[screen-devel] [bug #25089] screen produces zombies

[Axel Beckert]

Follow-up Comment #3, bug #25089 (project screen):

A coworker ran into this on Debian 10 Buster (amd64) with Screen 4.6.2 (Debian
package version 4.6.2-3) inside a (production) VM (ESX) and I was able to
reproduce this under my user on the same machine.

I was able to reproduce it by opening many (like 20 or 30 or so) screen
windows and then closing some of them (not the most recently created ones)
again. Not every closed shell became a zombie processes, though.

Looks like this at the moment:

→ ps auxwwwf | egrep 'Z|defunct|SCREEN' | fgrep -v grep
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
usera    32453  0.0  0.7  23776 15320 ?        Ss   Sep11  17:09 SCREEN
usera    22527  0.0  0.0      0     0 ?        Zs   Oct07   0:00  \_ [bash]
<defunct>
usera    17308  0.0  0.0      0     0 ?        Zs   Oct08   0:00  \_ [bash]
<defunct>
userb    17774  0.0  0.6  20440 13972 ?        Ss   Oct09   0:08 SCREEN
userb    17876  0.0  0.0      0     0 ?        Zs   Oct09   0:00  \_ [zsh]
<defunct>
userb    17902  0.0  0.0      0     0 ?        Zs   Oct09   0:00  \_ [zsh]
<defunct>
userb    17927  0.0  0.0      0     0 ?        Zs   Oct09   0:00  \_ [zsh]
<defunct>
userb    18124  0.0  0.0      0     0 ?        Zs   Oct09   0:00  \_ [zsh]
<defunct>
userb    18150  0.0  0.0      0     0 ?        Zs   Oct09   0:00  \_ [zsh]
<defunct>
userb    18174  0.0  0.0      0     0 ?        Zs   Oct09   0:00  \_ [zsh]
<defunct>
userb    20071  0.0  0.0      0     0 ?        Zs   Oct09   0:00  \_ [zsh]
<defunct>
userb    20104  0.0  0.0      0     0 ?        Zs   Oct09   0:00  \_ [zsh]
<defunct>
userb    20137  0.0  0.0      0     0 ?        Zs   Oct09   0:00  \_ [zsh]
<defunct>

Those process zombies have no according screen window anymore, i.e. are not
"screen zombie windows". From the user perspective it seems as if the
according shell has exited as expected and the according screen window has
been closed.

Maybe also worth to note is that these zombies process are not just there for
a few minutes and then get cleaned up. Most of them (but not all) stay for at
least days, if not until I exit the screen session.  

I though wasn't yet able to reproduce it on any other machine, neither with
the same Debian release, the same Debian screen package (but on real
hardware). So I'm still unsure what actually triggers this issue, but it only
seems to surface under specific conditions, but it's still unclear under which
conditions.

"usera" has rather short and not fancy .screenrc file:

   → cat ~usera/.screenrc
   autodetach on
   hardstatus alwayslastline "%{+b dr}${USER}@%H: %n %t | %w | %l | %c:%s"
   startup_message off
   
   screen 0

And userb has no .screenrc at all on this host.

The relevant parts of the /etc/screenrc present on that system (actually
Debian's default /etc/screenrc) are:

→ egrep -v '^$|^#' /etc/screenrc
deflogin on
vbell on
vbell_msg "   Wuff  ----  Wuff!!  "
defscrollback 1024
bind ^k
bind ^\
bind \\ quit
bind K kill
bind I login on
bind O login off
bind } history
termcapinfo vt100 dl=5\E[M
hardstatus off
termcapinfo xterm*|rxvt*|kterm*|Eterm* hs:ts=\E]0;:fs=\007:ds=\E]0;\007
hardstatus string "%h%? users: %u%?"
termcapinfo xterm*|linux*|rxvt*|Eterm* OP
termcapinfo xterm 'is=\E[r\E[m\E[2J\E[H\E[?7h\E[?1;4;6l'
defnonblock 5


It also seems to be independent of the login shell. My coworker uses bash, I
use zsh, and the issue is present with both.

I also wondered what this box might have in common with the original
reporter's box, but the only very vague connection is that a VM might be also
considered a "slow" machine.

So I also tried it on another, less potent VM (Xen this time) with Debian 10
Buster amd64, but wasn't able to reproduce this issue there either.

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