Introduce HandlerCallbackPtr alias

Summary:
This introduces the following aliases:
- `HandlerCallbackPtr<T>` → `std::unique_ptr<HandlerCallback<T>>`
- `HandlerCallbackBase::Ptr` → `std::unique_ptr<HandlerCallbackBase`
- `HandlerCallback<T>::Ptr` (same as `HandlerCallbackPtr<T>`)

This allows a no-effect codemod after which we can change the underlying type from `unique_ptr` to something similar to `folly::Executor::KeepAlive`. Such a change would effectively allow shared ownership rather than unique ownership and consequently let us invoke asynchronous functionality safely in `HandlerCallback::result` (required for `ServiceInterceptor::onResponse`).

Reviewed By: iahs

Differential Revision: D55932261

fbshipit-source-id: b58fe5cb839d8f20eca55fae5abcdda9dfabaa55

thrift/lib/cpp2/async/AsyncProcessor.h

@@ -1164,6 +1164,12 @@ class ServerInterface : public virtual AsyncProcessorFactory,
   void setNameOverride(std::string name) { nameOverride_ = std::move(name); }
 };
 
+template <class T>
+class HandlerCallback;
+
+template <class T>
+using HandlerCallbackPtr = std::unique_ptr<HandlerCallback<T>>;
+
 /**
  * HandlerCallback class for async callbacks.
  *
@@ -1201,6 +1207,8 @@ class HandlerCallbackBase {
   }
 
  public:
+  using Ptr = std::unique_ptr<HandlerCallbackBase>;
+
   HandlerCallbackBase() : eb_(nullptr), reqCtx_(nullptr), protoSeqId_(0) {}
 
   HandlerCallbackBase(
@@ -1366,6 +1374,7 @@ class HandlerCallback : public HandlerCallbackBase {
   using cob_ptr = typename Helper::CobPtr;
 
  public:
+  using Ptr = std::unique_ptr<HandlerCallback<T>>;
   using ResultType = std::decay_t<typename Helper::InputType>;
 
  public:
@@ -1413,6 +1422,7 @@ class HandlerCallback<void> : public HandlerCallbackBase {
   using cob_ptr = SerializedResponse (*)(ContextStack*);
 
  public:
+  using Ptr = std::unique_ptr<HandlerCallback<void>>;
   using ResultType = void;
 
   HandlerCallback() : cp_(nullptr) {}
@@ -1466,6 +1476,8 @@ template <typename InteractionIf, typename Response>
 class HandlerCallback<TileAndResponse<InteractionIf, Response>> final
     : public HandlerCallback<Response> {
  public:
+  using Ptr = std::unique_ptr<HandlerCallback>;
+
   void result(TileAndResponse<InteractionIf, Response>&& r) {
     if (this->fulfillTilePromise(std::move(r.tile))) {
       if constexpr (!std::is_void_v<Response>) {