boost::process::child 关闭输入流后不会退出

Question

在下面的示例中，我尝试将一些数据写入子进程，子进程处理数据并将其写入文件。关闭流后，父进程无限期地等待子进程完成。我不知道如何表明我已完成写入数据并希望子进程停止读取并完成它正在做的任何事情。根据调用终止 would send a SIGKILL 的文档，我认为这不是我想要的。

我错过了什么？我检查了但我宁愿先尝试使实际代码与同步 IO 一起工作。

#include <boost/process.hpp>
#include <iostream>


namespace bp = boost::process;


int main(int argc, char **argv)
{
    boost::process::opstream in{};
    boost::process::child child("/path/to/test.py", bp::std_in < in);

    in << "test1\n";
    in << "test2\n";
    in << "test3\n";
    in << std::flush;

    std::cerr << "Closing the stream…\n";
    in.close();
    std::cerr << "Waiting for the child to exit…\n";
    child.wait(); // Parent seems to hang here.

    return 0;
}

test.py 只是将数据写入文件，如下所示：

#!/usr/local/homebrew/opt/python@3.8/bin/python3

import sys

with open("/tmp/test.txt", "w") as f:
    for line in sys.stdin:
        f.write(line)

Answer 1

文档警告说，对子进程使用同步 IO 很容易出现死锁。

下面是对异步 IO 的最小改写：

#include <boost/process.hpp>
#include <iostream>

namespace bp = boost::process;

int main() {
    boost::asio::io_context ioc;
    bp::async_pipe in{ioc};
    bp::child child("./test.py", bp::std_in < in, bp::std_out.close());

    for (auto msg : { "test1\n", "test2\n", "test3\n" }) {
        write(in, bp::buffer(msg, strlen(msg)));
    }

    std::cerr << "Closing the pipe…\n";
    in.close();
    std::cerr << "Waiting for the child to exit…\n";
    ioc.run(); // already awaits completion

    child.wait(); // Parent seems to hang here.
}

你可以通过做一些延迟让它更真实：

#include <boost/process.hpp>
#include <iostream>

using namespace std::chrono_literals;
namespace bp = boost::process;

int main() {
    boost::asio::io_context ioc;
    bp::async_pipe in{ioc};
    bp::child child("./test.py", bp::std_in < in, bp::std_out.close());

    std::thread th([&] {
        for (auto msg : { "test1\n", "test2\n", "test3\n" }) {
            write(in, bp::buffer(msg, strlen(msg)));
            std::this_thread::sleep_for(1s);
        }

        std::cerr << "Closing the pipe…\n";
        in.close();
    });

    std::cerr << "Waiting for the child to exit…\n";
    ioc.run(); // already awaits completion
    th.join();

    child.wait(); // Parent seems to hang here.
}

对于成熟的异步 IO，请参阅其他示例：

Answer 2

在查看源代码后，我发现至少在这种情况下关闭流并没有关闭关联的管道。手动执行此操作确实解决了问题：

...
in.close();
in.pipe().close();
child.wait(); // Does not hang.

boost::process::child 关闭输入流后不会退出

boost::process::child will not exit after closing input stream

c++

boost

boost-process