[WIP] Implement P2079R10 parallel_scheduler

include/beman/execution/detail/parallel_scheduler.hpp

@@ -0,0 +1,130 @@
+// include/beman/execution/detail/parallel_scheduler.hpp            -*-C++-*-
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef INCLUDED_BEMAN_EXECUTION_DETAIL_PARALLEL_SCHEDULER
+#define INCLUDED_BEMAN_EXECUTION_DETAIL_PARALLEL_SCHEDULER
+
+#include <beman/execution/detail/common.hpp>
+#ifdef BEMAN_HAS_IMPORT_STD
+import std;
+#else
+#include <cstddef>
+#include <concepts>
+#include <exception>
+#include <memory>
+#include <optional>
+#include <span>
+#include <type_traits>
+#include <utility>
+#endif
+#ifdef BEMAN_HAS_MODULES
+import beman.execution.detail.completion_signatures;
+import beman.execution.detail.get_completion_scheduler;
+import beman.execution.detail.get_forward_progress_guarantee;
+import beman.execution.detail.operation_state;
+import beman.execution.detail.receiver;
+import beman.execution.detail.scheduler;
+import beman.execution.detail.scheduler_t;
+import beman.execution.detail.sender;
+import beman.execution.detail.set_error;
+import beman.execution.detail.set_stopped;
+import beman.execution.detail.set_value;
+#else
+#include <beman/execution/detail/completion_signatures.hpp>
+#include <beman/execution/detail/get_completion_scheduler.hpp>
+#include <beman/execution/detail/get_forward_progress_guarantee.hpp>
+#include <beman/execution/detail/operation_state.hpp>
+#include <beman/execution/detail/receiver.hpp>
+#include <beman/execution/detail/scheduler.hpp>
+#include <beman/execution/detail/scheduler_t.hpp>
+#include <beman/execution/detail/sender.hpp>
+#include <beman/execution/detail/set_error.hpp>
+#include <beman/execution/detail/set_stopped.hpp>
+#include <beman/execution/detail/set_value.hpp>
+#endif
+
+// ----------------------------------------------------------------------------
+
+namespace beman::execution::system_context_replaceability {
+
+struct receiver_proxy {
+    virtual ~receiver_proxy() = default;
+
+    virtual auto set_value() noexcept -> void                     = 0;
+    virtual auto set_error(::std::exception_ptr) noexcept -> void = 0;
+    virtual auto set_stopped() noexcept -> void                   = 0;
+
+    template <class P, class Query>
+        requires(::std::same_as<P, ::std::remove_cv_t<P>> && ::std::is_object_v<P> && !::std::is_array_v<P>)
+    auto try_query(Query) noexcept -> ::std::optional<P> {
+        // TODO(P2079R10): forward supported receiver environment queries
+        // through this proxy, especially get_stop_token_t -> inplace_stop_token
+        return ::std::nullopt;
+    }
+};
+
+struct bulk_item_receiver_proxy : receiver_proxy {
+    virtual auto execute(::std::size_t, ::std::size_t) noexcept -> void = 0;
+};
+
+struct parallel_scheduler_backend {
+    virtual ~parallel_scheduler_backend() = default;
+
+    virtual auto schedule(receiver_proxy&, ::std::span<::std::byte>) noexcept -> void = 0;
+    virtual auto schedule_bulk_chunked(::std::size_t, bulk_item_receiver_proxy&, ::std::span<::std::byte>) noexcept
+        -> void = 0;
+    virtual auto schedule_bulk_unchunked(::std::size_t, bulk_item_receiver_proxy&, ::std::span<::std::byte>) noexcept
+        -> void = 0;
+};
+
+// TODO(P2079R10): provide the project-supported link-time replaceability hook.
+auto query_parallel_scheduler_backend() -> ::std::shared_ptr<parallel_scheduler_backend>;
+
+} // namespace beman::execution::system_context_replaceability
+
+namespace beman::execution {
+
+class parallel_scheduler {
+    using backend_type = ::beman::execution::system_context_replaceability::parallel_scheduler_backend;
+
+  public:
+    using scheduler_concept = ::beman::execution::scheduler_t;
+
+    class sender;
+
+    parallel_scheduler()  = delete;
+    ~parallel_scheduler() = default;
+
+    parallel_scheduler(const parallel_scheduler&) noexcept                    = default;
+    parallel_scheduler(parallel_scheduler&&) noexcept                         = default;
+    auto operator=(const parallel_scheduler&) noexcept -> parallel_scheduler& = default;
+    auto operator=(parallel_scheduler&&) noexcept -> parallel_scheduler&      = default;
+
+    auto operator==(const parallel_scheduler& other) const noexcept -> bool {
+        return this->backend_ == other.backend_;
+    }
+
+    static constexpr auto query(::beman::execution::get_forward_progress_guarantee_t) noexcept
+        -> ::beman::execution::forward_progress_guarantee {
+        return ::beman::execution::forward_progress_guarantee::parallel;
+    }
+
+    auto schedule() const noexcept -> sender;
+    // TODO(P2079R10): customize bulk_chunked and bulk_unchunked for this scheduler.
+
+  private:
+    explicit parallel_scheduler(::std::shared_ptr<backend_type> backend) noexcept : backend_(::std::move(backend)) {}
+
+    ::std::shared_ptr<backend_type> backend_;
+
+    friend auto get_parallel_scheduler() -> parallel_scheduler;
+};
+
+// TODO(P2079R10): implement using system_context_replaceability::query_parallel_scheduler_backend().
+auto get_parallel_scheduler() -> parallel_scheduler;
+
+} // namespace beman::execution
+
+// ----------------------------------------------------------------------------
+
+#endif // INCLUDED_BEMAN_EXECUTION_DETAIL_PARALLEL_SCHEDULER

include/beman/execution/execution.hpp

@@ -39,6 +39,7 @@ import beman.execution.detail.let;
 import beman.execution.detail.matching_sig;
 import beman.execution.detail.on;
 import beman.execution.detail.operation_state;
+import beman.execution.detail.parallel_scheduler;
 import beman.execution.detail.prop;
 import beman.execution.detail.read_env;
 import beman.execution.detail.receiver;
@@ -101,6 +102,7 @@ import beman.execution.detail.write_env;
 #include <beman/execution/detail/matching_sig.hpp>
 #include <beman/execution/detail/on.hpp>
 #include <beman/execution/detail/operation_state.hpp>
+#include <beman/execution/detail/parallel_scheduler.hpp>
 #include <beman/execution/detail/prop.hpp>
 #include <beman/execution/detail/read_env.hpp>
 #include <beman/execution/detail/receiver.hpp>

src/beman/execution/CMakeLists.txt

@@ -133,6 +133,7 @@ target_sources(
                 ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/on_stop_request.hpp
                 ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/operation_state.hpp
                 ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/operation_state_task.hpp
+                ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/parallel_scheduler.hpp
                 ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/product_type.hpp
                 ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/prop.hpp
                 ${PROJECT_SOURCE_DIR}/include/beman/execution/detail/query_with_default.hpp
@@ -327,6 +328,7 @@ if(BEMAN_USE_MODULES)
                     on.cppm
                     operation_state_task.cppm
                     operation_state.cppm
+                    parallel_scheduler.cppm
                     product_type.cppm
                     prop.cppm
                     query_with_default.cppm

src/beman/execution/execution-detail.cppm

@@ -59,6 +59,7 @@ export import beman.execution.detail.non_assignable;
 export import beman.execution.detail.nothrow_callable;
 export import beman.execution.detail.notify;
 export import beman.execution.detail.operation_state_task;
+export import beman.execution.detail.parallel_scheduler;
 export import beman.execution.detail.product_type;
 export import beman.execution.detail.query_with_default;
 export import beman.execution.detail.queryable;

src/beman/execution/execution.cppm

@@ -43,6 +43,7 @@ import beman.execution.detail.never_stop_token;
 import beman.execution.detail.nostopstate;
 import beman.execution.detail.on;
 export import beman.execution.detail.operation_state; // [exec.opstate], operation states
+import beman.execution.detail.parallel_scheduler;
 import beman.execution.detail.prop;
 import beman.execution.detail.read_env;
 import beman.execution.detail.run_loop;
@@ -237,6 +238,10 @@ export using ::beman::execution::stopped_as_error;
 // [exec.run.loop], run_loop
 export using ::beman::execution::run_loop;
 
+// [exec.parallel.scheduler], parallel scheduler
+export using ::beman::execution::parallel_scheduler;
+export using ::beman::execution::get_parallel_scheduler;
+
 // [exec.consumers], consumers
 export using ::beman::execution::sync_wait_t;
 export using ::beman::execution::sync_wait_with_variant_t;

src/beman/execution/parallel_scheduler.cppm

@@ -0,0 +1,19 @@
+module;
+// src/beman/execution/parallel_scheduler.cppm                       -*-C++-*-
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <beman/execution/detail/parallel_scheduler.hpp>
+
+export module beman.execution.detail.parallel_scheduler;
+
+namespace beman::execution {
+export using beman::execution::parallel_scheduler;
+export using beman::execution::get_parallel_scheduler;
+
+namespace system_context_replaceability {
+export using beman::execution::system_context_replaceability::receiver_proxy;
+export using beman::execution::system_context_replaceability::bulk_item_receiver_proxy;
+export using beman::execution::system_context_replaceability::parallel_scheduler_backend;
+export using beman::execution::system_context_replaceability::query_parallel_scheduler_backend;
+} // namespace system_context_replaceability
+} // namespace beman::execution