I have a rather complex code with std::async calls and std::packaged_task, that fails to execute to the very end. I simplified it to the minimal reproducable example.
Two async functions are called one after another, inside of which there are packaged_tasks executed asynchronously using std::async. Then both we wait for both async functions to finish, using corresponding future.wait() method. The execution stops at futY.wait(); and second packaged_task is never executed (No second Inside handler func log).
#include <iostream> // std::cout
#include <future> // std::packaged_task, std::future
#include <exception>
#include <vector>
#include <list>
#include <memory>
#include <functional>
std::list<std::function<bool(const std::vector<int> &data)>> handlers_;
std::future<std::vector<int>> countup(int from, int to) {
std::function<std::vector<int>(std::exception_ptr, std::vector<int>)> func = [=] (std::exception_ptr ex, std::vector<int> data) {
std::cout << "Inside handler func " << from << " " << to << std::endl;
if (ex != nullptr) {
std::rethrow_exception(ex);
}
return data;
};
auto packageP = std::make_shared<std::packaged_task<std::vector<int>(std::exception_ptr, std::vector<int>)>>(func);
auto fut = packageP->get_future();
handlers_.push_back([packageP] (const std::vector<int> &data) mutable -> bool {
std::cout << "Calling handler with data, size: " << data.size() << std::endl;
(*packageP)(nullptr, data);
return data.size();
}
);
auto fut2 = std::async(std::launch::async, [=, &handlers_] {
std::cout << "Before handler called " << from << to << std::endl;
std::vector<int> vec ( to, from );
auto res = (*handlers_.begin())(vec);
std::cout << "Handler result " << res << " for " << from << " " << to << std::endl;
});
std::cout << "Called async in countup for " << from << " " << to << std::endl;
return fut;
}
int main ()
{
auto futX = std::async(std::launch::async, [] {
auto fut1 = std::async(std::launch::async, [] {
auto fut2 = countup(0, 2);
std::cout << "Called X countup and waiting to finish" << std::endl;
fut2.wait();
auto vec = fut2.get();
std::cout << "The X countup returned" << std::endl;
});
std::cout << "Called X async internal and waiting to finish" << std::endl;
fut1.wait();
return 2;
});
std::cout << "Called async X and waiting to finish" << std::endl;
auto futY = std::async(std::launch::async, [] {
auto fut1 = std::async(std::launch::async, [] {
auto fut2 = countup(0, 3);
std::cout << "Called Y countup and waiting to finish" << std::endl;
fut2.wait();
auto vec = fut2.get();
std::cout << "The Y countup returned " << std::endl;
});
std::cout << "Called Y async internal and waiting to finish" << std::endl;
fut1.wait();
return 3;
});
std::cout << "Called async Y and waiting to finish" << std::endl;
futX.wait();
std::cout << "After async X and waiting to finish" << std::endl;
futY.wait();
std::cout << "After async Y and waiting to finish" << std::endl;
int valueX = futX.get(); // wait for the task to finish and get result
int valueY = futY.get(); // wait for the task to finish and get result
std::cout << "The countdown lasted for " << valueX << " " << valueY << " seconds" << std::endl;
return 0;
}
The log is the following:
Called async X and waiting to finish
Called async Y and waiting to finish
Called X async internal and waiting to finish
Called Y async internal and waiting to finish
Called async in countup for Before handler called 02
0 2
Calling handler with data, size: 2
Inside handler func 0Called async in countup for 2
Handler result 01 for 0 2
Before handler called 03
3Calling handler with data, size:
Called X countup and waiting to finish
The X countup returned
3
After async X and waiting to finish
Called Y countup and waiting to finish
Could you please clarify why the code is never executed till the end?
You are re-using the same packaged task when you call (*handlers_.begin()), and I think you have a data race when you handlers_.push_back.
You don't need the global handlers_, you can just capture a local handler.
#include <iostream> // std::cout
#include <future> // std::packaged_task, std::future
#include <exception>
#include <vector>
#include <list>
#include <memory>
#include <functional>
std::future<std::vector<int>> countup(int from, int to) {
auto func = [=] (std::exception_ptr ex, std::vector<int> data) {
std::cout << "Inside handler func " << from << " " << to << std::endl;
if (ex != nullptr) {
std::rethrow_exception(ex);
}
return data;
};
auto packageP = std::make_shared<std::packaged_task<std::vector<int>(std::exception_ptr, std::vector<int>)>>(func);
auto fut = packageP->get_future();
auto handler = [packageP] (const std::vector<int> &data) mutable -> bool {
std::cout << "Calling handler with data, size: " << data.size() << std::endl;
(*packageP)(nullptr, data);
return data.size();
};
auto fut2 = std::async(std::launch::async, [=]() mutable {
std::cout << "Before handler called " << from << to << std::endl;
std::vector<int> vec ( to, from );
auto res = handler(vec);
std::cout << "Handler result " << res << " for " << from << " " << to << std::endl;
});
std::cout << "Called async in countup for " << from << " " << to << std::endl;
return fut;
}
int main ()
{
auto futX = std::async(std::launch::async, [] {
auto fut1 = std::async(std::launch::async, [] {
auto fut2 = countup(0, 2);
std::cout << "Called X countup and waiting to finish" << std::endl;
fut2.wait();
auto vec = fut2.get();
std::cout << "The X countup returned" << std::endl;
});
std::cout << "Called X async internal and waiting to finish" << std::endl;
fut1.wait();
return 2;
});
std::cout << "Called async X and waiting to finish" << std::endl;
auto futY = std::async(std::launch::async, [] {
auto fut1 = std::async(std::launch::async, [] {
auto fut2 = countup(0, 3);
std::cout << "Called Y countup and waiting to finish" << std::endl;
fut2.wait();
auto vec = fut2.get();
std::cout << "The Y countup returned " << std::endl;
});
std::cout << "Called Y async internal and waiting to finish" << std::endl;
fut1.wait();
return 3;
});
std::cout << "Called async Y and waiting to finish" << std::endl;
futX.wait();
std::cout << "After async X and waiting to finish" << std::endl;
futY.wait();
std::cout << "After async Y and waiting to finish" << std::endl;
int valueX = futX.get(); // wait for the task to finish and get result
int valueY = futY.get(); // wait for the task to finish and get result
std::cout << "The countdown lasted for " << valueX << " " << valueY << " seconds" << std::endl;
return 0;
}