c++ - Cygwin-打开命名的fifo时导致阻塞，导致另一个线程在打开常规文件时阻塞

Question

Cygwin中命名管道的阻止打开会导致另一个线程在尝试打开任何文件(包括简单文本文件)时挂起。以下代码在cygwin 3.1.6(0.340 / 5/3)上重现了该问题，并且在RHEL 7上工作正常(不挂起)。

#include <unistd.h>
#include <limits.h>
#include <stdio.h>
#include <thread>
#include <sys/stat.h>
#include <fcntl.h>

void openFile() {
    int fd;

    printf("inside new thread\n");
    sleep(10); // Ensure main thread reaches call to open()
    
    printf("opening a simple file\n");
    if((fd = open("simpleFile", 0600)) == -1) { // simpleFile is a simple text file in the filesystem
        printf("failed opening a simple file\n");
    }
    printf("simple file opened successfully\n");
    close(fd);
    printf("simple file closed\n");
}

int main(int argc, char *argv[]) {
    int fd;
    char readBuffer[PIPE_BUF];
    
    printf("creating named pipe\n");
    if (mkfifo("namedPipe", 0600)) {
        printf("creating named pipe failed\n");
    }
    
    printf("creating thread\n");
    std::thread pipeCreator = std::thread(openFile);
    
    printf("opening named pipe for read\n");
    fd = open("namedPipe", O_RDONLY); // Block will only release when we echo something into namedPipe
    printf("reading from named pipe\n");
    if (read(fd, readBuffer, PIPE_BUF) == -1) {
        printf("error reading from pipe\n");
    }
    printf("read successfully from named pipe\n");
    
    pipeCreator.join();
    
    return 0;
}

运行此打印:

creating named pipe
creating thread
opening named pipe for read
inside new thread
opening a simple file

然后阻塞，直到namedPipe的另一侧打开。
释放后，它将写入其余的打印件:

reading from named pipe
simple file opened successfully
read successfully from named pipe
simple file closed

在RHEL上，这将输出预期的结果:

creating named pipe
creating thread
opening named pipe for read
inside new thread
opening a simple file
simple file opened successfully
simple file closed

然后只有主线程挂起，直到将某些内容回显到namedPipe中。
我们正在研究一种不会阻止的变通方法，但这涉及到忙碌的等待，这并不是很好。
谁能解释这种行为？

Answer 1

在Cygwin上，open syscall在整个syscall期间锁定文件描述符表。这意味着所有open系统调用实际上都是顺序的。
参见 syscalls.cc/open() :

extern "C" int
open (const char *unix_path, int flags, ...)
{
      . . .
      cygheap_fdnew fd;  // <-- here

和 cygheap.h :

class cygheap_fdnew : public cygheap_fdmanip
{
 public:
  cygheap_fdnew (int seed_fd = -1, bool lockit = true)
  {
    if (lockit)
      cygheap->fdtab.lock ();  // <-- here
    . . .

我没有解决这个问题的简便方法，但是我想至少在使用fifo的情况下(一旦创建了描述符)就应该可以解锁fd表，因为 fifo会阻塞 fhandler_fifo。您可以在 cygwin-developers上进一步讨论。