Memory leak when reloading the app with Results/List Object listeners

[dkpalmer]

Goals

Use Results/List Object listeners during development of a React Native app. This requires restarting the simulator or force-closing the app on device after a certain number of reloads.

Expected Results

Reloading the app during development (DEV mode) should not leak.

Actual Results

Memory leaks when dev-reloading the app with listeners on RealmResults and List objects. It does not leak on listeners on the Realm instance.

Steps to Reproduce

• Call addListener on either a RealmResults object or a List object.
• Reload the app (CMD+R)
• See that memory climb

Code Sample

I created a fresh React Native app to demonstrate the issue at https://github.com/dkpalmer/realm-rn-reload-leak/. Inside App.js there are 3 cases within componentWillMount:

Does not leak when reloading: (1) Realm listener

Does leak when reloading: (2) Results listener and (3) List object listener

Assuming you have the RN CLI and Xcode installed, app can be run by cloning, installing dependencies, and running react-native run-ios.

Version of Realm and Tooling

• Realm JS SDK Version: 2.27.0 and 2.28.0
• Node or React Native: RN 0.59
• Client OS & Version: iOS
• Which debugger for React Native: DEV Perf Monitor

[dkpalmer]

Also, I have tried looking at this from the RN side about any sort of cleanup opportunities before the bundle gets reloaded. Unfortunately, it doesn't look like there is a good option on that side: facebook/react-native#9773

[dkpalmer]

Quick bump. Does anyone have any insights on the difference between a Realm Listener and a Results/Object listener and why the latter two would retain after reloading the JavaScript bundle, and/or suggestions on how to work around this?

[pedrobertao]

I have similar a problem with my project. On Android when we started to use listeners the App would just crash after a couple of reloads.

[kneth]

From facebook/react-native#9773 it sounds virtually impossible to make a solution which works every time.

Removing all listeners in componentDidMount() (before everything else) might be a work-around.

[dkpalmer]

@kneth Do you have insights why Results and List object listeners do not leak?

[kneth]

@dkpalmer Results and List are based by Realm (the underlaying database engine) so they are native (C++) resources. Such resources are probably reclaimed by the operating system - even at reload. But the listener lives in the JS engine, and seems to survive a reload. I have label this issue as a bug as I believe listeners should be reclaimed too.

[SDSLeon]

Still an issue. Found that old JSContext is not destroyed when addListener is present on Results;

[bmunkholm]

This may be resolved with our Hermes support. Will have to verify.

[kneth]

We have released v11.0.0 which has a complete rewrite of the integration with JSC. I am closing the issue, and ask you to upgrade. If the issue is still observed, please create a new issue.