Optimize RequestData shallow copying

Summary:
The shallow copy code path currently iterates and copies all values manually. Instead of benefiting from the underlying data collection's implementation of self-copy.
At the very least, it's simpler looking code.

Inheriting unique_ptr is usually discouraged, but in this specific (internal impl detail) use case, I can't foresee any risk.

Reviewed By: yfeldblum

Differential Revision: D15599260

fbshipit-source-id: 73edea27a27e8fbc8b057cf3e974ee3d83a6e760

folly/io/async/Request.cpp

@@ -282,15 +282,8 @@ RequestContext::setShallowCopyContext() {
   auto child = std::make_shared<RequestContext>();
 
   if (parent) {
-    auto ret = folly::acquireLocked(as_const(parent->state_), child->state_);
-    auto& parentLock = std::get<0>(ret);
-    auto& childLock = std::get<1>(ret);
-    childLock->callbackData_ = parentLock->callbackData_;
-    childLock->requestData_.reserve(parentLock->requestData_.size());
-    for (const auto& entry : parentLock->requestData_) {
-      childLock->requestData_.insert(std::make_pair(
-          entry.first, RequestData::constructPtr(entry.second.get())));
-    }
+    auto locks = folly::acquireLocked(as_const(parent->state_), child->state_);
+    *std::get<1>(locks) = *std::get<0>(locks);
   }
 
   // Do not use setContext to avoid global set/unset

folly/io/async/Request.h

@@ -98,7 +98,16 @@ class RequestData {
   struct DestructPtr {
     void operator()(RequestData* ptr);
   };
-  using SharedPtr = std::unique_ptr<RequestData, DestructPtr>;
+  struct SharedPtr : public std::unique_ptr<RequestData, DestructPtr> {
+    SharedPtr() = default;
+    using std::unique_ptr<RequestData, DestructPtr>::unique_ptr;
+    SharedPtr(const SharedPtr& other) : SharedPtr(constructPtr(other.get())) {}
+    SharedPtr& operator=(const SharedPtr& other) {
+      return operator=(constructPtr(other.get()));
+    }
+    SharedPtr(SharedPtr&&) = default;
+    SharedPtr& operator=(SharedPtr&&) = default;
+  };
 
   // Initialize the pseudo-shared ptr, increment the counter
   static SharedPtr constructPtr(RequestData* ptr);