Launching player from trackma leaves zombie process on linux.

[ahmubashshir]

Steps to reproduce

• Open trackma.
• Play something.
• Close player.
• Check for zombie process with zps -l

sh-5.0$ zps -l
PID   	PPID  	STATE	            NAME COMMAND
4097215	4088507	Z 	             vlc 
sh-5.0$ ps -p 4088507
    PID TTY          TIME CMD
4088507 pts/3    00:00:01 trackma-gtk

Cause

Player was started using subprocess.Popen(), but not calling .wait() or .poll() on Popen()ed process leaves a zombie.

Possible workarounds: [downsides may be caused by my implementation]

• Double fork then os.execvp
  • Upside: launches the player as a child of init, so it is reaped as soon as the process ends.
  • Downside: leaves a fork()ed zombie process* * fixed after calling os.waitpid on first fork
• subprocess.Popen().wait() in a thread
  • Upside: After launching, the thread waits for the child to terminate, then reaps the child.
  • Downside: Can't Close the UI unless palyer is closed too.
• Ignore SIGCHLD

[ahmubashshir]

I went with 3rd method because it was easier and cleaner.

[ahmubashshir]

I found a bug which occurs if we ignore SIGCHLD. If parent exits while child is running, child exits too. So I used fork then waitpid; fork again in forked child then os.execv to prevent zombie process without relying on ignoring SIGCHLD. I'll create a new pr with this fix.

[z411]

Did some testing and using Popen the zombie process gets reaped within seconds of being closed. Apparently Python 3 should automatically reap zombie processes whenever they get garbage collected. What Python version are you using? Do you get a different behavior?

[ahmubashshir]

I'm using python 3.8.6, on manjaro.

By the way... they were never reaped :|. Whenever I tried to reap all zombies with zps, trackma was killed too, even after closing the player a few hours before running zps. And from this issue, subprocess only reaps previous process on new Popen

On afeaef7ee034fe937019822c9f52b9e8ae84432f after closing vlc [closed on 01:22].

[z411]

Python 3.8.6 here and Arch Linux here. Weird since our configuration is similar, I tried with vlc and it gets reaped after around 5-10 seconds. This must be why I never noticed it. That said I guess we shouldn't have a zombie process in the first place at all, so I'll look into fixing this.

[z411]

I tried with the ignore SIGCLD method again (after moving processes out of the engine) and it works perfectly for me. I don't get the bug you described and I don't get a zombie process either. Could you try my prevent-zombie branch?

[ahmubashshir]

Player dies as soon as trackma quits if sigchld is ignored.

On Sat, Nov 21, 2020 at 22:50, z411 [email protected] wrote:

I tried with the ignore SIGCLD method again (after moving processes out of the engine) and it works perfectly for me. I don't get the bug you described and I don't get a zombie process either. Could you try my the prevent-zombie branch?

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[ahmubashshir]

1

[z411]

I seriously can't reproduce this...

https://streamable.com/3p345m

Could you try with a clean config, and ideally with the tracker disabled? I'll try to find other people to try and see if they can reproduce it.

[ahmubashshir]

Though I Started trackma with XDG_CONFIG_HOME=/tmp/ch XDG_DATA_HOME=/tmp/dd trackma-gtkand tracker was disabled, Vlc again closed on its own after closing trackma.

On Sat, Nov 21, 2020 at 23:28, z411 [email protected] wrote:

I seriously can't reproduce this...

https://streamable.com/3p345m

Could you try with a clean config, and ideally with the tracker disabled?

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[FichteFoll]

Running just python -m trackma.ui.cli play 2 should be enough to reproduce the second case, right? Because that simply opens mpv for me and does not kill it.

I can not reproduce the first case as well

Open trackma. Play something. Close player. Check for zombie process with zps -l

zps -l output is empty. On the prevent-zombie branch ofc, since I can't play anything on master due to #525.

Also Arch, btw.

[ahmubashshir]

Running just python -m trackma.ui.cli play 2 should be enough to reproduce the second case, right? Because that simply opens mpv for me and does not kill it.

I can not reproduce the first case as well

Open trackma. Play something. Close player. Check for zombie process with zps -l

zps -l output is empty. On the prevent-zombie branch ofc, since I can't play anything on master due to #525.

Also Arch, btw.

python -m trackma.ui.cli play 2 quits after launching player, doesn't it? so child process is reparented to init(1) that's why zps -l is empty :3 btw... I confirmed now... This only happens in Gtk UI. https://streamable.com/l887xl