Android: data race on ArgConverter's process-wide long-conversion cache when workers start or stop concurrently #93

Summary

ArgConverter::s_type_long_operations_cache is a single, process-wide robin_hood::unordered_map. Each runtime finds, inserts and erases its own entry from its own thread, and nothing synchronizes those accesses:

• find / insert: GetTypeLongCache(), on the first long conversion in that runtime.
• erase: onDisposeEnv() / onDisposeRuntime(), when the runtime is disposed.

When several workers start or stop at once, one thread can rehash the table while another is reading or writing it. That is a data race on the map, and in practice a heap use-after-free.

Affects both binding layers:

• NativeScript/ffi/jni/napi/conversion/ArgConverter.{h,cpp}: the map is keyed by napi_env.
• NativeScript/ffi/jni/jsi/conversion/ArgConverter.{h,cpp}: the map is keyed by rt.identity().

How it was found

A worker stress test, run while testing #91 and #92 on an API 36 arm64 emulator. HWASan was enabled locally by uncommenting the -fsanitize=hwaddress lines in platforms/android/test-app/runtime/CMakeLists.txt; it is off by default. The test:

• 12 workers at a time are created and terminated after 20–300 ms;
• each worker churns Java objects and forces JS and Java GC;
• the main runtime forces GC every 25 ms.

HWASan reported a memory error inside ArgConverter::GetTypeLongCache, in both the napi and jsi trees. On the old code (without #91), the crash happened in 1 of 3 napi runs and 1 of 2 jsi runs. It is independent of the GC-delivery race that #91 fixes.

Expected

Each runtime's cache is reachable only by that runtime, and creating or disposing one runtime can't corrupt another's entry. Possible fixes:

• make the cache per-runtime state, reached the way MetadataNode's per-runtime cache is;
• or guard the map with a mutex. This is cheap: a lookup happens once per long conversion, and the pointer could be cached after that.

Environment

• Branch: android-react-native at 64c7fed
• Emulator: Android API 36, arm64, debug build of platforms/android/test-app with HWASan enabled locally
• Engine: V8