Cygwin - 阻止打开命名的 fifo 会导致另一个线程阻止打开常规文件

Question

在 Cygwin 中阻止打开命名管道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。下面的代码重现了 cygwin 3.1.6(0.340/5/3) 上的问题，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，直到打开 namedPipe 的另一侧。 一旦被释放，它就会写入其余的打印：

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

在 RHEL 上，这会打印出预期的结果：

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

只有这样，主线程才会挂起，直到某些内容回显到 namedPipe。

我们正在研究一种不会阻塞的解决方法，但这涉及到忙于等待，这不是很好。 谁能解释这种行为？

Answer 1

在 Cygwin 上，open 系统调用会在整个系统调用期间锁定文件描述符表。这意味着所有open 系统调用基本上都是顺序化的。

见syscalls.cc/open():

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

还有cygheap.h:

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

我认为解决这个问题没有简单的方法，但我想至少在 fifo 的情况下创建描述符后应该可以解锁 fd 表（请参阅fhandler_fifo），因为fifo 阻塞open。您可以在cygwin-developers 上进一步讨论。