perf: Unload iframe before removing (#3796)

To drastically reduce the amount of memory leaked if the iframe cannot
be garbage collected properly, this PR modifies `terminateJob` to
navigate the iframe away from the Snaps execution environment before
removing it, effectively unloading it.

Before:

```
Explicit Allocations

15.36 MB (100.0%) -- explicit
└──15.36 MB (100.0%) -- window-objects/top(none)/detached
   ├──15.26 MB (99.40%) -- window(http://localhost:6363/)
   │  ├───7.67 MB (49.97%) ++ js-realm(moz-nullprincipal:{45eed705-e0da-4e2b-b0bf-e7ac336c93f4}?http://localhost:6363, http://localhost:6363/)
   │  ├───5.76 MB (37.49%) ++ js-realm(moz-nullprincipal:{8cb3b86e-774f-4a1a-bf6a-eeac2636fe9f}?http://localhost:6363, http://localhost:6363/)
   │  ├───1.83 MB (11.94%) ++ dom
   │  └───0.00 MB (00.00%) ++ (2 tiny)
   └───0.09 MB (00.60%) ++ window(chrome://devtools/content/shared/webextension-fallback.html#webextension-flask@metamask.io)
```

After:

```
Explicit Allocations

0.14 MB (100.0%) -- explicit
└──0.14 MB (100.0%) -- window-objects/top(none)/detached/window(about:blank)
   ├──0.06 MB (46.17%) ++ js-realm(moz-nullprincipal:{0114dc8e-3f5e-4a2c-8008-c4219c84dcc0}?moz-extension://1465affa-73bf-4b77-9103-8afb21b2fa9d, about:blank)
   ├──0.06 MB (46.17%) ++ js-realm(moz-nullprincipal:{7e2fd14e-3040-493d-862a-9c207eae5213}?moz-extension://1465affa-73bf-4b77-9103-8afb21b2fa9d, about:blank)
   ├──0.01 MB (07.16%) ++ dom
   └──0.00 MB (00.50%) ++ (2 tiny)
```

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Improves termination flow and reduces iframe memory retention by
making job termination asynchronous and explicitly unloading iframes
before removal.
> 
> - Changes `AbstractExecutionService.terminateJob` to return a
`Promise<void>` and awaits it in `terminateSnap` and when init times out
in `#initStreams`
> - In `IframeExecutionService`, `terminateJob` now navigates the iframe
to `about:blank` (with a short `withTimeout`-guarded wait for `load`)
before removing it
> - Updates `NodeProcessExecutionService` and `WebViewExecutionService`
`terminateJob` implementations to async (no functional change beyond
signature)
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
c5ec278. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: FrederikBolding

packages/snaps-controllers/src/services/AbstractExecutionService.ts

@@ -153,7 +153,9 @@ export abstract class AbstractExecutionService<WorkerType>
    *
    * @param job - The object corresponding to the job to be terminated.
    */
-  protected abstract terminateJob(job: TerminateJobArgs<WorkerType>): void;
+  protected abstract terminateJob(
+    job: TerminateJobArgs<WorkerType>,
+  ): Promise<void>;
 
   /**
    * Terminates the Snap with the specified ID and deletes all its associated
@@ -197,7 +199,7 @@ export abstract class AbstractExecutionService<WorkerType>
       }
     });
 
-    this.terminateJob(job);
+    await this.terminateJob(job);
 
     this.#jobs.delete(snapId);
     this.#status.delete(snapId);
@@ -258,7 +260,7 @@ export abstract class AbstractExecutionService<WorkerType>
 
     if (result === hasTimedOut) {
       // For certain environments, such as the iframe we may have already created the worker and wish to terminate it.
-      this.terminateJob({ id: snapId });
+      await this.terminateJob({ id: snapId });
 
       const status = this.#status.get(snapId);
       if (status === 'created') {

packages/snaps-controllers/src/services/iframe/IframeExecutionService.ts

@@ -2,6 +2,7 @@ import type { BasePostMessageStream } from '@metamask/post-message-stream';
 import { WindowPostMessageStream } from '@metamask/post-message-stream';
 import { createWindow } from '@metamask/snaps-utils';
 
+import { withTimeout } from '../../utils';
 import type {
   ExecutionServiceArgs,
   TerminateJobArgs,
@@ -29,8 +30,28 @@ export class IframeExecutionService extends AbstractExecutionService<Window> {
     this.iframeUrl = iframeUrl;
   }
 
-  protected terminateJob(jobWrapper: TerminateJobArgs<Window>): void {
-    document.getElementById(jobWrapper.id)?.remove();
+  protected async terminateJob(
+    jobWrapper: TerminateJobArgs<Window>,
+  ): Promise<void> {
+    const iframe = document.getElementById(
+      jobWrapper.id,
+    ) as HTMLIFrameElement | null;
+
+    if (!iframe) {
+      return;
+    }
+
+    iframe.id = '';
+
+    await withTimeout(
+      new Promise<void>((resolve) => {
+        iframe.addEventListener('load', () => resolve(), { once: true });
+        iframe.src = 'about:blank';
+      }),
+      10,
+    );
+
+    iframe.remove();
   }
 
   protected async initEnvStream(snapId: string): Promise<{

packages/snaps-controllers/src/services/node-js/NodeProcessExecutionService.ts

@@ -36,7 +36,9 @@ export class NodeProcessExecutionService extends AbstractExecutionService<ChildP
     return Promise.resolve({ worker, stream });
   }
 
-  protected terminateJob(jobWrapper: TerminateJobArgs<ChildProcess>): void {
+  protected async terminateJob(
+    jobWrapper: TerminateJobArgs<ChildProcess>,
+  ): Promise<void> {
     jobWrapper.worker?.kill();
   }
 }

packages/snaps-controllers/src/services/webview/WebViewExecutionService.ts

@@ -53,7 +53,9 @@ export class WebViewExecutionService extends AbstractExecutionService<WebViewInt
     return { worker: webView, stream };
   }
 
-  protected terminateJob(jobWrapper: TerminateJobArgs<WebViewInterface>): void {
+  protected async terminateJob(
+    jobWrapper: TerminateJobArgs<WebViewInterface>,
+  ): Promise<void> {
     this.#removeWebView(jobWrapper.id);
   }
 }