Fabric: Using RW-like shared lock in ShadowTreeRegistry

Summary:
`ShadowTree` class is already thread-safe, so we don't need to (we should not) guard concurent access to it.
We should guard concurrent write-access to the collection of them though.
Note that unordered_map is "thread-compatible" collection, so concurrent reading access is okay.

Reviewed By: mdvacca

Differential Revision: D13269745

fbshipit-source-id: 4779626018da0e42b81a835e538f6c1d1a8e25f7

ReactCommon/fabric/uimanager/ShadowTreeRegistry.cpp

@@ -9,13 +9,15 @@ namespace facebook {
 namespace react {
 
 void ShadowTreeRegistry::add(std::unique_ptr<ShadowTree> &&shadowTree) const {
-  std::lock_guard<std::mutex> lock(mutex_);
+  std::unique_lock<folly::SharedMutex> lock(mutex_);
+
   registry_.emplace(shadowTree->getSurfaceId(), std::move(shadowTree));
 }
 
 std::unique_ptr<ShadowTree> ShadowTreeRegistry::remove(
     SurfaceId surfaceId) const {
-  std::lock_guard<std::mutex> lock(mutex_);
+  std::unique_lock<folly::SharedMutex> lock(mutex_);
+
   auto iterator = registry_.find(surfaceId);
   auto shadowTree = std::unique_ptr<ShadowTree>(iterator->second.release());
   registry_.erase(iterator);
@@ -25,7 +27,8 @@ std::unique_ptr<ShadowTree> ShadowTreeRegistry::remove(
 bool ShadowTreeRegistry::get(
     SurfaceId surfaceId,
     std::function<void(const ShadowTree &shadowTree)> callback) const {
-  std::lock_guard<std::mutex> lock(mutex_);
+  std::shared_lock<folly::SharedMutex> lock(mutex_);
+
   auto iterator = registry_.find(surfaceId);
 
   if (iterator == registry_.end()) {

ReactCommon/fabric/uimanager/ShadowTreeRegistry.h

@@ -5,6 +5,9 @@
 
 #pragma once
 
+#include <folly/SharedMutex.h>
+#include <shared_mutex>
+
 #include <react/core/ReactPrimitives.h>
 #include <react/uimanager/ShadowTree.h>
 
@@ -46,7 +49,7 @@ class ShadowTreeRegistry final {
       std::function<void(const ShadowTree &shadowTree)> callback) const;
 
  private:
-  mutable std::mutex mutex_;
+  mutable folly::SharedMutex mutex_;
   mutable std::unordered_map<SurfaceId, std::unique_ptr<ShadowTree>>
       registry_; // Protected by `mutex_`.
 };