coro async-stack: make AsyncStackFrame and AsyncStackRoot immobile

Summary:
Make `AsyncStackFrame` and `AsyncStackRoot` immobile by explicitly deleting
copy/move constructors and assignment operators. These structures are
pointed-to by other parts of the async stack infrastructure, so moving them
after they are set up would invalidate pointers and corrupt the async stack
trace.

Changes:

- **AsyncStackFrame**: Delete copy/move ctor/assign, add `clear()` member
  function that resets all members to their default state (as an alternative
  to re-assignment).
- **AsyncStackRoot**: Delete copy/move ctor/assign, add explicit default
  constructor. (AsyncStackRoot was already implicitly non-copyable/non-movable
  due to its `std::atomic` member, but making it explicit is better practice.)
- **AsyncStack.cpp**: Refactor `makeDetachedRootFrame()` which returned
  `AsyncStackFrame` by value. Now uses a static variable with a lambda-based
  initializer to avoid requiring move semantics.
- **AsyncGenerator.h**: The `CleanupAwaitable` class previously embedded an
  `AsyncStackFrame` as a direct member, but needed to be movable for the
  `co_withAsyncStack` customization point. Introduced a nested `Awaiter` class
  returned by `CleanupAwaitable::operator co_await()` that holds the
  `AsyncStackFrame` as a direct member. This avoids heap allocation (no
  `unique_ptr`/`make_unique`) while keeping `CleanupAwaitable` itself movable,
  since the immobile `AsyncStackFrame` now lives in the awaiter which is
  materialized directly in the coroutine frame via guaranteed copy elision.
  Also replaced `asyncFrame_ = {}` in `clearContext()` with
  `asyncFrame_.clear()`.

All changes applied to both `fbcode/folly/` and `xplat/folly/` copies.

Reviewed By: iahs

Differential Revision: D95416569

fbshipit-source-id: 9c78fdf04c688193ec9db2c94acb081546f75d01

Author: yfeldblum

third-party/folly/src/folly/coro/AsyncGenerator.h

@@ -255,24 +255,34 @@ class [[nodiscard]] AsyncGenerator {
 
   class [[nodiscard]] CleanupAwaitable {
    public:
-    bool await_ready() noexcept { return !scopeExit_; }
-
-    template <typename Promise>
-    FOLLY_NOINLINE auto await_suspend(
-        coroutine_handle<Promise> continuation) noexcept {
-      asyncFrame_.setReturnAddress();
-      scopeExit_.promise().setContext(
-          continuation, &asyncFrame_, executor_.get_alias());
-      if constexpr (detail::promiseHasAsyncFrame_v<Promise>) {
-        folly::pushAsyncStackFrameCallerCallee(
-            continuation.promise().getAsyncFrame(), asyncFrame_);
-        return scopeExit_;
-      } else {
-        folly::resumeCoroutineWithNewAsyncStackRoot(scopeExit_);
+    struct Awaiter {
+      bool await_ready() noexcept { return !scopeExit_; }
+
+      template <typename Promise>
+      FOLLY_NOINLINE auto await_suspend(
+          coroutine_handle<Promise> continuation) noexcept {
+        asyncFrame_.setReturnAddress();
+        scopeExit_.promise().setContext(
+            continuation, &asyncFrame_, executor_.get_alias());
+        if constexpr (detail::promiseHasAsyncFrame_v<Promise>) {
+          folly::pushAsyncStackFrameCallerCallee(
+              continuation.promise().getAsyncFrame(), asyncFrame_);
+          return scopeExit_;
+        } else {
+          folly::resumeCoroutineWithNewAsyncStackRoot(scopeExit_);
+        }
       }
-    }
 
-    void await_resume() noexcept {}
+      void await_resume() noexcept {}
+
+      coroutine_handle<detail::ScopeExitTaskPromiseBase> scopeExit_;
+      folly::Executor::KeepAlive<> executor_;
+      folly::AsyncStackFrame asyncFrame_;
+    };
+
+    Awaiter operator co_await() && noexcept {
+      return Awaiter{scopeExit_, std::move(executor_), {}};
+    }
 
    private:
     friend CleanupSemiAwaitable;
@@ -288,7 +298,6 @@ class [[nodiscard]] AsyncGenerator {
     }
 
     coroutine_handle<detail::ScopeExitTaskPromiseBase> scopeExit_;
-    folly::AsyncStackFrame asyncFrame_;
     folly::Executor::KeepAlive<> executor_;
   };
 
@@ -784,7 +793,7 @@ class AsyncGeneratorPromise final
     executor_ = {};
     cancelToken_ = {};
     hasCancelTokenOverride_ = false;
-    asyncFrame_ = {};
+    asyncFrame_.clear();
   }
 
   friend coroutine_handle<ScopeExitTaskPromiseBase> tag_invoke(

third-party/folly/src/folly/coro/CMakeLists.txt

@@ -34,7 +34,6 @@ folly_add_library(
     folly_coro_auto_cleanup_fwd
     folly_coro_base_promise
     folly_coro_coroutine
-    folly_coro_current_executor
     folly_coro_detail_malloc
     folly_coro_detail_manual_lifetime
     folly_coro_invoke
@@ -149,6 +148,7 @@ folly_add_library(
     folly_coro_via_if_async
     folly_coro_with_async_stack
     folly_executors_manual_executor
+    folly_executors_sequenced_executor
     folly_fibers_core
     folly_lang_must_use_immediately
     folly_synchronization_baton
@@ -371,6 +371,7 @@ folly_add_library(
   HEADERS
     GmockHelpers.h
   EXPORTED_DEPS
+    folly_coro_blocking_wait
     folly_coro_coroutine
     folly_coro_gtest_helpers
     folly_coro_result
@@ -455,6 +456,7 @@ folly_add_library(
   EXPORTED_DEPS
     folly_coro_via_if_async
     folly_exception_wrapper
+    folly_portability
 )
 
 folly_add_library(
@@ -625,7 +627,6 @@ folly_add_library(
     folly_cancellation_token
     folly_coro_base_promise
     folly_coro_coroutine
-    folly_coro_current_executor
     folly_coro_detail_malloc
     folly_coro_detail_traits
     folly_coro_inline_task
@@ -742,7 +743,11 @@ folly_add_library(
   EXPORTED_DEPS
     folly_coro_task_wrapper
     folly_coro_via_if_async
+    folly_lang_assume
+    folly_result_result
+    folly_result_try
     folly_result_value_only_result
+    folly_unit
 )
 
 folly_add_library(

third-party/folly/src/folly/tracing/AsyncStack-inl.h

@@ -102,6 +102,12 @@ inline void AsyncStackFrame::setParentFrame(AsyncStackFrame& frame) noexcept {
   parentFrame = &frame;
 }
 
+inline void AsyncStackFrame::clear() noexcept {
+  parentFrame = nullptr;
+  instructionPointer = nullptr;
+  stackRoot = nullptr;
+}
+
 inline AsyncStackRoot* AsyncStackFrame::getStackRoot() noexcept {
   return stackRoot;
 }

third-party/folly/src/folly/tracing/AsyncStack.cpp

@@ -181,16 +181,13 @@ AsyncStackRoot& getCurrentAsyncStackRoot() noexcept {
   return *root;
 }
 
-static AsyncStackFrame makeDetachedRootFrame() noexcept {
-  AsyncStackFrame frame;
-  frame.setReturnAddress(detached_task());
-  return frame;
+AsyncStackFrame::AsyncStackFrame(MakeDetachedRootFrame) noexcept {
+  setReturnAddress(detached_task());
 }
 
-static AsyncStackFrame detachedRootFrame = makeDetachedRootFrame();
-
 AsyncStackFrame& getDetachedRootAsyncStackFrame() noexcept {
-  return detachedRootFrame;
+  static auto frame = AsyncStackFrame{AsyncStackFrame::MakeDetachedRootFrame{}};
+  return frame;
 }
 
 #if FOLLY_HAS_COROUTINES

third-party/folly/src/folly/tracing/AsyncStack.h

@@ -317,6 +317,13 @@ void deactivateSuspendedLeaf(AsyncStackFrame& leafFrame) noexcept;
 struct AsyncStackFrame {
  public:
   AsyncStackFrame() = default;
+  ~AsyncStackFrame() = default;
+  AsyncStackFrame(const AsyncStackFrame&) = delete;
+  AsyncStackFrame(AsyncStackFrame&&) = delete;
+  AsyncStackFrame& operator=(const AsyncStackFrame&) = delete;
+  AsyncStackFrame& operator=(AsyncStackFrame&&) = delete;
+
+  void clear() noexcept;
 
   // The parent frame is the frame of the async operation that is logically
   // the caller of this frame.
@@ -343,6 +350,9 @@ struct AsyncStackFrame {
   void* getReturnAddress() const noexcept;
 
  private:
+  struct MakeDetachedRootFrame {};
+  explicit AsyncStackFrame(MakeDetachedRootFrame) noexcept;
+
   friend AsyncStackRoot;
 
   friend AsyncStackFrame& getDetachedRootAsyncStackFrame() noexcept;
@@ -414,6 +424,13 @@ struct AsyncStackFrame {
 // list of contexts) is obtained by calling getCurrentAsyncStackRoot().
 struct AsyncStackRoot {
  public:
+  AsyncStackRoot() = default;
+  ~AsyncStackRoot() = default;
+  AsyncStackRoot(const AsyncStackRoot&) = delete;
+  AsyncStackRoot(AsyncStackRoot&&) = delete;
+  AsyncStackRoot& operator=(const AsyncStackRoot&) = delete;
+  AsyncStackRoot& operator=(AsyncStackRoot&&) = delete;
+
   // Sets the top-frame to be 'frame' and also updates the cached
   // 'frame.stackRoot' to be 'this'.
   //