Tony tables showing async_scope_association by ispeters · Pull Request #49 · ispeters/async_scope

ispeters · 2025-05-23T04:13:34Z

The result of our afternoon pairing on what nest's implementation would look like if we brought back async_scope_association.

The result of our afternoon pairing on what `nest`'s implementation would look like if we brought back `async_scope_association`.

I *think* this makes _`nest-data`_ and its opstate for both rvalue and lvalue connections with and without an association. Things to consider: - what if `connect` throws? - what if the _`wrap-sender`_'s move-constructor throws? - what if the _`wrap-sender`_'s copy-constructor throws? - what if the association's copy-constructor throws? - what if the association's copy-constructor fails? Icky things: - you *must* call `nest-data::get_assoc` before `nest-data::get_sender` - I'm not entirely sure the rvalue case is safe if the sender's move-constructor throws Some of the problems would be solved if _`nest-data`_ stored an optional<wrap-sender> instead of a possibly-uninitialized wrap-sender.

This diff introduces what I think is a good interface for the association-enhanced _`nest-data`_. The central change is that _`nest-data`_ now has this method: ```cpp using scope_ref = unique_ptr<wrap_sender, deleter>; pair<assoc_t, scope_ref> release() && noexcept; ``` where _`deleter`_ is a deleter that invokes the destructor on the to-be-deleted pointer when it's not-null. This gives us a way to move the association out of the source _`nest-data`_ and a reference to its sender that we can also move from, with an attached guarantee that the source sender will have its destructor invoked in all cases. The one weird thing about this interface is that the returned `pair` is only valid so long as the source _`nest-data`_ instance still exists since the `scope_ref`'s destructor will reach into the source instance to destroy the sender therein, which is clearly not allowed if the source instance has already been destroyed.

ispeters · 2025-05-26T22:43:01Z

+struct @_spawn-state_@ : @_spawn-state-base_@ {
+    using @_op-t_@ = decltype(connect(declval<Sender>(), @_spawn-receiver_@{nullptr}));
+
+    @_spawn-state_@(Alloc alloc, Sender&& sndr, Token token) noexcept(...)
+                : alloc(alloc),
+                  op(connect(std::move(sndr), @_spawn-receiver_@{this})),
+                  token(std::move(token)) {}
+    void @_run_@() {
+        if (@_token_@.try_associate())
+            op.start();
+        else
+            @_destroy_@();
+    }
+    void @_complete_@() override {
+        auto token = std::move(this->@_token_@);
+
+        @_destroy_@();
+
+        token.disassociate();
+    }
+
+private:
+    using @_alloc-t_@ = typename allocator_traits<Alloc>::template rebind_alloc<@_spawn-state_@>;
+
+    @_alloc-t_@ alloc;
+    @_op-t_@ op;
+    Token @_token_@;
+
+    void @_destroy_@() noexcept {
+        auto alloc = std::move(this->alloc);
+
+        allocator_traits<@_alloc-t_@>::destroy(alloc, this);
+        allocator_traits<@_alloc-t_@>::deallocate(alloc, this, 1);
+    }
+


To match below, there are a few things that need cleaning up:

more noexcept

more expo-only.

Suggested change

struct @_spawn-state_@ : @_spawn-state-base_@ {

using @_op-t_@ = decltype(connect(declval<Sender>(), @_spawn-receiver_@{nullptr}));

@_spawn-state_@(Alloc alloc, Sender&& sndr, Token token) noexcept(...)

: alloc(alloc),

op(connect(std::move(sndr), @_spawn-receiver_@{this})),

token(std::move(token)) {}

void @_run_@() {

if (@_token_@.try_associate())

op.start();

else

@_destroy_@();

}

void @_complete_@() override {

auto token = std::move(this->@_token_@);

@_destroy_@();

token.disassociate();

}

private:

using @_alloc-t_@ = typename allocator_traits<Alloc>::template rebind_alloc<@_spawn-state_@>;

@_alloc-t_@ alloc;

@_op-t_@ op;

Token @_token_@;

void @_destroy_@() noexcept {

auto alloc = std::move(this->alloc);

allocator_traits<@_alloc-t_@>::destroy(alloc, this);

allocator_traits<@_alloc-t_@>::deallocate(alloc, this, 1);

}

struct @_spawn-state_@ : @_spawn-state-base_@ {

using @_op-t_@ = connect_result_t<Sender, @_spawn-receiver_@>;

@_spawn-state_@(Alloc a, Sender&& sndr, Token t) noexcept(...)

: @_alloc_@(std::move(a)),

@_op_@(connect(std::move(sndr), @_spawn-receiver_@{this})),

@_token_@(std::move(t)) {}

void @_run_@() {

if (@_token_@.try_associate())

op.start();

else

@_destroy_@();

}

void @_complete_@() noexcept override {

auto t = std::move(@_token_@);

@_destroy_@();

t.disassociate();

}

private:

using @_alloc-t_@ = // exposition only

typename allocator_traits<Alloc>::template rebind_alloc<@_spawn-state_@>;

@_alloc-t_@ @_alloc_@; // exposition only

@_op-t_@ @_op_@; // exposition only

Token @_token_@; // exposition only

void @_destroy_@() noexcept {

auto a = std::move(@_alloc_@);

allocator_traits<@_alloc-t_@>::destroy(a, this);

allocator_traits<@_alloc-t_@>::deallocate(a, this, 1);

}

ispeters · 2025-05-26T22:46:37Z

+template <class Alloc, async_scope_token Token, sender Sender>
+struct @_spawn-state_@ : @_spawn-state-base_@ {
+    using @_op-t_@ = decltype(connect(declval<Sender>(), @_spawn-receiver_@{nullptr}));
+    using @_assoc-t_@ = remove_cvref_t<decltype(declval<Token&>().try_associate())>;
+
+    @_spawn-state_@(Alloc alloc, Sender&& sndr, Token token) noexcept(...)
+                : alloc(alloc),
+                  op(connect(std::move(sndr), @_spawn-receiver_@{this})),
+                  assoc(token.try_associate()) {}
+    void @_run_@() {
+        if (assoc)
+            op.start();
+        else
+            @_destroy_@();
+    }
+    void @_complete_@() override {
+        auto assoc = std::move(this->assoc);
+        @_destroy_@();
+    }
+
+private:
+    using @_alloc-t_@ = typename allocator_traits<Alloc>::template rebind_alloc<@_spawn-state_@>;
+
+    @_alloc-t_@ alloc;
+    @_op-t_@ op;
+    @_assoc-t_@ assoc;
+
+    void @_destroy_@() noexcept {
+        auto alloc = std::move(this->alloc);
+
+        allocator_traits<@_alloc-t_@>::destroy(alloc, this);
+        allocator_traits<@_alloc-t_@>::deallocate(alloc, this, 1);
+    }
+
+};


I few thoughts:

I think we can simplify even a little more,

we should add some noexcepts to both sides of the comparison, and

the members should be in italics and expo-only.

Suggested change

template <class Alloc, async_scope_token Token, sender Sender>

struct @_spawn-state_@ : @_spawn-state-base_@ {

using @_op-t_@ = decltype(connect(declval<Sender>(), @_spawn-receiver_@{nullptr}));

using @_assoc-t_@ = remove_cvref_t<decltype(declval<Token&>().try_associate())>;

@_spawn-state_@(Alloc alloc, Sender&& sndr, Token token) noexcept(...)

: alloc(alloc),

op(connect(std::move(sndr), @_spawn-receiver_@{this})),

assoc(token.try_associate()) {}

void @_run_@() {

if (assoc)

op.start();

else

@_destroy_@();

}

void @_complete_@() override {

auto assoc = std::move(this->assoc);

@_destroy_@();

}

private:

using @_alloc-t_@ = typename allocator_traits<Alloc>::template rebind_alloc<@_spawn-state_@>;

@_alloc-t_@ alloc;

@_op-t_@ op;

@_assoc-t_@ assoc;

void @_destroy_@() noexcept {

auto alloc = std::move(this->alloc);

allocator_traits<@_alloc-t_@>::destroy(alloc, this);

allocator_traits<@_alloc-t_@>::deallocate(alloc, this, 1);

}

};

template <class Alloc, async_scope_token Token, sender Sender>

struct @_spawn-state_@ : @_spawn-state-base_@ {

using @_op-t_@ = connect_result_t<Sender, @_spawn-receiver_@>;

using @_assoc-t_@ = remove_cvref_t<decltype(declval<Token&>().try_associate())>;

@_spawn-state_@(Alloc a, Sender&& sndr, Token t) noexcept(...)

: @_alloc_@(std::move(a)),

@_op_@(connect(std::move(sndr), @_spawn-receiver_@{this})) {

@_assoc_@ = t.try_associate();

}

void @_run_@() noexcept {

if (@_assoc_@)

@_op_@.start();

else

@_destroy_@();

}

void @_complete_@() noexcept override {

auto a = std::move(@_assoc_@);

@_destroy_@();

}

private:

using @_alloc-t_@ = // exposition only

typename allocator_traits<Alloc>::template rebind_alloc<@_spawn-state_@>;

@_assoc-t_@ @_assoc_@; // exposition only

@_alloc-t_@ @_alloc_@; // exposition only

@_op-t_@ @_op_@; // exposition only

void @_destroy_@() noexcept { // exposition only

auto a = std::move(@_alloc_@);

allocator_traits<@_alloc-t_@>::destroy(a, this);

allocator_traits<@_alloc-t_@>::deallocate(a, this, 1);

}

};

ispeters · 2025-05-27T01:07:50Z

+        else
+            @_destroy_@();
+    }
+    void @_complete_@() noexcept(is_nothrow_move_constructible_v<Token>) override {


The token concept requires that tokens be no-throw move-constructible so we can leave this off.

Suggested change

void @_complete_@() noexcept(is_nothrow_move_constructible_v<Token>) override {

void @_complete_@() noexcept override {

ispeters · 2025-05-27T01:08:43Z

+        @_assoc_@ = t.try_associate();
+    }
+
+    void @_run_@() {


This can be noexcept because all the constituent ops are. Also, it's expo-only.

Suggested change

void @_run_@() {

void @_run_@() noexcept { // exposition only

ispeters · 2025-05-27T01:09:13Z

+        else
+            @_destroy_@();
+    }
+    void @_complete_@() noexcept(is_nothrow_move_constructible_v<@_assoc_t_@>) override {


Same as above, the association has to be no-throw move-constructible, and this is expo-only.

Suggested change

void @_complete_@() noexcept(is_nothrow_move_constructible_v<@_assoc_t_@>) override {

void @_complete_@() noexcept override { // exposition only

ispeters added 4 commits May 22, 2025 21:11

Tony tables showing async_scope_association

8f600aa

The result of our afternoon pairing on what `nest`'s implementation would look like if we brought back `async_scope_association`.

More Tony tables

2541bb5

jesswong reviewed May 26, 2025

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Comment thread asyncscope.md Outdated

ispeters and others added 2 commits May 26, 2025 13:47

Fix typo in from-parts nest-data constructor

508ad6d

Add before/after for spawn

88f22b2

ispeters commented May 26, 2025

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Add noexcept clause and exposition only comments to spawn

2fc55b8

ispeters commented May 27, 2025

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Remove noexcept on spawn-state and simplify code

9091254

ispeters commented May 27, 2025

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Tony tables showing async_scope_association#49

Tony tables showing async_scope_association#49
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	void @_complete_@() noexcept(is_nothrow_move_constructible_v<Token>) override {
	void @_complete_@() noexcept override {

	void @_run_@() {
	void @_run_@() noexcept { // exposition only

	void @_complete_@() noexcept(is_nothrow_move_constructible_v<@_assoc_t_@>) override {
	void @_complete_@() noexcept override { // exposition only

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