Method start

include/exec/detail/shared.hpp:177–209

    }

    constexpr void start() noexcept
    {
      // Scenario: there are no more split senders, this is the only operation state, the
      // underlying operation has not yet been started, and the receiver's stop token is already
      // in the "stop requested" state. Then registering the stop callback will call
      // __local_state::operator() on *this synchronously. It may also be called asynchronously
      // at any point after the callback is registered. Beware. We are guaranteed, however, that
      // __local_state::operator() will not complete the operation or decrement the shared state's
      // ref count until after *this has been added to the waiters list.
      auto const __stok = STDEXEC::get_stop_token(STDEXEC::get_env(__rcvr_));
      __on_stop_.emplace(__stok, *this);

      // We haven't put __state in the waiters list yet and we are holding a ref count to
      // __sh_state_, so nothing can happen to the __sh_state_ here.

      // Start the shared op. As an optimization, skip it if the receiver's stop token has already
      // been signaled.
      if (!__stok.stop_requested())
      {
        __sh_state_->__try_start();
        if (__sh_state_->__try_add_waiter(this, __stok))
        {
          // successfully added the waiter
          return;
        }
      }

      // Otherwise, failed to add the waiter because of a stop-request.
      // Complete synchronously with set_stopped().
      __on_stop_.reset();
      std::exchange(__sh_state_, {})->__detach();
      STDEXEC::set_stopped(static_cast<_Receiver&&>(__rcvr_));
    }

    // Stop request callback:
    constexpr void operator()() noexcept