task<> scheduler affinity (facebookexperimental#290)

This commit implements scheduler affinity -- aka "sticky" scheduling -- in `unifex::task<>`. The idea is that it is impossible for a child operation to cause the current coroutine to resume on the wrong execution context.

* `task<>`-based coroutines track and propagate the current scheduler
* `at_coroutine_exit` remembers current scheduler from when the cleanup action is scheduled
* `schedule` always returns an instance of `sender_for<schedule, the_sender>`,
  which is also a `scheduler_provider`
* scheduler affinity when co_await-ing senders in a `task<>`-returning coroutine
* scheduler affinity when co_await-ing awaitables in a `task<>`-returning coroutine
* awaitables and senders that are `blocking_kind::always_inline` don't get a thunk
* More senders and awaitables support compile-time blocking queries
* `co_await schedule(sched)` is magic in a `task<>`-returning coroutine: it changes execution
  context and schedules a cleanup action to transition back to the original scheduler

Move implementation of special co_await behavior of scheduler senders out of task.hpp

Hoist untyped RAII containers for coroutine_handle<> out of task<> and its awaiter (facebookexperimental#329)

While looking at the binary size impact of adopting coroutines with
`unifex::task<>`, I noticed that a number of operations on
`coroutine_handle<T>` are expressed in `unifex::task<>` as if they
depend on `T` when they don't.  The consequence is extra code.

This diff creates a `coro_holder` class that uniquely owns a
`coroutine_handle<>` and makes `unifex::task<>` inherit from it.  We
technically lose some type safety, but it's still correct by
construction.  This change saves about 1.5 kilobytes in one of our apps.

Similar to the above, I noticed binary duplication due to false
template parameter dependencies in `unifex::task<>`'s awaiter type.

This diff hoists a non-type-specific RAII container for a
`coroutine_handle<>` that stores the handle as a `std::uintptr_t` so
that `task`'s awaiter can use the low bit as a dirty flag.  This change
saves another ~1.5 kilobytes in one of our apps.

Fix scheduler affinity (facebookexperimental#405)

* Fix scheduler affinity

We have been storing a `task<>`'s scheduler as an `any_scheduler_ref`,
which has proven to be a source of use-after-free bugs.  This change
switches all the `any_scheduler_ref`s to `any_scheduler`s, fixing the
lifetime issues.

Make task<>'s thunk-on-resume unstoppable (facebookexperimental#495)

* Make task<>'s thunk-on-resume unstoppable

When awaiting an async Sender that swallows done signals (such as
let_done(never_sender{}, just)), the user-level code looks like it
swallows done signals:
```
// never cancels
co_await let_done(never_sender{}, just);
```

However, `task<>`'s Scheduler affinity implementation transforms the
above code into this:
```
co_await typed_via(let_done(never_sender{}, just), <current scheduler>);
```

The `schedule()` operation inside the injected `typed_via` can emit done
if the current stop token has had stop requested, leading to very
non-obvious cancellation behaviour that can't be worked around.

This diff introduces a pair of regression tests that capture the above
scenario, and the analogous scenario of awaiting an async Awaitable that
completes with done.  The next diff will fix these failing tests.

* Change task<>'s thunk-on-resume to be unstoppable

This diff fixes the broken tests in the previous diff.

Respect blocking_kind in `let_value()` (facebookexperimental#381)

* `let_value()` would always assume `blocking_kind::maybe`, which
  results in potentially unnecessary reschedule on resumption
* replicate `blocking_kind` customization from `finally()`

fix `variant_sender` blocking kind (facebookexperimental#474)

add `unifex::v2::async_scope` (facebookexperimental#463)

* simpler than `unifex::v1::async_scope` (`nest()` and `join()`)
* does not support cancellation

fixing linter error (facebookexperimental#414)

move deduction guide to namespace scope for gcc-10

in scheduler concept, check copy_constructability after requiring call to schedule()

work around gcc-10 bugs

avoid warning about missing braces in initializer

back out change to awaiter_type_t

Co-authored-by: Eric Niebler <eniebler@boost.org>
Co-authored-by: Ian Petersen <ispeters@gmail.com>
Co-authored-by: Ondrej Lehecka <lehecka@fb.com>

include/unifex/allocate.hpp

@@ -22,6 +22,7 @@
 #include <unifex/sender_concepts.hpp>
 #include <unifex/tag_invoke.hpp>
 #include <unifex/bind_back.hpp>
+#include <unifex/blocking.hpp>
 
 #include <memory>
 #include <type_traits>
@@ -105,6 +106,10 @@ namespace _alloc {
           static_cast<Self&&>(s).sender_, (Receiver &&) r};
     }
 
+    friend constexpr auto tag_invoke(tag_t<unifex::blocking>, const type& self) noexcept {
+      return blocking(self.sender_);
+    }
+
     Sender sender_;
   };
 } // namespace _alloc

include/unifex/any_scheduler.hpp

@@ -198,6 +198,10 @@ struct _with<CPOs...>::any_scheduler {
     return _sender{this};
   }
 
+  type_index type() const noexcept {
+    return _get_type_index(impl_);
+  }
+
   friend _equal_to_fn;
   friend bool operator==(const any_scheduler& left, const any_scheduler& right) noexcept {
     return _equal_to(left.impl_, right);
@@ -210,15 +214,44 @@ struct _with<CPOs...>::any_scheduler {
   any_scheduler_impl<CPOs...> impl_;
 };
 
-template <typename... ReceiverCPOs>
-using any_scheduler_ref_impl = any_ref_t<_schedule_and_connect<ReceiverCPOs...>>;
+template <typename... CPOs>
+using any_scheduler_ref_impl =
+    any_ref_t<
+        _schedule_and_connect<CPOs...>,
+        _get_type_index,
+        overload<bool(const this_&, const any_scheduler_ref<CPOs...>&) noexcept>(_equal_to)>;
+
+#if defined(__GLIBCXX__)
+template <typename>
+inline constexpr bool _is_tuple = false;
+
+template <typename... Ts>
+inline constexpr bool _is_tuple<std::tuple<Ts...>> = true;
+
+template <typename... Ts>
+inline constexpr bool _is_tuple<std::tuple<Ts...> const> = true;
+#endif
 
 template <typename... CPOs>
 struct _with<CPOs...>::any_scheduler_ref {
+#if !defined(__GLIBCXX__)
   template (typename Scheduler)
-    (requires (!same_as<const Scheduler, const any_scheduler_ref>) AND scheduler<Scheduler>)
+    (requires (!same_as<const Scheduler, const any_scheduler_ref>) AND
+      scheduler<Scheduler>)
   /* implicit */ any_scheduler_ref(Scheduler& sched) noexcept
     : impl_(sched) {}
+#else
+  // Under-constrained implicit tuple converting constructor from a
+  // single argument doesn't exclude instances of the tuple type
+  // itself, so it is considered for copy/move constructors, leading
+  // to constraint recursion with the any_scheduler_ref constructor
+  // below.
+  template (typename Scheduler)
+    (requires (!same_as<const Scheduler, const any_scheduler_ref>) AND
+      (!_is_tuple<Scheduler>) AND scheduler<Scheduler>)
+  /* implicit */ any_scheduler_ref(Scheduler& sched) noexcept
+    : impl_(sched) {}
+#endif
 
   struct _sender {
     template <template <class...> class Variant, template <class...> class Tuple>
@@ -257,14 +290,26 @@ struct _with<CPOs...>::any_scheduler_ref {
     return _sender{this};
   }
 
+  type_index type() const noexcept {
+    return _get_type_index(impl_);
+  }
+
+  // Shallow equality comparison by default, for regularity:
   friend bool operator==(const any_scheduler_ref& left, const any_scheduler_ref& right) noexcept {
     return left.impl_ == right.impl_;
   }
   friend bool operator!=(const any_scheduler_ref& left, const any_scheduler_ref& right) noexcept {
     return !(left == right);
   }
 
+  // Deep equality comparison:
+  friend _equal_to_fn;
+  bool equal_to(const any_scheduler_ref& that) const noexcept {
+    return _equal_to(impl_, that);
+  }
+
 private:
+
   any_scheduler_ref_impl<CPOs...> impl_;
 };
 

include/unifex/async_trace.hpp

@@ -103,7 +103,7 @@ namespace _async_trace {
       return operation<Receiver>{(Receiver &&) r};
     }
 
-    friend blocking_kind tag_invoke(tag_t<blocking>, const sender&) noexcept {
+    friend auto tag_invoke(tag_t<blocking>, const sender&) noexcept {
       return blocking_kind::always_inline;
     }
   };

include/unifex/at_coroutine_exit.hpp

@@ -19,9 +19,13 @@
 #include <unifex/tag_invoke.hpp>
 #include <unifex/await_transform.hpp>
 #include <unifex/continuations.hpp>
+#include <unifex/scheduler_concepts.hpp>
 #include <unifex/stop_token_concepts.hpp>
 #include <unifex/unstoppable_token.hpp>
 #include <unifex/get_stop_token.hpp>
+#include <unifex/inline_scheduler.hpp>
+#include <unifex/any_scheduler.hpp>
+#include <unifex/blocking.hpp>
 
 #if UNIFEX_NO_COROUTINES
 # error "Coroutine support is required to use this header"
@@ -115,11 +119,19 @@ struct _cleanup_promise_base {
   }
 #endif
 
-  friend unstoppable_token tag_invoke(tag_t<get_stop_token>, const _cleanup_promise_base&) noexcept {
+  friend unstoppable_token
+  tag_invoke(tag_t<get_stop_token>, const _cleanup_promise_base&) noexcept {
     return unstoppable_token{};
   }
 
+  friend any_scheduler
+  tag_invoke(tag_t<get_scheduler>, const _cleanup_promise_base& p) noexcept {
+    return p.sched_;
+  }
+
+  inline static constexpr inline_scheduler _default_scheduler{};
   continuation_handle<> continuation_{};
+  any_scheduler sched_{_default_scheduler};
   bool isUnhandledDone_{false};
 };
 
@@ -145,6 +157,15 @@ struct _die_on_done_rec {
       UNIFEX_ASSERT(!"A cleanup action tried to cancel. Calling terminate...");
       std::terminate();
     }
+
+    template(typename CPO)
+      (requires is_receiver_query_cpo_v<CPO> AND
+                is_callable_v<CPO, const Receiver&>)
+    friend auto tag_invoke(CPO cpo, const type& p)
+        noexcept(is_nothrow_callable_v<CPO, const Receiver&>)
+        -> callable_result_t<CPO, const Receiver&> {
+      return cpo(p.rec_);
+    }
   };
 };
 
@@ -177,7 +198,7 @@ struct _die_on_done {
           _die_on_done_rec_t<Receiver>{(Receiver&&) rec});
     }
 
-    Sender sender_;
+    UNIFEX_NO_UNIQUE_ADDRESS Sender sender_;
   };
 };
 
@@ -229,7 +250,7 @@ struct _cleanup_promise : _cleanup_promise_base {
     return unifex::await_transform(*this, _die_on_done_fn{}((Value&&) value));
   }
 
-  std::tuple<Ts&...> args_;
+  UNIFEX_NO_UNIQUE_ADDRESS std::tuple<Ts&...> args_;
 };
 
 template <typename... Ts>
@@ -251,14 +272,24 @@ struct [[nodiscard]] _cleanup_task {
   }
 
   template <typename Promise>
-  bool await_suspend(coro::coroutine_handle<Promise> parent) noexcept {
+  bool await_suspend_impl_(Promise& parent) noexcept {
     continuation_.promise().continuation_ =
-        exchange_continuation(parent.promise(), continuation_);
+        exchange_continuation(parent, continuation_);
+    continuation_.promise().sched_ = get_scheduler(parent);
     return false;
   }
 
+  template <typename Promise>
+  bool await_suspend(coro::coroutine_handle<Promise> parent) noexcept {
+    return await_suspend_impl_(parent.promise());
+  }
+
   std::tuple<Ts&...> await_resume() noexcept {
-    return std::exchange(continuation_, {}).promise().args_;
+    return std::move(std::exchange(continuation_, {}).promise().args_);
+  }
+
+  friend constexpr auto tag_invoke(tag_t<blocking>, const _cleanup_task&) noexcept {
+    return blocking_kind::always_inline;
   }
 
 private:
@@ -275,7 +306,7 @@ namespace _at_coroutine_exit {
   public:
     template (typename Action, typename... Ts)
       (requires callable<std::decay_t<Action>, std::decay_t<Ts>...>)
-    _cleanup_task<Ts...> operator()(Action&& action, Ts&&... ts) const {
+    _cleanup_task<std::decay_t<Ts>...> operator()(Action&& action, Ts&&... ts) const {
       return _fn::at_coroutine_exit((Action&&) action, (Ts&&) ts...);
     }
   } at_coroutine_exit{};

include/unifex/await_transform.hpp

@@ -85,13 +85,13 @@ struct _awaitable_base<Promise, Value>::type {
   struct _rec {
   public:
     explicit _rec(_expected<Value>* result, coro::coroutine_handle<Promise> continuation) noexcept
-    : result_(result)
-    , continuation_(continuation)
+      : result_(result)
+      , continuation_(continuation)
     {}
 
     _rec(_rec&& r) noexcept
-    : result_(std::exchange(r.result_, nullptr))
-    , continuation_(std::exchange(r.continuation_, nullptr))
+      : result_(std::exchange(r.result_, nullptr))
+      , continuation_(std::exchange(r.continuation_, nullptr))
     {}
 
     template(class... Us)
@@ -185,13 +185,16 @@ struct _awaitable<Promise, Sender>::type
 template <typename Promise, typename Sender>
 using _as_awaitable = typename _awaitable<Promise, Sender>::type;
 
-inline const struct _fn {
+struct _fn {
   // Call custom implementation if present.
   template(typename Promise, typename Value)
     (requires tag_invocable<_fn, Promise&, Value>)
   auto operator()(Promise& promise, Value&& value) const
     noexcept(is_nothrow_tag_invocable_v<_fn, Promise&, Value>)
     -> tag_invoke_result_t<_fn, Promise&, Value> {
+    static_assert(detail::_awaitable<tag_invoke_result_t<_fn, Promise&, Value>>,
+        "The return type of a customization of unifex::await_transform() "
+        "must satisfy the awaitable concept.");
     return unifex::tag_invoke(_fn{}, promise, (Value&&)value);
   }
 
@@ -218,7 +221,7 @@ inline const struct _fn {
       return (Value&&) value;
     }
   }
-} await_transform {};
+};
 
 } // namespace _await_tfx
 
@@ -231,7 +234,7 @@ inline const struct _fn {
 //
 // Coroutine promise_types can implement their .await_transform() methods to
 // forward to this customisation point to enable use of type customisations.
-using _await_tfx::await_transform;
+inline constexpr _await_tfx::_fn await_transform {};
 
 } // namespace unifex
 

include/unifex/blocking.hpp

@@ -22,10 +22,11 @@
 
 namespace unifex {
 
-enum class blocking_kind {
+namespace _block {
+enum class _enum {
   // No guarantees about the timing and context on which the receiver will
   // be called.
-  maybe,
+  maybe = 0,
 
   // Always completes asynchronously.
   // Guarantees that the receiver will not be called on the current thread
@@ -44,8 +45,46 @@ enum class blocking_kind {
   always_inline
 };
 
-namespace _blocking {
-inline const struct _fn {
+struct blocking_kind {
+  template <_enum Kind>
+  using constant = std::integral_constant<_enum, Kind>;
+
+  blocking_kind() = default;
+
+  constexpr blocking_kind(_enum kind) noexcept
+    : value(kind)
+  {}
+
+  template <_enum Kind>
+  constexpr blocking_kind(constant<Kind>) noexcept
+    : value(Kind)
+  {}
+
+  constexpr operator _enum() const noexcept {
+    return value;
+  }
+
+  constexpr _enum operator()() const noexcept {
+    return value;
+  }
+
+  friend constexpr bool operator==(blocking_kind a, blocking_kind b) noexcept {
+    return a.value == b.value;
+  }
+
+  friend constexpr bool operator!=(blocking_kind a, blocking_kind b) noexcept {
+    return a.value != b.value;
+  }
+
+  static constexpr constant<_enum::maybe> maybe {};
+  static constexpr constant<_enum::never> never {};
+  static constexpr constant<_enum::always> always {};
+  static constexpr constant<_enum::always_inline> always_inline {};
+
+  _enum value{};
+};
+
+struct _fn {
   template(typename Sender)
     (requires tag_invocable<_fn, const Sender&>)
   constexpr auto operator()(const Sender& s) const
@@ -55,12 +94,32 @@ inline const struct _fn {
   }
   template(typename Sender)
     (requires (!tag_invocable<_fn, const Sender&>))
-  constexpr blocking_kind operator()(const Sender&) const noexcept {
+  constexpr auto operator()(const Sender&) const noexcept {
+    return blocking_kind::maybe;
+  }
+};
+
+namespace _cfn {
+  template <_enum Kind>
+  static constexpr auto _kind(blocking_kind::constant<Kind> kind) noexcept {
+    return kind;
+  }
+  static constexpr auto _kind(blocking_kind) noexcept {
     return blocking_kind::maybe;
   }
-} blocking{};
-} // namespace _blocking
-using _blocking::blocking;
+
+  template <typename T>
+  constexpr auto cblocking() noexcept {
+    using blocking_t = remove_cvref_t<decltype(_fn{}(UNIFEX_DECLVAL(T&)))>;
+    return _cfn::_kind(blocking_t{});
+  }
+}
+
+} // namespace _block
+
+inline constexpr _block::_fn blocking {};
+using _block::_cfn::cblocking;
+using _block::blocking_kind;
 
 } // namespace unifex