如何在 C++ 上使用 posix_spawn 创建非标准文件描述符的管道

当我尝试posix_spawn在 MacOS 上启动一个进程（最初是 Chrome）时，遇到了一种情况。此代码应该也适用于 Linux，但我尚未测试。

我想为描述符 1（STDOUT）、2（STDERR）以及 3（写入）和 4（读取）创建管道。

它不适用于 Chrome，我正在尝试用一个简单的代码来测试这种情况：

#!/usr/bin/env python3
import os
import sys
import time
import fcntl

print("fcntl 3:", fcntl.fcntl(3, fcntl.F_GETFD))
print("fcntl 4:", fcntl.fcntl(4, fcntl.F_GETFD))

fd4 = os.fdopen(4, 'wb')
while True:
    print('Cycle')
    time.sleep(2)
    while chunk := os.read(3, 1024):
        print('REQ:', chunk.decode('utf8', errors='backslashreplace'), file=sys.stderr)
        fd4.write(b'FOO')

我正在尝试检查描述符是否可访问。描述符 3 已准备好读取，并且可以从中读取数据。

但是描述符 4 无法访问，尝试打开它或使用 fcntl 进行测试会导致错误“OSError: [Errno 9] Bad file descriptor”。Chrome 也会在同样的尝试中失败。

posix_spawn以下是导致这种情况的C++ 代码（稍微简化了） ：

#include "pipes.h"

#include <cerrno>
#include <cstring>
#include <spawn.h>
#include <stdio.h>
#include <sys/poll.h>
#include <sys/wait.h>

extern "C" char** environ;

constexpr int CDP_WRITE_FILENO = 3;
constexpr int CDP_READ_FILENO = 4;

void RunBrowser(const char* exePath, const std::vector<const char*>& launchArgs) {
    posix_spawn_file_actions_t childFdActions{};
    posix_spawn_file_actions_init(&childFdActions);
    DEFER(&childFdActions) {
        posix_spawn_file_actions_destroy(&childFdActions);
    };

    int stdoutPipe[2];
    int stderrPipe[2];
    int cdpWritePipe[2];
    int cdpReadPipe[2];
    if (pipe(stdoutPipe) || pipe(stderrPipe) || pipe(cdpWritePipe) || pipe(cdpReadPipe)) {
        throw TBrowserError() << "Could not create a pipe: " << std::strerror(errno);
    }
    posix_spawn_file_actions_addclose(&childFdActions, stdoutPipe[0]);
    posix_spawn_file_actions_addclose(&childFdActions, stderrPipe[0]);
    posix_spawn_file_actions_addclose(&childFdActions, cdpWritePipe[1]);
    posix_spawn_file_actions_addclose(&childFdActions, cdpReadPipe[0]);
    posix_spawn_file_actions_adddup2(&childFdActions, stdoutPipe[1], STDOUT_FILENO);
    posix_spawn_file_actions_adddup2(&childFdActions, stderrPipe[1], STDERR_FILENO);
    posix_spawn_file_actions_adddup2(&childFdActions, cdpWritePipe[0], CDP_WRITE_FILENO);
    posix_spawn_file_actions_adddup2(&childFdActions, cdpReadPipe[1], CDP_READ_FILENO);
    posix_spawn_file_actions_addclose(&childFdActions, stdoutPipe[1]);
    posix_spawn_file_actions_addclose(&childFdActions, stderrPipe[1]);
    posix_spawn_file_actions_addclose(&childFdActions, cdpWritePipe[0]);
    posix_spawn_file_actions_addclose(&childFdActions, cdpReadPipe[1]);

    pid_t childPid{};
    if (const int ret = posix_spawn(&childPid, exePath,
                                    &childFdActions, nullptr,
                                    const_cast<char* const*>(launchArgs.data()),
                                    environ)) {
        throw TBrowserError() << "Failed to spawn a process: " << std::strerror(ret);
    }

    close(stdoutPipe[1]);
    close(stderrPipe[1]);
    close(cdpWritePipe[0]);
    close(cdpReadPipe[1]);

    int status;
    do {
        const int ret = waitpid(childPid, &status, WUNTRACED | WCONTINUED);
        if (ret == -1) {
            kill(childPid, SIGKILL);
            throw TBrowserError() << "waitpid error: -1";
        }

        if (WIFEXITED(status)) {
            const auto retCode = WEXITSTATUS(status);
            if (retCode == 0) {
                break;
            }
            throw TBrowserError() << std::format("Browser process failed with exit code {}", retCode);
        }

        if (WIFSIGNALED(status)) {
            throw TBrowserError() << "Browser process killed by signal " << WTERMSIG(status);
        }

        if (WIFSTOPPED(status)) {
            std::cerr << "Browser process stopped by signal " << WSTOPSIG(status) << std::endl;
        }

        if (WIFCONTINUED(status)) {
            std::cerr << "Browser process is continued" << std::endl;
        }
    } while (WIFEXITED(status) || WIFSIGNALED(status));
}