fix potential memory leak in Concast, add tests (#11358)

Author: phryneas

src/testing/matchers/index.d.ts

@@ -43,6 +43,10 @@ interface ApolloCustomMatchers<R = void, T = {}> {
     | ProfiledHook<any, any>
     ? (count: number, options?: NextRenderOptions) => Promise<R>
     : { error: "matcher needs to be called on a ProfiledComponent instance" };
+
+  toBeGarbageCollected: T extends WeakRef<any>
+    ? () => Promise<R>
+    : { error: "matcher needs to be called on a WeakRef instance" };
 }
 
 declare global {

src/testing/matchers/index.ts

@@ -2,10 +2,12 @@ import { expect } from "@jest/globals";
 import { toMatchDocument } from "./toMatchDocument.js";
 import { toHaveSuspenseCacheEntryUsing } from "./toHaveSuspenseCacheEntryUsing.js";
 import { toRerender, toRenderExactlyTimes } from "./ProfiledComponent.js";
+import { toBeGarbageCollected } from "./toBeGarbageCollected.js";
 
 expect.extend({
   toHaveSuspenseCacheEntryUsing,
   toMatchDocument,
   toRerender,
   toRenderExactlyTimes,
+  toBeGarbageCollected,
 });

src/testing/matchers/toBeGarbageCollected.ts

@@ -0,0 +1,59 @@
+import type { MatcherFunction } from "expect";
+
+// this is necessary because this file is picked up by `tsc` (it's not a test),
+// but our main `tsconfig.json` doesn't include `"ES2021.WeakRef"` on purpose
+declare class WeakRef<T extends WeakKey> {
+  constructor(target: T);
+  deref(): T | undefined;
+}
+
+export const toBeGarbageCollected: MatcherFunction<[weakRef: WeakRef<any>]> =
+  async function (actual) {
+    const hint = this.utils.matcherHint("toBeGarbageCollected");
+
+    if (!(actual instanceof WeakRef)) {
+      throw new Error(
+        hint +
+          "\n\n" +
+          `Expected value to be a WeakRef, but it was a ${typeof actual}.`
+      );
+    }
+
+    let pass = false;
+    let interval: NodeJS.Timeout | undefined;
+    let timeout: NodeJS.Timeout | undefined;
+    await Promise.race([
+      new Promise<void>((resolve) => {
+        timeout = setTimeout(resolve, 1000);
+      }),
+      new Promise<void>((resolve) => {
+        interval = setInterval(() => {
+          global.gc!();
+          pass = actual.deref() === undefined;
+          if (pass) {
+            resolve();
+          }
+        }, 1);
+      }),
+    ]);
+
+    clearInterval(interval);
+    clearTimeout(timeout);
+
+    return {
+      pass,
+      message: () => {
+        if (pass) {
+          return (
+            hint +
+            "\n\n" +
+            "Expected value to not be cache-collected, but it was."
+          );
+        }
+
+        return (
+          hint + "\n\n Expected value to be cache-collected, but it was not."
+        );
+      },
+    };
+  };