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`stop-when`: the fused token's callback can run twice, or be skipped, when the first token is already stopped

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@workingBanana 已經在處理了。

開始於 2026年10月12日。

  • #2327 來自 @workingBanana —— 未關閉

評估

難度
3/5
預估耗時
1-2 天
新手友好度
35/100
Issue 類型
缺陷
描述清晰度
描述清楚
活躍度
停滯
技術堆疊
cpp
領域
backend-api-design

研究方向

The issue identifies __stop_when_::__fused_token<T1, T2>::callback_type and gives a reproducer using stdexec/execution.hpp. Read the callback member declarations and constructor initialization order, then run the reproducer to check immediate invocation when the first token is already stopped. Done means the callback runs once at registration and is not called again when the second token stops. Pull request #2327 is already open against this issue.

由索引模型根據 Issue 內容生成。

描述

Summary

__stop_when_::__fused_token<T1, T2>::callback_type declares its once-flag __called_ after the two inner callbacks:

_Fn                 __fn_;
__cb1_t             __cb1_;
__cb2_t             __cb2_;
__std::atomic<bool> __called_{false};   // initialized last

If the first token is already stopped, __cb1_'s constructor invokes the callback immediately. That callback does __called_.exchange(true) on an object whose lifetime hasn't started yet. Then __called_'s initializer stores false. So the result depends on the indeterminate value:

  • if it was 0: the function runs at registration, the flag is reset, and the function runs a second time when the second token is stopped;
  • if it was non-zero: the function is skipped at registration. It runs only if the second token is stopped later, and is lost otherwise.

Both violate [exec.stop.when]: the callback must be invoked when either token is stopped, and at most once.

Introduced in https://github.com/NVIDIA/stdexec/pull/2234

Reproducer

#include <stdexec/execution.hpp>
#include <cstdio>

namespace ex = STDEXEC;

int main() {
  ex::inplace_stop_source a, b;
  a.request_stop(); // the sender's token is already stopped
  int calls = 0, at_registration = -1;
  auto probe = ex::read_env(ex::get_stop_token) | ex::then([&](auto token) {
    auto fn = [&]() noexcept { ++calls; };
    ex::stop_callback_for_t<decltype(token), decltype(fn)> cb(token, fn); // must run: a is stopped
    at_registration = calls;
    b.request_stop(); // must not run again
  });
  // The inner stop-when sees a stoppable receiver token (b), so its child gets a fused token.
  ex::sync_wait(ex::__stop_when(ex::__stop_when(probe, a.get_token()), b.get_token()));
  std::printf("calls at registration: %d (expected 1), in total: %d (expected 1)\n",
              at_registration, calls);
  return at_registration == 1 && calls == 1 ? 0 : 1;
}

Then output will be: calls at registration: 1 (expected 1), in total: 2 (expected 1). It should be 1 and 1.

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  3. Fork 儲存庫，在一個分支上完成修改。
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