Summary:
Right now, when two threads require two NativeModules, both threads fight for the same `std::mutex`. Why? Because NativeModule require can read from/write to the shared `std::unordered_map<std::string, TurboModuleHolder*>`.
**A Few Thoughts:**
- All threads should be able to read from the TurboModule cache concurrently, without issue. Only writes into the cache require exclusive access. *With our current setup, both reads and writes acquire exclusive access.*
- During the lifetime of an application, there will only be tens of NativeModule create events (i.e: writes into the TurboModule cache). However, there may be hundreds, if not thousands of TurboModule cache lookups. *We don't need to serialize those hundreds/thousands of TurboModule cache reads.*
This is a potential optimization opportunity for the TurboModule infra.
## Changes
This diff introduces a `std::shared_mutex` inside `RCTTurboModuleManager`. We use either the regular `std::mutex` or the `std::shared_mutex`, depending on whether `RCTTurboModuleSharedMutexInitEnabled()` is `YES`.
Changelog: [Internal]
Reviewed By: fkgozali
Differential Revision: D23413118
fbshipit-source-id: 0880413c691b141db98a2715648f0c3e05983307
Ramanpreet Nara
f5c246d50e