1 Changes

1.1 R6

In revision 4 of this paper, Lewis Baker discovered a problem with using nest() as the basis operation for implementing spawn() (and spawn_future()) when the counting_scope that tracks the spawned work is being used to protect against out-of-lifetime accesses to the allocator provided to spawn(). Revision 5 of this paper raised Lewis’s concerns and presented several solutions. Revision 6 has selected the solution originally presented as “option 4”: define a new set of refcounting basis operations and define nest(), spawn(), and spawn_future() in terms of them.

1.1.1 The Problem

What follows is a description, taken from revision 5, section 6.5.1, of the problem with using nest() as the basis operation for implementing spawn() (a similar problem exists for spawn_future() but spawn() is simpler to explain).

When a spawned operation completes, the order of operations was as follows:

1. The spawned operation completes by invoking set_value() or set_stopped() on a receiver, rcvr, provided by spawn() to the nest-sender.
2. rcvr destroys the nest-sender’s operation-state by invoking its destructor.
3. rcvr deallocates the storage previously allocated for the just-destroyed operation-state using a copy of the allocator that was chosen when spawn() was invoked. Assume this allocator was passed to spawn() in the optional environment argument.

Note that in step 2, above, the destruction of the nest-sender’s operation-state has the side effect of decrementing the associated counting_scope’s count of outstanding operations. If the scope has a join-sender waiting and this decrement brings the count to zero, the code waiting on the join-sender to complete may start to destroy the allocator while step 3 is busy using it.

1.1.2 Some Solutions

Revision 5 presented the following possible solutions:

1. Do nothing; declare that counting_scope can’t be used to protect memory allocators.
2. Remove allocator support from spawn() and spawn_future() and require allocation with ::operator new.
3. Make spawn() and spawn_future() basis operations of async_scope_tokens (alongside nest()) so that the derement in step 2 can be deferred until after step 3 completes.
4. Define a new set of refcounting basis operations and define nest(), spawn(), and spawn_future() in terms of them.
5. Treat nest-senders as RAII handles to “scope references” and change how spawn() is defined to defer the decrement. (There are a few implementation possibilities here.)
6. Give async_scope_tokens a new basis operation that can wrap an allocator in a new allocator wrapper that increments the scope’s refcount in allocate() and decrements it in deallocate().

1.1.3 LEWG Discussion in St Louis

The authors opened the discussion by recommending option 6. By the end of the discussion, the authors’ expressed preferences were: “4 & 6 are better than 5; 5 is better than 3.” The biggest concern with option 4 was the time required to rework the paper in terms of the new basis operation.

The room took the following two straw polls:

1. In P3149R5 strike option 1 from 6.5.2 (option 1 would put the responsibility to coordinate the lifetime of the memory resource on the end user)

   SF	F	N	A	SA
   10	2	3	1	1

   Attendance: 21 in-person + 10 remote

   # of Authors: 2

   Authors’ position: 2x SF

   Outcome: Consensus in favor

   SA: I’m SA because I don’t think async scope needs to protect memory allocations or resources, it’s fine for this not to be a capability and I think adding this capability will add complexity, and that’ll mean it doesn’t make C++26.

2. In P3149R5 strike option 2 from 6.5.2 (option 2 would prevent spawn from supporting allocators)

   SF	F	N	A	SA
   8	4	2	2	0

   Attendance: 21 in-person + 10 remote

   # of Authors: 2

   Authors’ position: 2x SF

   Outcome: Consensus in favor

   WA: As someone who was weakly against I’m not ready to rule out this possibility yet.

Ultimately, the authors chose option 4, leading to revision 6 of the paper changing from this:

template <class Token, class Sender>
concept async_scope_token =
    copyable<Token> &&
    is_nothrow_move_constructible_v<Token> &&
    is_nothrow_move_assignable_v<Token> &&
    is_nothrow_copy_constructible_v<Token> &&
    is_nothrow_copy_assignable_v<Token> &&
    sender<Sender> &&
    requires(Token token, Sender&& snd) {
      { token.nest(std::forward<Sender>(snd)) } -> sender;
    };

with execution::nest() forwarding to the nest() method on the provided token and spawn() and spawn_future() being expressed in terms of nest(), to this:

template <class Assoc>
concept async_scope_association =
    semiregular<Assoc> &&
    requires(const Assoc& assoc) {
        { static_cast<bool>(assoc) } noexcept;
    };

template <class Token>
concept async_scope_token =
    copyable<Token> &&
    requires(Token token) {
        { token.try_associate() } -> async_scope_association;
    };

with nest(), spawn(), and spawn_future() all being expressed in terms of the async_scope_token concept.

1.2 R5

• Clarify that the nest-sender’s operation state must destroy its child operation state before decrementing the scope’s reference count.
• Add naming discussion.
• Discuss a memory allocator lifetime concern raised by Lewis Baker and several options for resolving it.

1.3 R4

• Permit caller of spawn_future() to provide a stop token in the optional environment argument.
• Remove [[nodiscard]].
• Make simple_counting_scope::token::token() and counting_scope::token::token() explicit and exposition-only.
• Remove redundant concept async_scope.
• Remove last vestiges of let_async_scope.
• Add some wording to a new Specification section

1.4 R3

• Update slide code to be exception safe
• Split the async scope concept into a scope and token; update counting_scope to match
• Rename counting_scope to simple_counting_scope and give the name counting_scope to a scope with a stop source
• Add example for recursively spawned work using let_async_scope and counting_scope

1.5 R2

• Update counting_scope::nest() to explain when the scope’s count of outstanding senders is decremented and remove counting_scope::joined(), counting_scope::join_started(), and counting_scope::use_count() on advice of SG1 straw poll:

  forward P3149R1 to LEWG for inclusion in C++26 after P2300 is included in C++26, with notes:

  1. the point of refcount decrement to be moved after the child operation state is destroyed
  2. a future paper should explore the design for cancellation of scopes
  3. observers (joined, join_started, use_count) can be removed

  SF	F	N	A	SA
  10	14	2	0	1

  Consensus

  SA: we are moving something without wide implementation experience, the version with experience has cancellation of scopes

• Add a fourth state to counting_scope so that it can be used as a data-member safely

1.6 R1

• Add implementation experience
• Incorporate pre-meeting feedback from Eric Niebler

1.7 R0

• First revision

2 Introduction

[P2300R7] lays the groundwork for writing structured concurrent programs in C++ but it leaves three important scenarios under- or unaddressed:

1. progressively structuring an existing, unstructured concurrent program;
2. starting a dynamic number of parallel tasks without “losing track” of them; and
3. opting in to eager execution of sender-shaped work when appropriate.

This paper describes the utilities needed to address the above scenarios within the following constraints:

• No detached work by default; as specified in [P2300R7], the start_detached and ensure_started algorithms invite users to start concurrent work with no built-in way to know when that work has finished.
  • Such so-called “detached work” is undesirable; without a way to know when detached work is done, it is difficult know when it is safe to destroy any resources referred to by the work. Ad hoc solutions to this shutdown problem add unnecessary complexity that can be avoided by ensuring all concurrent work is “attached”.
  • [P2300R7]’s introduction of structured concurrency to C++ will make async programming with C++ much easier but experienced C++ programmers typically believe that async C++ is “just hard” and that starting async work means starting detached work (even if they are not thinking about the distinction between attached and detached work) so adapting to a post-[P2300R7] world will require unlearning many deprecated patterns. It is thus useful as a teaching aid to remove the unnecessary temptation of falling back on old habits.
• No dependencies besides [P2300R7]; it will be important for the success of [P2300R7] that existing code bases can migrate from unstructured concurrency to structured concurrency in an incremental way so tools for progressively structuring code should not take on risk in the form of unnecessary dependencies.

The proposed solution comes in the following parts:

• template <class Assoc> concept async_scope_association;
• template <class Token> concept async_scope_token;
• sender auto nest(sender auto&& snd, async_scope_token auto token);
• void spawn(sender auto&& snd, async_scope_token auto token, auto&& env);
• sender auto spawn_future(sender auto&& snd, async_scope_token auto token, auto&& env);
• Proposed in [P3296R2]: sender auto let_async_scope(callable auto&& senderFactory);
• struct simple_counting_scope; and
• struct counting_scope.

2.1 Implementation experience

The general concept of an async scope to manage work has been deployed broadly at Meta. Code written with Folly’s coroutine library, [folly::coro], uses [folly::coro::AsyncScope] to safely launch awaitables. Most code written with Unifex, an implementation of an earlier version of the Sender/Receiver model proposed in [P2300R7], uses [unifex::v1::async_scope], although experience with the v1 design led to the creation of [unifex::v2::async_scope], which has a smaller interface and a cleaner definition of responsibility.

As an early adopter of Unifex, [rsys] (Meta’s cross-platform voip client library) became the entry point for structured concurrency in mobile code at Meta. We originally built rsys with an unstructured asynchrony model built around posting callbacks to threads in order to optimize for binary size. However, this came at the expense of developer velocity due to the increasing cost of debugging deadlocks and crashes resulting from race conditions.

We decided to adopt Unifex and refactor towards a more structured architecture to address these problems systematically. Converting an unstructured production codebase to a structured one is such a large project that it needs to be done in phases. As we began to convert callbacks to senders/tasks, we quickly realized that we needed a safe place to start structured asynchronous work in an unstructured environment. We addressed this need with unifex::v1::async_scope paired with an executor to address a recurring pattern:

// Abstraction for thread that has the ability
// to execute units of work.
class Executor {
public:
    virtual void add(Func function) noexcept = 0;
};

// Example class
class Foo {
    std::shared_ptr<Executor> exec_;

public:
    void doSomething() {
        auto asyncWork = [&]() {
            // do something
        };
        exec_->add(asyncWork);
    }
};

// Utility class for executing async work on an
// async_scope and on the provided executor
class ExecutorAsyncScopePair {
    unifex::v1::async_scope scope_;
    ExecutorScheduler exec_;

public:
    void add(Func func) {
        scope_.detached_spawn_call_on(exec_, func);
    }

    auto cleanup() {
        return scope_.cleanup();
    }
};

// Example class
class Foo {
    std::shared_ptr<ExecutorAsyncScopePair> exec_;

public:
    ~Foo() {
        sync_wait(exec_->cleanup());
    }

    void doSomething() {
        auto asyncWork = [&]() {
            // do something
        };

        exec_->add(asyncWork);
    }
};

This broadly worked but we discovered that the above design coupled with the v1 API allowed for too many redundancies and conflated too many responsibilities (scoping async work, associating work with a stop source, and transferring scoped work to a new scheduler).

We learned that making each component own a distinct responsibility will minimize the confusion and increase the structured concurrency adoption rate. The above example was an intuitive use of async_scope because the concept of a “scoped executor” was familiar to many engineers and is a popular async pattern in other programming languages. However, the above design abstracted away some of the APIs in async_scope that explicitly asked for a scheduler, which would have helped challenge the assumption engineers made about async_scope being an instance of a “scoped executor”.

Cancellation was an unfamiliar topic for engineers within the context of asynchronous programming. The v1::async_scope provided both cleanup() and complete() to give engineers the freedom to decide between canceling work or waiting for work to finish. The different nuances on when this should happen and how it happens ended up being an obstacle that engineers didn’t want to deal with.

Over time, we also found redundancies in the way v1::async_scope and other algorithms were implemented and identified other use cases that could benefit from a different kind of async scope. This motivated us to create v2::async_scope which only has one responsibility (scope), and nest which helped us improve maintainability and flexibility of Unifex.

The unstructured nature of cleanup()/complete() in a partially structured codebase introduced deadlocks when engineers nested the cleanup()/complete() sender in the scope being joined. This risk of deadlock remains with v2::async_scope::join() however, we do think this risk can be managed and is worth the tradeoff in exchange for a more coherent architecture that has fewer crashes. For example, we have experienced a significant reduction in these types of deadlocks once engineers understood that join() is a destructor-like operation that needs to be run only by the scope’s owner. Since there is no language support to manage async lifetimes automatically, this insight was key in preventing these types of deadlocks. Although this breakthrough was a result of strong guidance from experts, we believe that the simpler design of v2::async_scope would make this a little easier.

We strongly believe that async_scope was necessary for making structured concurrency possible within rsys, and we believe that the improvements we made with v2::async_scope will make the adoption of P2300 more accessible.

3 Motivation

3.1 Motivating example

Let us assume the following code:

namespace ex = std::execution;

struct work_context;
struct work_item;
void do_work(work_context&, work_item*);
std::vector<work_item*> get_work_items();

int main() {
    static_thread_pool my_pool{8};
    work_context ctx; // create a global context for the application

    std::vector<work_item*> items = get_work_items();
    for (auto item : items) {
        // Spawn some work dynamically
        ex::sender auto snd = ex::transfer_just(my_pool.get_scheduler(), item) |
                              ex::then([&](work_item* item) { do_work(ctx, item); });
        ex::start_detached(std::move(snd));
    }
    // `ctx` and `my_pool` are destroyed
}

In this example we are creating parallel work based on the given input vector. All the work will be spawned on the local static_thread_pool object, and will use a shared work_context object.

Because the number of work items is dynamic, one is forced to use start_detached() from [P2300R7] (or something equivalent) to dynamically spawn work. [P2300R7] doesn’t provide any facilities to spawn dynamic work and return a sender (i.e., something like when_all but with a dynamic number of input senders).

Using start_detached() here follows the fire-and-forget style, meaning that we have no control over, or awareness of, the completion of the async work that is being spawned.

At the end of the function, we are destroying the work_context and the static_thread_pool. But at that point, we don’t know whether all the spawned async work has completed. If any of the async work is incomplete, this might lead to crashes.

[P2300R7] doesn’t give us out-of-the-box facilities to use in solving these types of problems.

This paper proposes the counting_scope and [P3296R2]’s let_async_scope facilities that would help us avoid the invalid behavior. With counting_scope, one might write safe code this way:

namespace ex = std::execution;

struct work_context;
struct work_item;
void do_work(work_context&, work_item*);
std::vector<work_item*> get_work_items();

int main() {
    static_thread_pool my_pool{8};
    work_context ctx;         // create a global context for the application
    ex::counting_scope scope; // create this *after* the resources it protects

    // make sure we always join
    unifex::scope_guard join = [&]() noexcept {
        // wait for all nested work to finish
        this_thread::sync_wait(scope.join()); // NEW!
    };

    std::vector<work_item*> items = get_work_items();
    for (auto item : items) {
        // Spawn some work dynamically
        ex::sender auto snd = ex::transfer_just(my_pool.get_scheduler(), item) |
                              ex::then([&](work_item* item) { do_work(ctx, item); });

        // start `snd` as before, but associate the spawned work with `scope` so that it can
        // be awaited before destroying the resources referenced by the work (i.e. `my_pool`
        // and `ctx`)
        ex::spawn(std::move(snd), scope.get_token()); // NEW!
    }

    // `ctx` and `my_pool` are destroyed *after* they are no longer referenced
}

With [P3296R2]’s let_async_scope, one might write safe code this way:

namespace ex = std::execution;

struct work_context;
struct work_item;
void do_work(work_context&, work_item*);
std::vector<work_item*> get_work_items();

int main() {
    static_thread_pool my_pool{8};
    work_context ctx; // create a global context for the application

    this_thread::sync_wait(
            ex::let_async_scope(ex::just(get_work_items()), [&](auto scope, auto& items) {
                for (auto item : items) {
                    // Spawn some work dynamically
                    ex::sender auto snd = ex::transfer_just(my_pool.get_scheduler(), item) |
                                          ex::then([&](work_item* item) { do_work(ctx, item); });

                    // start `snd` as before, but associate the spawned work with `scope` so that it
                    // can be awaited before destroying the resources referenced by the work (i.e.
                    // `my_pool` and `ctx`)
                    ex::spawn(std::move(snd), scope); // NEW!
                }
                return just();
            }));

    // `ctx` and `my_pool` are destroyed *after* they are no longer referenced
}

Simplifying the above into something that fits in a Tony Table to highlight the differences gives us:

namespace ex = std::execution;

struct context;
ex::sender auto work(const context&);

int main() {
  context ctx;

  ex::sender auto snd = work(ctx);

  // fire and forget
  ex::start_detached(std::move(snd));

  // `ctx` is destroyed, perhaps before
  // `snd` is done
}

namespace ex = std::execution;

struct context;
ex::sender auto work(const context&);

int main() {
  context ctx;
  ex::counting_scope scope;

  ex::sender auto snd = work(ctx);

  try {
      // fire, but don't forget
      ex::spawn(std::move(snd), scope.get_token());
  } catch (...) {
      // do something to handle exception
  }

  // wait for all work nested within scope
  // to finish
  this_thread::sync_wait(scope.join());

  // `ctx` is destroyed once nothing
  // references it
}

namespace ex = std::execution;

struct context;
ex::sender auto work(const context&);

int main() {
  context ctx;
  this_thread::sync_wait(ex::just()
      | ex::let_async_scope([&](auto scope) {
        ex::sender auto snd = work(ctx);

        // fire, but don't forget
        ex::spawn(std::move(snd), scope.get_token());
      }));

  // `ctx` is destroyed once nothing
  // references it
}

Please see below for more examples.

3.2 counting_scope and let_async_scope are a step forward towards Structured Concurrency

Structured Programming [Dahl72] transformed the software world by making it easier to reason about the code, and build large software from simpler constructs. We want to achieve the same effect on concurrent programming by ensuring that we structure our concurrent code. [P2300R7] makes a big step in that direction, but, by itself, it doesn’t fully realize the principles of Structured Programming. More specifically, it doesn’t always ensure that we can apply the single entry, single exit point principle.

The start_detached sender algorithm fails this principle by behaving like a GOTO instruction. By calling start_detached we essentially continue in two places: in the same function, and on different thread that executes the given work. Moreover, the lifetime of the work started by start_detached cannot be bound to the local context. This will prevent local reasoning, which will make the program harder to understand.

To properly structure our concurrency, we need an abstraction that ensures that all async work that is spawned has a defined, observable, and controllable lifetime. This is the goal of counting_scope and let_async_scope.

4 Examples of use

4.1 Spawning work from within a task

Use let_async_scope in combination with a system_context from [P2079R2] to spawn work from within a task:

namespace ex = std::execution;

int main() {
    ex::system_context ctx;
    int result = 0;

    ex::scheduler auto sch = ctx.scheduler();

    ex::sender auto val = ex::just() | ex::let_async_scope([sch](ex::async_scope_token auto scope) {
        int val = 13;

        auto print_sender = ex::just() | ex::then([val]() noexcept {
            std::cout << "Hello world! Have an int with value: " << val << "\n";
        });

        // spawn the print sender on sch
        //
        // NOTE: if spawn throws, let_async_scope will capture the exception
        //       and propagate it through its set_error completion
        ex::spawn(ex::on(sch, std::move(print_sender)), scope);

        return ex::just(val);
    }) | ex::then([&result](auto val) { result = val });

    this_thread::sync_wait(ex::on(sch, std::move(val)));

    std::cout << "Result: " << result << "\n";
}

// 'let_async_scope' ensures that, if all work is completed successfully, the result will be 13
// `sync_wait` will throw whatever exception is thrown by the callable passed to `let_async_scope`

4.2 Starting work nested within a framework

In this example we use the counting_scope within a class to start work when the object receives a message and to wait for that work to complete before closing.

namespace ex = std::execution;

struct my_window {
    class close_message {};

    ex::sender auto some_work(int message);

    ex::sender auto some_work(close_message message);

    void onMessage(int i) {
        ++count;
        ex::spawn(ex::on(sch, some_work(i)), scope);
    }

    void onClickClose() {
        ++count;
        ex::spawn(ex::on(sch, some_work(close_message{})), scope);
    }

    my_window(ex::system_scheduler sch, ex::counting_scope::token scope)
        : sch(sch)
        , scope(scope) {
        // register this window with the windowing framework somehow so that
        // it starts receiving calls to onClickClose() and onMessage()
    }

    ex::system_scheduler sch;
    ex::counting_scope::token scope;
    int count{0};
};

int main() {
    // keep track of all spawned work
    ex::counting_scope scope;
    ex::system_context ctx;
    try {
        my_window window{ctx.get_scheduler(), scope.get_token()};
    } catch (...) {
        // do something with exception
    }
    // wait for all work nested within scope to finish
    this_thread::sync_wait(scope.join());
    // all resources are now safe to destroy
    return window.count;
}

4.3 Starting parallel work

In this example we use let_async_scope to construct an algorithm that performs parallel work. Here foo launches 100 tasks that concurrently run on some scheduler provided to foo, through its connected receiver, and then the tasks are asynchronously joined. This structure emulates how we might build a parallel algorithm where each some_work might be operating on a fragment of data.

namespace ex = std::execution;

ex::sender auto some_work(int work_index);

ex::sender auto foo(ex::scheduler auto sch) {
    return ex::just() | ex::let_async_scope([sch](ex::async_scope_token auto scope) {
        return ex::schedule(sch) | ex::then([] { std::cout << "Before tasks launch\n"; }) |
               ex::then([=] {
                   // Create parallel work
                   for (int i = 0; i < 100; ++i) {
                       // NOTE: if spawn() throws, the exception will be propagated as the
                       //       result of let_async_scope through its set_error completion
                       ex::spawn(ex::on(sch, some_work(i)), scope);
                   }
               });
    }) | ex::then([] { std::cout << "After tasks complete successfully\n"; });
}

4.4 Listener loop in an HTTP server

This example shows how one can write the listener loop in an HTTP server, with the help of coroutines. The HTTP server will continuously accept new connection and start work to handle the requests coming on the new connections. While the listening activity is bound in the scope of the loop, the lifetime of handling requests may exceed the scope of the loop. We use counting_scope to limit the lifetime of the request handling without blocking the acceptance of new requests.

namespace ex = std::execution;

task<size_t> listener(int port, io_context& ctx, static_thread_pool& pool) {
    size_t count{0};
    listening_socket listen_sock{port};

    co_await ex::let_async_scope(ex::just(), [&](ex::async_scope_token auto scope) -> task<void> {
        while (!ctx.is_stopped()) {
            // Accept a new connection
            connection conn = co_await async_accept(ctx, listen_sock);
            count++;

            // Create work to handle the connection in the scope of `work_scope`
            conn_data data{std::move(conn), ctx, pool};
            ex::sender auto snd = ex::just(std::move(data)) |
                                  ex::let_value([](auto& data) { return handle_connection(data); });

            ex::spawn(std::move(snd), scope);
        }
    });

    // At this point, all the request handling is complete
    co_return count;
}

[libunifex] has a very similar example HTTP server at [io_uring HTTP server] that compiles and runs on Linux-based machines with io_uring support.

4.5 Pluggable functionality through composition

This example is based on real code in rsys, but it reduces the real code to slideware and ports it from Unifex to the proposed std::execution equivalents. The central abstraction in rsys is a Call, but each integration of rsys has different needs so the set of features supported by a Call varies with the build configuration. We support this configurability by exposing the equivalent of the following method on the Call class:

template <typename Feature>
Handle<Feature> Call::get();

and it’s used like this in app-layer code:

unifex::task<void> maybeToggleCamera(Call& call) {
    Handle<Camera> camera = call.get<Camera>();

    if (camera) {
        co_await camera->toggle();
    }
}

A Handle<Feature> is effectively a part-owner of the Call it came from.

The team that maintains rsys and the teams that use rsys are, unsurprisingly, different teams so rsys has to be designed to solve organizational problems as well as technical problems. One relevant design decision the rsys team made is that it is safe to keep using a Handle<Feature> after the end of its Call’s lifetime; this choice adds some complexity to the design of Call and its various features but it also simplifies the support relationship between the rsys team and its many partner teams because it eliminates many crash-at-shutdown bugs.

namespace rsys {

class Call {
public:
    unifex::nothrow_task<void> destroy() noexcept {
        // first, close the scope to new work and wait for existing work to finish
        scope_->close();
        co_await scope_->join();

        // other clean-up tasks here
    }

    template <typename Feature>
    Handle<Feature> get() noexcept;

private:
    // an async scope shared between a call and its features
    std::shared_ptr<std::execution::counting_scope> scope_;
    // each call has its own set of threads
    ExecutionContext context_;

    // the set of features this call supports
    FeatureBag features_;
};

class Camera {
public:
    std::execution::sender auto toggle() {
        namespace ex = std::execution;

        return ex::just() | ex::let_value([this]() {
            // this callable is only invoked if the Call's scope is in
            // the open or unused state when nest() is invoked, making
            // it safe to assume here that:
            //
            //  - scheduler_ is not a dangling reference to the call's
            //    execution context
            //  - Call::destroy() has not progressed past starting the
            //    join-sender so all the resources owned by the call
            //    are still valid
            //
            // if the nest() attempt fails because the join-sender has
            // started (or even if the Call has been completely destroyed)
            // then the sender returned from toggle() will safely do
            // nothing before completing with set_stopped()

            return ex::schedule(scheduler_) | ex::then([this]() {
                // toggle the camera
            });
        }) | ex::nest(callScope_->get_token());
    }

private:
    // a copy of this camera's Call's scope_ member
    std::shared_ptr<ex::counting_scope> callScope_;
    // a scheduler that refers to this camera's Call's ExecutionContext
    Scheduler scheduler_;
};

} // namespace rsys

4.6 Recursively spawning work until completion

Below are three ways you could recursively spawn work on a scope using let_async_scope or counting_scope.

4.6.1 let_async_scope with spawn()

struct tree {
    std::unique_ptr<tree> left;
    std::unique_ptr<tree> right;
    int data;
};

auto process(ex::scheduler auto sch, auto scope, tree& t) noexcept {
    return ex::schedule(sch) | then([sch, &]() {
        if (t.left)
            ex::spawn(process(sch, scope, t.left.get()), scope);
        if (t.right)
            ex::spawn(process(sch, scope, t.right.get()), scope);
        do_stuff(t.data);
    }) | ex::let_error([](auto& e) {
        // log error
        return just();
    });
}

int main() {
    ex::scheduler sch;
    tree t = make_tree();
    // let_async_scope will ensure all new work will be spawned on the
    // scope and will not be joined until all work is finished.
    // NOTE: Exceptions will not be surfaced to let_async_scope; exceptions
    // will be handled by let_error instead.
    this_thread::sync_wait(ex::just() | ex::let_async_scope([&, sch](auto scope) {
        return process(sch, scope, t);
    }));
}

4.6.2 let_async_scope with spawn_future()

struct tree {
    std::unique_ptr<tree> left;
    std::unique_ptr<tree> right;
    int data;
};

auto process(ex::scheduler auto sch, auto scope, tree& t) {
    return ex::schedule(sch) | ex::let_value([sch, &]() {
        unifex::any_sender_of<> leftFut = ex::just();
        unifex::any_sender_of<> rightFut = ex::just();
        if (t.left) {
            leftFut = ex::spawn_future(scope, process(sch, scope, t.left.get()));
        }

        if (t.right) {
            rightFut = ex::spawn_future(scope, process(sch, scope, t.right.get()));
        }

        do_stuff(t.data);
        return ex::when_all(leftFut, rightFut) | ex::then([](auto&&...) noexcept {});
    });
}

int main() {
    ex::scheduler sch;
    tree t = make_tree();
    // let_async_scope will ensure all new work will be spawned on the
    // scope and will not be joined until all work is finished
    // NOTE: Exceptions will be surfaced to let_async_scope which will
    // call set_error with the exception_ptr
    this_thread::sync_wait(ex::just() | ex::let_async_scope([&, sch](auto scope) {
        return process(sch, scope, t);
    }));
}

4.6.3 counting_scope

struct tree {
    std::unique_ptr<tree> left;
    std::unique_ptr<tree> right;
    int data;
};

auto process(ex::counting_scope_token scope, ex::scheduler auto sch, tree& t) noexcept {
    return ex::schedule(sch) | ex::then([sch, &]() noexcept {
        if (t.left)
            ex::spawn(process(scope, sch, t.left.get()), scope);

        if (t.right)
            ex::spawn(process(scope, sch, t.right.get()), scope);

        do_stuff(t.data);
    }) | ex::let_error([](auto& e) {
        // log error
        return just();
    });
}

int main() {
    ex::scheduler sch;
    tree t = make_tree();
    ex::counting_scope scope;
    ex::spawn(process(scope.get_token(), sch, t), scope.get_token());
    this_thread::sync_wait(scope.join());
}

5 Async Scope, usage guide

An async scope is a type that implements a “bookkeeping policy” for senders that have been associated with the scope. Depending on the policy, different guarantees can be provided in terms of the lifetimes of the scope and any associated senders. The counting_scope described in this paper defines a policy that has proven useful while progressively adding structure to existing, unstructured code at Meta, but other useful policies are possible. By defining nest(), spawn(), and spawn_future() in terms of the more fundamental async scope token interface, and leaving the implementation of the abstract interface to concrete token types, this paper’s design leaves the set of policies open to extension by user code or future standards.

An async scope token’s implementation of the async_scope_token concept:

• must allow an arbitrary sender to be wrapped without eagerly starting the sender;
• must not add new value or error completions when wrapping a sender;
• may fail to associate a new sender by returning an association that converts to false from try_associate();
• may fail to associate a new sender by eagerly throwing an exception from either try_associate() or wrap();

More on these items can be found below in the sections below.

5.1 Definitions

namespace { // exposition-only

template <class Env>
struct spawn-env; // exposition-only

template <class Env>
struct spawn-receiver { // exposition-only
    void set_value() noexcept;
    void set_stopped() noexcept;

    const spawn-env<Env>& get_env() const noexcept;
};

template <class Env>
struct future-env; // exposition-only

template <valid-completion-signatures Sigs>
struct future-sender; // exposition-only

template <sender Sender, class Env>
using future-sender-t = // exposition-only
    future-sender<completion_signatures_of_t<Sender, future-env<Env>>>;

}

template <class Assoc>
concept async_scope_association =
    semiregular<Assoc> &&
    requires(const Assoc& assoc) {
        { static_cast<bool>(assoc) } noexcept;
    };

template <class Token>
concept async_scope_token =
    copyable<Token> &&
    requires(Token token) {
        { token.try_associate() } -> async_scope_association;
    };

template <async_scope_token Token>
using association-from = decltype(declval<Token&>().try_associate()); // exposition-only

template <async_scope_token Token, sender Sender>
using wrapped-sender-from = decay_t<decltype(declval<Token&>().wrap(declval<Sender>()))>; // @@_exposition-only_@

template <sender Sender, async_scope_token Token>
struct nest-sender { // exposition-only
    nest-sender(Sender&& sender, Token token);

    ~nest-sender();

private:
    optional<wrapped-sender-from<Token, Sender>> sender_;
    association-from<Token> token;
};

template <sender Sender, async_scope_token Token>
auto nest(Sender&& snd, Token token)
    noexcept(is_nothrow_constructible_v<nest-sender<Sender, Token>, Sender, Token>)
    -> nest-sender<Sender, Token>;

template <sender Sender, async_scope_token Token, class Env = empty_env>
void spawn(Sender&& snd, Token token, Env env = {})
    requires sender_to<decltype(token.wrap(forward<Sender>(snd))),
                       spawn-receiver<End>>;

template <sender Sender, async_scope_token Token, class Env = empty_env>
future-sender-t<Sender, Env> spawn_future(Sender&& snd, Token token, Env env = {});

struct simple_counting_scope {
    simple_counting_scope() noexcept;
    ~simple_counting_scope();

    // simple_counting_scope is immovable and uncopyable
    simple_counting_scope(const simple_counting_scope&) = delete;
    simple_counting_scope(simple_counting_scope&&) = delete;
    simple_counting_scope& operator=(const simple_counting_scope&) = delete;
    simple_counting_scope& operator=(simple_counting_scope&&) = delete;

    struct token;

    struct assoc {
        assoc() noexcept = default;

        assoc(const assoc&) noexcept;

        assoc(assoc&&) noexcept;

        ~assoc();

        assoc& operator=(assoc) noexcept;

        explicit operator bool() const noexcept;

    private:
        friend token;

        explicit assoc(simple_counting_scope*) noexcept; // exposition-only

        simple_counting_scope* scope_{}; // exposition-only
    };

    struct token {
        template <sender Sender>
        Sender&& wrap(Sender&& snd) const noexcept;

        assoc try_associate() const;

    private:
        friend simple_counting_scope;

        explicit token(simple_counting_scope* s) noexcept; // exposition-only

        simple_counting_scope* scope_; // exposition-only
    };

    token get_token() noexcept;

    void close() noexcept;

    struct join-sender; // exposition-only

    join-sender join() noexcept;
};

struct counting_scope {
    counting_scope() noexcept;
    ~counting_scope();

    // counting_scope is immovable and uncopyable
    counting_scope(const counting_scope&) = delete;
    counting_scope(counting_scope&&) = delete;
    counting_scope& operator=(const counting_scope&) = delete;
    counting_scope& operator=(counting_scope&&) = delete;

    template <sender Sender>
    struct wrapper-sender; // exposition-only

    struct token {
        template <sender Sender>
        wrapper-sender<Sender> wrap(Sender&& snd) const;

        async_scope_association auto try_associate() const;

    private:
        friend counting_scope;

        explicit token(counting_scope* s) noexcept; // exposition-only

        counting_scope* scope_; // exposition-only
    };

    token get_token() noexcept;

    void close() noexcept;

    void request_stop() noexcept;

    struct join-sender; // exposition-only

    join-sender join() noexcept;
};

5.2 execution::async_scope_association

template <class Assoc>
concept async_scope_association =
    semiregular<Assoc> &&
    requires(const Assoc& assoc) {
        { static_cast<bool>(assoc) } noexcept;
    };

An async scope association is an RAII handle type that represents a possible association between a sender and an async scope. If the scope association contextually converts to true then the object is “engaged” and represents an association; otherwise, the object is “disengaged” and represents the lack of an association. Async scope associations are copyable but, when copying an engaged association, the resulting copy may be disengaged because the underlying async scope may decline to create a new association.

5.3 execution::async_scope_token

template <class Token>
concept async_scope_token =
    copyable<Token> &&
    requires(Token token) {
        { token.try_associate() } -> async_scope_association;
    };

An async scope token is a non-owning handle to an async scope. The try_associate() method on a token attempts to create a new association with the scope; try_associate() returns an engaged association when the association is successful, and it may either return a disengaged association or throw an exception to indicate failure. Returning a disengaged association will generally lead to algorithms that operate on tokens behaving as if provided a sender that completes immediately with set_stopped(), leading to rejected work being discarded as a “no-op”. Throwing an exception will generally lead to that exception escaping from the calling algorithm.

Tokens also have a wrap() method that takes and returns a sender. The wrap() method gives the token an opportunity to modify the input sender’s behaviour in a scope-specific way. The proposed counting_scope uses this opportunity to associate the input sender with a stop token that the scope can use to request stop on all outstanding operations associated within the scope.

In order to provide the Strong Exception Guarantee, the algorithms proposed in this paper invoke token.wrap(snd) before invoking token.try_associate(). Other algorithms written in terms of async_scope_token should do the same.

The following sketch implementation of nest-sender illustrates how the methods on an async scope token iteract:

template <sender Sender, async_scope_token Token>
struct nest-sender {
    nest-sender(Sender&& s, Token t)
        : sender_(t.wrap(forward<Sender>(s))) {
        assoc_ = t.try_associate();
        if (!assoc_) {
            sender_.reset(); // assume no_throw destructor
        }
    }

    nest-sender(const nest-sender& other)
        requires copy_constructible<wrapped-sender-from<Token, Sender>>
        : assoc_(t.try_associate()) {
        if (assoc_) {
            sender_ = other.sender_;
        }
    }

    nest-sender(nest-sender&& other) noexcept = default;

    ~nest-sender() = default;

    // ... implement the sender concept in terms of Sender and sender_

private:
    association-from<Token> assoc_;
    optional<wrapped-sender-from<Token, Sender>> sender_;
};

An async scope token behaves like a reference-to-async-scope; tokens are no-throw copyable and movable, and it is undefined behaviour to invoke any methods on a token that has outlived its scope.

5.4 execution::nest

template <sender Sender, async_scope_token Token>
auto nest(Sender&& snd, Token token)
    noexcept(is_nothrow_constructible_v<nest-sender<Sender, Token>, Sender, Token>)
    -> nest-sender<Sender, Token>;

When successful, nest() creates an association with the given token’s scope and returns an “associated” sender that behaves the same as its input sender, with the following additional effects:

• the association ends when the returned sender is destroyed or, if it is connected, when the resulting operation state is destroyed; and
• whatever effects are added by the token’s wrap() method.

When unsuccessful, nest() will either return an “unassociated” sender or it will allow any thrown exceptions to escape.

When nest() returns an associated sender:

• connecting and starting the associated sender connects and starts the given sender; and
• the associated sender has exactly the same completions as the input sender.

When nest() returns an unassociated sender:

• the input sender is discarded and will never be connected or started; and
• the unassociated sender will only complete with set_stopped().

nest() simply constructs and returns a nest-sender. Given an async_scope_token, token, and a sender, snd, the nest-sender constructor performs the following operations in the following order:

1. store the result of token.wrap(snd) in a member variable
2. store the result of token.try_associate() in a member variable
   1. if the resulting association is disengaged then destroy the previously stored result of token.wrap(snd); the nest-sender under construction is an unassociated sender.
   2. otherwise, the nest-sender under construction is an associated sender.

Any exceptions thrown during the evaluation of the constructor are allowed to escape; nevertheless, nest() provides the Strong Exception Guarantee.

An associated nest-sender has many properties of an RAII handle:

• constructing an instance acquires a “resource” (the association with the scope)
• destructing an instance releases the same resource
• moving an instance into another transfers ownership of the resource from the source to the destination
• etc.

Copying a nest-sender is possible if the sender it is wrapping is copyable but the copying process is a bit unusual because of the async_scope_association it contains. If the sender, snd, provided to nest() is copyable then the resulting nest-sender is also copyable, with the following rules:

• copying an unassociated nest-sender invariably produces a new unassociated nest-sender; and
• copying an associated nest-sender proceeds as follows:
  1. copy the association from the source into the destination nest-sender
     • if the copied association is engaged then copy the wrapped sender from the source into the destination nest-sender; the destination is associated
     • otherwise, the destination is unassociated

When nest-sender has a copy constructor, it provides the Strong Exception Guarantee.

When connecting an unassociated nest-sender, the resulting operation-state completes immediately with set_stopped() when started.

When connecting an associated nest-sender, there are four possible outcomes:

1. the nest-sender is rvalue connected, which infallibly moves the sender’s association from the sender to the operation-state
2. the nest-sender is lvalue connected, in which case the sender’s association must be copied into the operation-state, which may:
   1. succeed by creating a new engaged association for the operation-state;
   2. fail by creating a new disengaged association for the operation-state, in which case the new operation-state behaves as if it were constructed from an unassociated nest-sender; or
   3. fail by throwing an exception, in which case the exception escapes from the call to connect.

An operation-state with its own association must invoke the association’s destructor as the last step of the operation-state’s destructor.

Note: the timing of when an associated operation-state ends its association with the scope is chosen to avoid exposing user code to dangling references. Scopes are expected to serve as mechanisms for signaling when it is safe to destroy shared resources being protected by the scope. Ending any given association with a scope may lead to that scope signaling that the protected resources can be destroyed so a nest-sender’s operation-state must not permit that signal to be sent until the operation-state is definitely finished accessing the shared resources, which is at the end of the operation-state’s destructor.

A call to nest() does not start the given sender and is not expected to incur allocations.

Regardless of whether the returned sender is associated or unassociated, it is multi-shot if the input sender is multi-shot and single-shot otherwise.

5.5 execution::spawn

namespace { // exposition-only

template <class Env>
struct spawn-env; // exposition-only

template <class Env>
struct spawn-receiver { // exposition-only
    void set_value() noexcept;
    void set_stopped() noexcept;

    const spawn-env<Env>& get_env() const noexcept;
};

}

template <sender Sender, async_scope_token Token, class Env = empty_env>
void spawn(Sender&& snd, Token token, Env env = {})
    requires sender_to<decltype(token.wrap(forward<Sender>(snd))),
                       spawn-receiver<End>>;

Attempts to associate the given sender with the given scope token’s scope. On success, the given sender is eagerly started. On failure, either the sender is discarded and no further work happens or spawn() throws.

Starting the given sender without waiting for it to finish requires a dynamic allocation of the sender’s operation-state. The following algorithm determines which Allocator to use for this allocation:

• If get_allocator(env) is valid and returns an Allocator then choose that Allocator.
• Otherwise, if get_allocator(get_env(snd)) is valid and returns an Allocator then choose that Allocator.
• Otherwise, choose std::allocator<>.

spawn() proceeds with the following steps in the following order:

1. the type of the object to dynamically allocate is computed, say op_t; op_t contains
   • an operation-state;
   • an allocator of the chosen type; and
   • an association of type decltype(token.try_associate()).
2. an op_t is dynamically allocated by the Allocator chosen as described above
3. the fields of the op_t are initialized in the following order:
   1. the operation-state within the allocated op_t is initialized with the result of connect(token.wrap(forward<Sender>(sender)), spawn-receiver{...});
   2. the allocator is initialized with a copy of the allocator used to allocate the op_t; and
   3. the association is initialized with the result of token.try_associate().
4. if the association in the op_t is engaged then the operation-state is started; otherwise, the op_t is destroyed and deallocated.

Any exceptions thrown during the execution of spawn() are allowed to escape; nevertheless, spawn() provides the Strong Exception Guarantee.

Upon completion of the operation-state, the spawn-receiver performs the following steps:

1. move the allocator and association from the op_t into local variables;
2. destroy the operation-state;
3. use the local copy of the allocator to deallocate the op_t;
4. destroy the local copy of the allocator; and
5. destroy the local copy of the association.

Performing step 5 last ensures that all possible references to resources protected by the scope, including possibly the allocator, are no longer in use before dissociating from the scope.

A spawn-receiver, sr, responds to get_env(sr) with an instance of a spawn-env<Env>, senv. The result of get_allocator(senv) is a copy of the Allocator used to allocate the operation-state. For all other queries, Q, the result of Q(senv) is Q(env).

This is similar to start_detached() from [P2300R7], but the scope may observe and participate in the lifetime of the work described by the sender. The simple_counting_scope and counting_scope described in this paper use this opportunity to keep a count of spawned senders that haven’t finished, and to prevent new senders from being spawned once the scope has been closed.

The given sender must complete with set_value() or set_stopped() and may not complete with an error; the user must explicitly handle the errors that might appear as part of the sender-expression passed to spawn().

User expectations will be that spawn() is asynchronous and so, to uphold the principle of least surprise, spawn() should only be given non-blocking senders. Using spawn() with a sender generated by on(sched, blocking-sender) is a very useful pattern in this context.

NOTE: A query for non-blocking start will allow spawn() to be constrained to require non-blocking start.

Usage example:

...
for (int i = 0; i < 100; i++)
    spawn(on(sched, some_work(i)), scope.get_token());

5.6 execution::spawn_future

namespace { // exposition-only

template <class Env>
struct future-env; // exposition-only

template <valid-completion-signatures Sigs>
struct future-sender; // exposition-only

template <sender Sender, class Env>
using future-sender-t = // exposition-only
    future-sender<completion_signatures_of_t<Sender, future-env<Env>>>;

}

template <sender Sender, async_scope_token Token, class Env = empty_env>
future-sender-t<Sender, Env> spawn_future(Sender&& snd, Token token, Env env = {});

Attempts to associate the given sender with the given scope token’s scope. On success, the given sender is eagerly started and spawn_future returns a future-sender that provides access to the result of the given sender. On failure, either spawn_future returns a future-sender that unconditionally completes with set_stopped() or it throws.

Similar to spawn(), starting the given sender involves a dynamic allocation of some state. spawn_future() chooses an Allocator for this allocation in the same way spawn() does: use the result of get_allocator(env) if that is a valid expression, otherwise use the result of get_allocator(get_env(snd)) if that is a valid expression, otherwise use a std::allocator<>.

Compared to spawn(), the dynamically allocated state is more complicated because it must contain storage for the result of the given sender, however it eventually completes, and synchronization facilities for resolving the race between the given sender’s production of its result and the returned sender’s consumption or abandonment of that result.

Unlike spawn(), spawn_future() returns a future-sender rather than void. The returned sender, fs, is a handle to the spawned work that can be used to consume or abandon the result of that work. The completion signatures of fs include set_stopped() and all the completion signatures of the spawned sender. When fs is connected and started, it waits for the spawned sender to complete and then completes itself with the spawned sender’s result.

The receiver, fr, that is connected to the given sender responds to get_env(fr) with an instance of future-env<Env>, fenv. The result of get_allocator(fenv) is a copy of the Allocator used to allocate the dynamically allocated state. The result of get_stop_token(fenv) is a stop token that will be “triggered” (i.e. signal that stop is requested) when:

• the returned future-sender is dropped;
• the returned future-sender receives a stop request; or
• the stop token returned from get_stop_token(env) is triggered if get_stop_token(env) is a valid expression.

For all other queries, Q, the result of Q(fenv) is Q(env).

spawn_future() proceeds with the following steps in the following order:

1. storage for the spawned sender’s state is dynamically allocated by the Allocator chosen as described above
2. the state for the spawned sender is constructed in the allocated storage
   • a subset of this state is an operation-state created by connecting the result of token.wrap(forward<Sender>(sender)) with a receiver
   • the last field to be initialized in the dynamically allocated state is an async scope association that is initialized with the result of token.try_associate()
     • if the resulting association is engaged then
       • the operation-state within the allocated state is started; and
       • a future-sender is returned that, when connected and started, will complete with the result of the eagerly-started work
     • otherwise
       • the dynamically-allocated state is destroyed and deallocated; and
       • a future-sender is returned that will complete with set_stopped()

Any exceptions thrown during the execution of spawn_future() are allowed to escape; nevertheless, spawn_future() provides the Strong Exception Guarantee.

Given a future-sender, fs, if fs is destroyed without being connected, or if it is connected and the resulting operation-state, fsop, is destroyed without being started, then the eagerly-started work is “abandoned”.

Abandoning the eagerly-started work means:

• a stop request is sent to the running operation-state;
• any result produced by the running operation-state is discarded when the operation completes; and
• after the operation completes, the dynamically-allocated state is “cleaned up”.

Cleaning up the dynamically-allocated state means doing the following, in order:

1. the allocator and association in the state are moved into local variables;
2. the state is destroyed;
3. the dynamic allocation is deallocated with the local copy of the allocator;
4. the local copy of the allocator is destroyed; and
5. the local copy of the association is destroyed.

When fsop is started, if fsop receives a stop request from its receiver before the eagerly-started work has completed then an attempt is made to abandon the eagerly-started work. Note that it’s possible for the eagerly-started work to complete while fsop is requesting stop; once the stop request has been delivered, either fsop completes with the result of the eagerly-started work if it’s ready, or it completes with set_stopped() without waiting for the eagerly-started work to complete.

When fsop is started and does not receive a stop request from its receiver, fsop completes after the eagerly-started work completes with the same completion. Once fsop completes, it cleans up the dynamically-allocated state.

spawn_future is similar to ensure_started() from [P2300R7], but the scope may observe and participate in the lifetime of the work described by the sender. The simple_counting_scope and counting_scope described in this paper use this opportunity to keep a count of given senders that haven’t finished, and to prevent new senders from being started once the scope has been closed.

Unlike spawn(), the sender given to spawn_future() is not constrained on a given shape. It may send different types of values, and it can complete with errors.

Usage example:

...
sender auto snd = spawn_future(on(sched, key_work()), token) | then(continue_fun);
for (int i = 0; i < 10; i++)
    spawn(on(sched, other_work(i)), token);
return when_all(scope.join(), std::move(snd));

5.7 execution::simple_counting_scope

struct simple_counting_scope {
    simple_counting_scope() noexcept;
    ~simple_counting_scope();

    // simple_counting_scope is immovable and uncopyable
    simple_counting_scope(const simple_counting_scope&) = delete;
    simple_counting_scope(simple_counting_scope&&) = delete;
    simple_counting_scope& operator=(const simple_counting_scope&) = delete;
    simple_counting_scope& operator=(simple_counting_scope&&) = delete;

    struct token;

    struct assoc {
        assoc() noexcept = default;

        assoc(const assoc&) noexcept;

        assoc(assoc&&) noexcept;

        ~assoc();

        assoc& operator=(assoc) noexcept;

        explicit operator bool() const noexcept;

    private:
        friend token;

        explicit assoc(simple_counting_scope*) noexcept; // exposition-only

        simple_counting_scope* scope_{}; // exposition-only
    };

    struct token {
        template <sender Sender>
        Sender&& wrap(Sender&& snd) const noexcept;

        assoc try_associate() const;

    private:
        friend simple_counting_scope;

        explicit token(simple_counting_scope* s) noexcept; // exposition-only

        simple_counting_scope* scope_; // exposition-only
    };

    token get_token() noexcept;

    void close() noexcept;

    struct join-sender; // exposition-only

    join-sender join() noexcept;
};

A simple_counting_scope maintains a count of outstanding operations and goes through several states durings its lifetime:

• unused
• open
• closed
• open-and-joining
• closed-and-joining
• unused-and-closed
• joined

The following diagram illustrates the simple_counting_scope’s state machine:

Note: a scope is “open” if its current state is unused, open, or open-and-joining; a scope is “closed” if its current state is closed, unused-and-closed, closed-and-joining, or joined.

Instances start in the unused state after being constructed. This is the only time the scope’s state can be set to unused. When the simple_counting_scope destructor starts, the scope must be in the unused, unused-and-closed, or joined state; otherwise, the destructor invokes std::terminate(). Permitting destruction when the scope is in the unused or unused-and-closed state ensures that instances of simple_counting_scope can be used safely as data-members while preserving structured functionality.

Connecting and starting a join-sender returned from join() moves the scope to either the open-and-joining or closed-and-joining state. Merely calling join() or connecting the join-sender does not change the scope’s state—the operation-state must be started to effect the state change. A started join-sender completes when the scope’s count of outstanding operations reaches zero, at which point the scope transitions to the joined state.

Calling close() on a simple_counting_scope moves the scope to the closed, unused-and-closed, or closed-and-joining state, and causes all future calls to try_associate() to return false.

Associating work with a simple_counting_scope can be done through simple_counting_scope’s token. simple_counting_scope’s token provides 2 methods: wrap(Sender&& s), and try_associate().

• wrap(Sender&&s) takes in a sender and returns it unmodified.
• try_associate() attempts to create a new association with the simple_counting_scope and will return an engaged association when successful, or a disengaged association otherwise. The requirements for try_associate()’s success are outlined below:
  1. While a scope is in the unused, open, or open-and-joining state, calls to token.try_associate() succeeds by incrementing the scope’s count of oustanding operations before returning an engaged association.
  2. While a scope is in the closed, unused-and-closed, closed-and-joining, or joined state, calls to token.try_associate() will return a disengaged assocation and will not increment the scope’s count of outstanding operations.

When a token’s try_associate() returns an engaged association, the destructor of the resulting association will undo the association by decrementing the scope’s count of oustanding operations.

• When a scope is in the open-and-joining or closed-and-joining state and an association’s destructor undoes the final scope association, the scope moves to the joined state and the outstanding join-sender completes.

The state transitions of a simple_counting_scope mean that it can be used to protect asynchronous work from use-after-free errors. Given a resource, res, and a simple_counting_scope, scope, obeying the following policy is enough to ensure that there are no attempts to use res after its lifetime ends:

• all senders that refer to res are associated with scope; and
• scope is destroyed (and therefore in the joined, unused, or unused-and-closed state) before res is destroyed.

It is safe to destroy a scope in the unused or unusued-and-closed state because there can’t be any work referring to the resources protected by the scope.

A simple_counting_scope is uncopyable and immovable so its copy and move operators are explicitly deleted. simple_counting_scope could be made movable but it would cost an allocation so this is not proposed.

5.7.1 simple_counting_scope::simple_counting_scope

simple_counting_scope() noexcept;

Initializes a simple_counting_scope in the unused state with the count of outstanding operations set to zero.

5.7.2 simple_counting_scope::~simple_counting_scope

~simple_counting_scope();

Checks that the simple_counting_scope is in the joined, unused, or unused-and-closed state and invokes std::terminate() if not.

5.7.3 simple_counting_scope::get_token

simple_counting_scope::token get_token() noexcept;

Returns a simple_counting_scope::token referring to the current scope, as if by invoking token{this}.

5.7.4 simple_counting_scope::close

void close() noexcept;

Moves the scope to the closed, unused-and-closed, or closed-and-joining state. After a call to close(), all future calls to try_associate() return false.

5.7.5 simple_counting_scope::join

struct join-sender; // exposition-only

join-sender join() noexcept;

Returns a join-sender. When the join-sender is connected to a receiver, r, it produces an operation-state, o. When o is started, the scope moves to either the open-and-joining or closed-and-joining state. o completes with set_value() when the scope moves to the joined state, which happens when the scope’s count of outstanding senders drops to zero. o may complete synchronously if it happens to observe that the count of outstanding senders is already zero when started; otherwise, o completes on the execution context associated with the scheduler in its receiver’s environment by asking its receiver, r, for a scheduler, sch, with get_scheduler(get_env(r)) and then starting the sender returned from schedule(sch). This requirement to complete on the receiver’s scheduler restricts which receivers a join-sender may be connected to in exchange for determinism; the alternative would have the join-sender completing on the execution context of whichever nested operation happens to be the last one to complete.

5.7.6 simple_counting_scope::assoc::assoc

assoc() noexcept = default;

explicit assoc(simple_counting_scope*) noexcept; // exposition-only

assoc(const assoc&) noexcept;

assoc(assoc&&) noexcept;

The default assoc constructor produces a disengaged association.

The private, exposition-only constructor accepting a simple_counting_scope* either:

• constructs a disengaged association if the given pointer is nullptr; or
• constructs an engaged association associated with the given scope.

The copy constructor either:

• infallibly copies a disengaged association; or
• attempts to create a new association as if by invoking get_token().try_associate() on the source association’s scope.

The move constructor either:

• infallibly produces a disengaged association from a disengaged assocation; or
• infallibly transfers the association from an engaged assocation to the new object, leaving the source disengaged.

5.7.7 simple_counting_scope::assoc::~assoc

~assoc();

The assoc destructor either:

• does nothing if the association is disengaged; or
• decrements the associated scope’s count of outstanding operations and, when the scope is in the open-and-joining or closed-and-joing state, moves the scope to the joined state and signals the outstanding join-sender to complete.

5.7.8 simple_counting_scope::assoc::operator=

assoc& operator=(assoc) noexcept;

The assignment operator behaves as if it is implemented as follows:

assoc& operator=(assoc rhs) noexcept
  swap(scope_, rhs.scope_);
  return *this;
}

where scope_ is a private member of type simple_counting_scope* that points to the association’s associated scope.

5.7.9 simple_counting_scope::assoc::operator bool

explicit operator bool() const noexcept;

Returns true when the association is engaged and false when it is disengaged.

5.7.10 simple_counting_scope::token::wrap

template <sender Sender>
Sender&& wrap(Sender&& s) const noexcept;

Returns the argument unmodified.

5.7.11 simple_counting_scope::token::try_associate

assoc try_associate() const;

The following atomic state change is attempted on the token’s scope:

• increment the scope’s count of outstanding operations; and
• move the scope to the open state if it was in the unused state.

The atomic state change succeeds and the method returns an enaged assoc if the scope is observed to be in the unused, open, or open-and-joining state; otherwise the scope’s state is left unchanged and the method returns a disengaged assoc.

5.8 execution::counting_scope

struct counting_scope {
    counting_scope() noexcept;
    ~counting_scope();

    // counting_scope is immovable and uncopyable
    counting_scope(const counting_scope&) = delete;
    counting_scope(counting_scope&&) = delete;
    counting_scope& operator=(const counting_scope&) = delete;
    counting_scope& operator=(counting_scope&&) = delete;

    template <sender Sender>
    struct wrapper-sender; // exposition-only

    struct token {
        template <sender Sender>
        wrapper-sender<Sender> wrap(Sender&& snd);

        async_scope_association auto try_associate() const;

    private:
        friend counting_scope;

        explicit token(counting_scope* s) noexcept; // exposition-only

        counting_scope* scope; // exposition-only
    };

    token get_token() noexcept;

    void close() noexcept;

    void request_stop() noexcept;

    struct join-sender; // exposition-only

    join-sender join() noexcept;
};

A counting_scope augments a simple_counting_scope with a stop source and gives to each of its associated wrapper-senders a stop token from its stop source. This extension of simple_counting_scope allows a counting_scope to request stop on all of its outstanding operations by requesting stop on its stop source.

Assuming an exposition-only stop_when(sender auto&&, stoppable_token auto) (explained below), counting_scope behaves as if it were implemented like so:

struct counting_scope {
    struct token {
        template <sender S>
        sender auto wrap(S&& snd) const
                noexcept(std::is_nothrow_constructible_v<std::remove_cvref_t<S>, S>) {
            return stop_when(std::forward<S>(snd), scope_->source_.get_token());
        }

        async_scope_association auto try_associate() const {
            return scope_->scope_.get_token().try_associate();
        }

    private:
        friend counting_scope;

        explicit token(counting_scope* scope) noexcept
            : scope_(scope) {}

        counting_scope* scope_;
    };

    token get_token() noexcept { return token{this}; }

    void close() noexcept { return scope_.close(); }

    void request_stop() noexcept { source_.request_stop(); }

    sender auto join() noexcept { return scope_.join(); }

private:
    simple_counting_scope scope_;
    inplace_stop_source source_;
};

stop_when(sender auto&& snd, stoppable_token auto stoken) is an exposition-only sender algorithm that maps its input sender, snd, to an output sender, osnd, such that, when osnd is connected to a receiver, r, the resulting operation-state behaves the same as connecting the original sender, snd, to r, except that snd will receive a stop request when either the token returned from get_stop_token(r) receives a stop request or when stoken receives a stop request.

Other than the use of stop_when() in counting_scope::token::wrap() and the addition of request_stop() to the interface, counting_scope has the same behavior and lifecycle as simple_counting_scope.

5.8.1 counting_scope::counting_scope

counting_scope() noexcept;

Initializes a counting_scope in the unused state with the count of outstanding operations set to zero.

5.8.2 counting_scope::~counting_scope

~counting_scope();

Checks that the counting_scope is in the joined, unused, or unused-and-closed state and invokes std::terminate() if not.

5.8.3 counting_scope::get_token

counting_scope::token get_token() noexcept;

Returns a counting_scope::token referring to the current scope, as if by invoking token{this}.

5.8.4 counting_scope::close

void close() noexcept;

Moves the scope to the closed, unused-and-closed, or closed-and-joining state. After a call to close(), all future calls to nest() that return normally return unassociated senders.

5.8.5 counting_scope::request_stop

void request_stop() noexcept;

Requests stop on the scope’s internal stop source. Since all senders nested within the scope have been given stop tokens from this internal stop source, the effect is to send stop requests to all outstanding (and future) nested operations.

5.8.6 counting_scope::join

struct join-sender; // exposition-only

join-sender join() noexcept;

Returns a join-sender that behaves the same as the result of simple_counting_scope::join(). Connecting and starting the join-sender moves the scope to the open-and-joining or closed-and-joining state; the join-sender completes when the scope’s count of outstanding operations drops to zero, at which point the scope moves to the joined state.

5.8.7 counting_scope::token::wrap

template <sender S>
struct wrapper-sender; // exposition-only

template <sender Sender>
wrapper-sender<Sender> wrap(Sender&& snd);

Returns a wrapper-sender<Sender>, osnd, that behaves in all ways the same as the input sender, snd, except that, when osnd is connected to a receiver, the resulting operation-state receives stop requests from both the connected receiver and the stop source in the token’s counting_scope.

5.8.8 counting_scope::token::try_associate

async_scope_association auto try_associate() const;

Returns an async_scope_association that is engaged if the token’s scope is open, and disengaged if it’s closed. try_associate() behaves as if its counting_scope owns a simple_counting_scope, scope, and the result is equivalent to the result of invoking scope.get_token().try_associate().

5.9 When to use counting_scope vs [P3296R2]’s let_async_scope

Although counting_scope and let_async_scope have overlapping use-cases, we specifically designed the two facilities to address separate problems. In short, counting_scope is best used in an unstructured context and let_async_scope is best used in a structured context.

We define “unstructured context” as:

• a place where using sync_wait would be inappropriate,
• and you can’t “solve by induction” (i.e you’re not in an async context where you can start the sender by “awaiting” it)

counting_scope should be used when you have a sender you want to start in an unstructured context. In this case, spawn(sender, scope.get_token()) would be the preferred way of starting asynchronous work. scope.join() needs to be called before the owning object’s destruction in order to ensure that the object’s lifetime lives at least until all asynchronous work completes. Note that exception safety needs to be handled explicitly in the use of counting_scope.

let_async_scope returns a sender, and therefore can only be started in one of 3 ways:

1. sync_wait
2. spawn on a counting_scope
3. co_await

let_async_scope will manage the scope for you, ensuring that the managed scope is always joined before let_async_scope completes. The algorithm frees the user from having to manage the coupling between the lifetimes of the managed scope and the resource(s) it protects with the limitation that the nested work must be fully structured. This behavior is a feature, since the scope being managed by let_async_scope is intended to live only until the sender completes. This also means that let_async_scope will be exception safe by default.

6 Design considerations

6.1 Shape of the given sender

6.1.1 Constraints on set_value()

It makes sense for spawn_future() and nest() to accept senders with any type of completion signatures. The caller gets back a sender that can be chained with other senders, and it doesn’t make sense to restrict the shape of this sender.

The same reasoning doesn’t necessarily follow for spawn() as it returns void and the result of the spawned sender is dropped. There are two main alternatives:

• do not constrain the shape of the input sender (i.e., dropping the results of the computation)
• constrain the shape of the input sender

The current proposal goes with the second alternative. The main reason is to make it more difficult and explicit to silently drop results. The caller can always transform the input sender before passing it to spawn() to drop the values manually.

Chosen: spawn() accepts only senders that advertise set_value() (without any parameters) in the completion signatures.

6.1.2 Handling errors in spawn()

The current proposal does not accept senders that can complete with error given to spawn(). This will prevent accidental error scenarios that will terminate the application. The user must deal with all possible errors before passing the sender to spawn(). i.e., error handling must be explicit.

Another alternative considered was to call std::terminate() when the sender completes with error.

Another alternative is to silently drop the errors when receiving them. This is considered bad practice, as it will often lead to first spotting bugs in production.

Chosen: spawn() accepts only senders that do not call set_error(). Explicit error handling is preferred over stopping the application, and over silently ignoring the error.

6.1.3 Handling stop signals in spawn()

Similar to the error case, we have the alternative of allowing or forbidding set_stopped() as a completion signal. Because the goal of counting_scope is to track the lifetime of the work started through it, it shouldn’t matter whether that the work completed with success or by being stopped. As it is assumed that sending the stop signal is the result of an explicit choice, it makes sense to allow senders that can terminate with set_stopped().

The alternative would require transforming the sender before passing it to spawn, something like spawn(std::move(snd) | let_stopped(just), s.get_token()). This is considered boilerplate and not helpful, as the stopped scenarios should be implicit, and not require handling.

Chosen: spawn() accepts senders that complete with set_stopped().

6.1.4 No shape restrictions for the senders passed to spawn_future() and nest()

Similarly to spawn(), we can constrain spawn_future() and nest() to accept only a limited set of senders. But, because we can attach continuations for these senders, we would be limiting the functionality that can be expressed. For example, the continuation can handle different types of values and errors.

Chosen: spawn_future() and nest() accept senders with any completion signatures.

6.2 P2300’s start_detached()

The spawn() algorithm in this paper can be used as a replacement for start_detached proposed in [P2300R7]. Essentially it does the same thing, but it also provides the given scope the opportunity to apply its bookkeeping policy to the given sender, which, in the case of counting_scope, ensures the program can wait for spawned work to complete before destroying any resources references by that work.

6.3 P2300’s ensure_started()

The spawn_future() algorithm in this paper can be used as a replacement for ensure_started proposed in [P2300R7]. Essentially it does the same thing, but it also provides the given scope the opportunity to apply its bookkeeping policy to the given sender, which, in the case of counting_scope, ensures the program can wait for spawned work to complete before destroying any resources references by that work.

6.4 Supporting the pipe operator

This paper doesn’t support the pipe operator to be used in conjunction with spawn() and spawn_future(). One might think that it is useful to write code like the following:

std::move(snd1) | spawn(s); // returns void
sender auto snd3 = std::move(snd2) | spawn_future(s) | then(...);

In [P2300R7] sender consumers do not have support for the pipe operator. As spawn() works similarly to start_detached() from [P2300R7], which is a sender consumer, if we follow the same rationale, it makes sense not to support the pipe operator for spawn().

On the other hand, spawn_future() is not a sender consumer, thus we might have considered adding pipe operator to it.

On the third hand, Unifex supports the pipe operator for both of its equivalent algorithms (unifex::spawn_detached() and unifex::spawn_future()) and Unifex users have not been confused by this choice.

To keep consistency with spawn() this paper doesn’t support pipe operator for spawn_future().

7 Naming

As is often true, naming is a difficult task. We feel more confident about having arrived at a reasonably good naming scheme than good names:

• There is some consensus that the default standard “scope” should be the one this paper calls counting_scope because it provides all of the obviously-useful features of a scope, while simple_counting_scope is the more spare type that only provides scoping facilities. Therefore, counting_scope should get the “nice” name, while simple_counting_scope should get a more cumbersome name that conveys fewer features in exchange for a smaller object size and fewer atomic operations.

• Most people seem to hate the name counting_scope because the “counting” is an implementation detail, there are arguments about whether it’s really “scoping” anything, and the name doesn’t really tell you what the type is for. The leading suggestion for a better name is to pick one that conveys that the type “groups together” or “keeps track of” “tasks”, “senders”, or “operations”. Examples of this scheme include task_pool, sender_group, and task_arena. We like the suggested pattern but seek LEWG’s feedback on:

  • Should we choose task or sender to desribe the thing being “grouped”? task feels friendlier, but might risk conveying that not all sender types are supported.
  • What word should we use to describe the “grouping”?
    • pool often means a pre-allocated group of resources that can be borrowed from and returned to, which isn’t appropriate.
    • group is either the most generic word for a group of things, or an unrelated mathematical object.
    • arena is used outside computing to mean a place where competitions happen, and within computing to refer to a memory allocation strategy.
    • Something else?

• The name-part token was selected by analogy to stop_token, but it feels like a loose analogy. Perhaps handle or ref (short for reference) would be better. ref is nice for being short and accurate.

• The likely use of the async_scope_token concept will be to constrain algorithms that accept a sender and a token with code like the following:

  template <sender Sender, async_scope_token Token>
  void foo(Sender, Token);

  We propose the token concept should be named async_ <new name of counting_scope> <new word for token>. Assuming we choose task_pool and ref, that would produce async_task_pool_ref, which would look like this:

  template <sender Sender, async_task_pool_ref Ref>
  void foo(Sender, Ref);

• The simple prefix does not convey much about how simple_counting_scope is “simple”. Suggestions for alternatives include:

  • fast by analogy to the fast-prefixed standard integer types, which are so-named because they’re expected to be efficient.
  • non_cancellable to speak to what’s “missing” relative to counting_scope, however, simple_counting_scope does not change the cancellability of senders nested within it and we worry that this suggestion might convey that senders nested within a non_cancellable scope might somehow lose cancellability.

7.1 async_scope_token

This is a concept that is satisfied by types that support nesting senders within themselves. It is primarily useful for constraining the arguments to spawn() and spawn_future() to give useful error messages for invalid invocations.

Since concepts don’t support existential quantifiers and thus can’t express “type T is an async_scope_token if there exists a sender, s, for which t.nest(s) is valid”, the async_scope_token concept must be parameterized on both the type of the token and the type of some particular sender and thus describes whether this token type is an async_scope_token in combination with this sender type. Given this limitation, perhaps the name should convey something about the fact that it is checking the relationship between two types rather than checking something about the scope’s type alone. Nothing satisfying comes to mind.

alternatives: task_pool_ref, task_pool_token, task_group_ref, sender_group_ref, task_group_token, sender_group_token, don’t name it and leave it as exposition-only

7.2 nest()

This provides a way to build a sender that is associated with a “scope”, which is a type that implements and enforces some bookkeeping policy regarding the senders nested within it. nest() does not allocate state, call connect, or call start.

It would be good for the name to indicate that it is a simple operation (insert, add, embed, extend might communicate allocation, which nest() does not do).

alternatives: wrap(), attach(), track(), add(), associate()

7.3 spawn()

This provides a way to start a sender that produces void and to associate the resulting async work with an async scope that can implement a bookkeeping policy that may help ensure the async work is complete before destroying any resources it is using. This allocates, connects, and starts the given sender.

It would be good for the name to indicate that it is an expensive operation.

alternatives: connect_and_start(), spawn_detached(), fire_and_remember()

7.4 spawn_future()

This provides a way to start work and later ask for the result. This will allocate, connect, and start the given sender, while resolving the race (using synchronization primitives) between the completion of the given sender and the start of the returned sender. Since the type of the receiver supplied to the result sender is not known when the given sender starts, the receiver will be type-erased when it is connected.

It would be good for the name to be ugly, to indicate that it is a more expensive operation than spawn().

alternatives: spawn_with_result()

7.5 simple_counting_scope

A simple_counting_scope represents the root of a set of nested lifetimes.

One mental model for this is a semaphore. It tracks a count of lifetimes and fires an event when the count reaches 0.

Another mental model for this is block syntax. {} represents the root of a set of lifetimes of locals and temporaries and nested blocks.

Another mental model for this is a container. This is the least accurate model. This container is a value that does not contain values. This container contains a set of active senders (an active sender is not a value, it is an operation).

alternatives: simple_async_scope, simple_task_pool, fast_task_pool, non_cancellable_task_pool, simple_task_group, simple_sender_group

7.6 counting_scope

Has all of the same behavior as simple_counting_scope, with the added functionality of cancellation; work nested in this scope can be asked to cancel en masse from the scope.

alternatives: async_scope, task_pool, task_group, sender_group

7.6.1 counting_scope::join()

This method returns a sender that, when started, prevents new senders from being nested within the scope and then waits for the scope’s count of outstanding senders to drop to zero before completing. It is somewhat analogous to std::thread::join() but does not block.

join() must be invoked, and the returned sender must be connected, started, and completed, before the scope may be destroyed so it may be useful to convey some of this importance in the name, although std::thread has similar requirements for its join().

join() is the biggest wart in this design; the need to manually manage the end of a scope’s lifetime stands out as less-than-ideal in C++, and there is some real risk that users will write deadlocks with join() so perhaps join() should have a name that conveys danger.

alternatives: complete(), close()

8 Specification

8.1 Async scope concepts

Add the following as a new subsection immediately after [exec.utils.tfxcmplsigs]:

namespace std::execution {

template <class Assoc>
concept async_scope_association =
    semiregular<Assoc> &&
    requires(const Assoc& assoc) {
        { static_cast<bool>(assoc) } noexcept;
    };
}

namespace std::execution {

template <class Token>
concept async_scope_token =
    copyable<Token> &&
    requires(Token token) {
        { token.try_associate() } -> async_scope_association;
    } &&

}

8.2 execution::nest

Add the following as a new subsection immediately after [exec.stopped.as.error]:

8.3 execution::spawn

spec here

8.4 execution::spawn_future

spec here

8.5 execution::simple_counting_scope

spec here

8.6 execution::counting_scope

spec here

9 Acknowledgements

Thanks to Lewis Baker, Robert Leahy, Dmitry Prokoptsev, Anthony Williams, and everyone else who contributed to discussions leading to this paper.

Thanks to Andrew Royes for unwavering support for the development and deployment of Unifex at Meta and for recognizing the importance of contributing this paper to the C++ Standard.

Thanks to Eric Niebler for the encouragement and support it took to get this paper published.

10 References