A base::flat_map/absl::flat_hash_map, or a shared “operation in progress” flag, is reached from two threads — the GPU IO thread and the GPU main thread, several async pipeline-creation threads, or multiple WebUSB DeviceImpl connections to one physical device. Access is unsynchronized.

A concurrent insert or erase triggers a rehash/reallocation while another thread is mid-read, dangling its iterator or handle → use-after-free / OOB in the GPU or device process. The flag variant races across per-connection instances: a destroyed connection leaks the shared flag as permanently “in progress”, wedging the device.

Wrap the container in a lock and route every access through it (base::Lock + GUARDED_BY, or MutexProtected<...> with Use/ConstUse); resolve create-races by destroying the losing duplicate; and track such state per-instance, clearing it on destruction.