
+  const wasmCursors = textWithCursor.cursors();
+  const cursors = wasmCursors.map(toCursorPosition);
+  for (const cursor of wasmCursors) {
+    cursor.free();
+  }
+
   return {
     text: textWithCursor.text(),
-    cursors: textWithCursor.cursors().map(toCursorPosition),
+    cursors,
   };
 }
 
@@ -338,9 +344,11 @@ function toCursorPosition(cursor: wasmCursorPosition): CursorPosition {
   };
 }
 
-function toSpanWithHistory(textWithHistory: wasmSpanWithHistory): SpanWithHistory {
-  return {
-    text: textWithHistory.text(),
-    history: textWithHistory.history(),
+function toSpanWithHistory(span: wasmSpanWithHistory): SpanWithHistory {
+  const result = {
+    text: span.text(),
+    history: span.history(),
   };
+  span.free();
+  return result;
 }
diff --git a/reconcile-js/src/wasm-leak-detector.ts b/reconcile-js/src/wasm-leak-detector.ts
new file mode 100644
index 0000000..a8bcec4
--- /dev/null
+++ b/reconcile-js/src/wasm-leak-detector.ts
@@ -0,0 +1,63 @@
+/**
+ * Test utility for detecting WASM memory leaks.
+ *
+ * wasm-bindgen registers every JS-side object with a `FinalizationRegistry`.
+ * This detector patches `FinalizationRegistry.prototype.register` to collect
+ * references to all WASM objects. After each test, {@link checkForWasmLeaks}
+ * inspects `__wbg_ptr` on every tracked object - a non-zero pointer means
+ * `.free()` was never called, i.e. a leak.
+ *
+ * Install once (before any WASM calls) and call {@link checkForWasmLeaks}
+ * in an `afterEach` hook.
+ */
+
+let trackedObjects: object[] = [];
+let originalRegister: Function | null = null;
+
+interface WasmBindgenObject {
+  __wbg_ptr: number;
+  constructor: { name?: string };
+}
+
+function isWasmBindgenObject(target: unknown): target is WasmBindgenObject {
+  return (
+    target !== null &&
+    typeof target === 'object' &&
+    '__wbg_ptr' in (target as Record<string, unknown>)
+  );
+}
+
+/**
+ * Patches `FinalizationRegistry.prototype.register` to track all wasm-bindgen
+ * objects. Safe to call multiple times (idempotent).
+ */
+export function installWasmLeakDetector(): void {
+  if (originalRegister) return;
+
+  originalRegister = FinalizationRegistry.prototype.register;
+
+  FinalizationRegistry.prototype.register = function (
+    target: object,
+    heldValue: unknown,
+    unregisterToken?: object
+  ) {
+    if (isWasmBindgenObject(target)) {
+      trackedObjects.push(target);
+    }
+    return originalRegister!.call(this, target, heldValue, unregisterToken);
+  };
+}
+
+/**
+ * Returns any tracked WASM objects whose `__wbg_ptr` is still non-zero
+ * (i.e. `.free()` was never called). Clears the tracked set afterwards.
+ */
+export function checkForWasmLeaks(): string[] {
+  const leaks = trackedObjects
+    .filter(isWasmBindgenObject)
+    .filter((obj) => obj.__wbg_ptr !== 0)
+    .map((obj) => `${obj.constructor?.name ?? 'Unknown'} (ptr=${obj.__wbg_ptr})`);
+
+  trackedObjects = [];
+  return leaks;
+}