Firebase Unity SDK – Native crash in Realtime Database (uS WebSocket thread)

[darren-beckett]

Firebase Unity SDK – Native crash in Realtime Database (uS WebSocket thread)

Summary

Unity Editor crashes with a native crash in FirebaseCppApp-13_7_0.dll (Windows) when using Firebase Realtime Database with multiple ValueChanged and ChildAdded/ChildRemoved listeners. The crash occurs on a worker thread in the uS (µWebSockets) layer, not in C#.

Environment

• Unity: 6000.3.0f1 (Unity 6)
• Firebase Unity SDK: 13.7.0 (Realtime Database)
• Platform: Windows 10 (64-bit), Unity Editor
• Scripting: .NET / Mono

Crash details

• Module: FirebaseCppApp-13_7_0.dll
• Thread: Worker thread (stack shows BaseThreadInitThunk / RtlUserThreadStart)
• Native stack (representative frames):
  • uS::TLS::Context::operator bool
  • uS::Node::...
  • uS::Socket::write
  • uS::Socket::freeMessage

So the failure is in the C++ WebSocket/TLS code, not in managed code.

Reproduction

1. Open a Unity project with Firebase Realtime Database (SDK 13.7.0).
2. Load a games list from the database and build UI for each game.
3. For each game, attach:
   • DatabaseReference.ValueChanged
   • DatabaseReference.ChildAdded (e.g. on a /Players child ref)
   • DatabaseReference.ChildRemoved
4. Trigger a “refresh” that:
   • Removes all listeners,
   • Fetches a new snapshot (GetValueAsync / equivalent),
   • Rebuilds the UI,
   • Re-attaches the same listeners for each game (either in one frame or over a few frames).
5. The crash can occur during or shortly after re-attaching many listeners, or when many ValueChanged callbacks are processed. It is intermittent but reproducible under load.

Mitigations tried (C# side)

• Coalescing UI rebuilds to at most one per frame.
• Not attaching listeners while a “refresh” is in progress.
• Deferring listener attachment by one frame after the UI rebuild.
• Staggering listener attachment (e.g. 5 games per frame).
• Delaying the single coalesced UI rebuild by one frame.
• In Editor only: adding an extra 2.5 s delay after the UI rebuild before starting listener attachment.

These reduce frequency but do not eliminate the crash; the failure remains in native code.

Request

Please check whether this uS WebSocket/TLS path is subject to known race conditions or use-after-free when many listeners are added/removed or when many callbacks are delivered, and whether a fix or workaround is available or planned for the Unity SDK.