Output lost for process with detached terminal (sometimes!)

[throwable-one]

Windows Version

Microsoft Windows [Version 10.0.19045.4046]

WSL Version

2.1.4.0

Are you using WSL 1 or WSL 2?

• WSL 2
• WSL 1

Kernel Version

5.15.146.1-2

Distro Version

Debian 12

Other Software

No response

Repro Steps

Compile and execute the following program that uses daemon(3) to detach from controlled terminal.

#include <unistd.h>
#include <stdio.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <fcntl.h>


int main() {
        if (daemon(0, 1) != 0) { // detach from terminal but leave stdout untouched!
                fprintf(stderr, "daemon error %d\n", errno);
                return 1;
        }

        int fd = open(ctermid(NULL), O_RDWR);
        if (fd == -1) {
                printf("I have no terminal\n");
                return 0;
        } else {
                fprintf(stderr, "I HAVE terminal, failed to daemonize??\n");
                close(fd);
                return 1;
        }

}

Compile it and run several times to make sure it works

$ cc 1.c
$ ./a.out
I have no terminal
$ ./a.out
I have no terminal
$ ./a.out
I have no terminal
$ ./a.out
$ I have no terminal

Everything works as expected.

Now, run it via wsl.exe

c:\>wsl.exe --exec "/home/link/a.out"

c:\>wsl.exe --exec "/home/link/a.out"
I have no terminal

c:\>wsl.exe --exec "/home/link/a.out"

c:\>wsl.exe --exec "/home/link/a.out"

Sometimes it work, sometimes doesn't. I assume that stdout is broken when process doesn't have a terminal.
This is probably the reason fake terminal with funny size exists for redirected processes: #10701

Expected Behavior

"I have no terminal" string printed

Actual Behavior

Sometimes it does, sometimes it doesn't

Diagnostic Logs

No response

[OneBlue]

@throwable-one: The behavior you're seeing is by design. As long as wsl.exe is running, the deamon process has a attached terminal, but when wsl.exe exits, the terminal is gone.

This is essentially a race condition based on how quickly the daemon process opens its terminal based on wsl.exe's lifetime. If had the parent process wait for its child before exiting, that would prevent wsl.exe from exiting, which should give you the behavior you want.