Public bug reported:

References:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=810660
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=866898


If X is running and you switch to a plain text console (ie ctrl-alt-f3), log in 
as a regular user, and then log out, the display switches back to the graphical 
login screen and locks up.  The system is still functioning (can ssh in, etc) 
but the video output is frozen.

The underlying cause is:
https://gitlab.freedesktop.org/xorg/xserver/issues/492

However, the problem can at least be worked around by making
clear_console switch to tty 6 and back instead of switching to 1 and
back.  This patch corrects the problem:


--- bash-4.4.18.orig/debian/clear_console.c     2019-03-28 12:09:02.415907787 
-0400
+++ bash-4.4.18/debian/clear_console.c  2019-03-28 12:08:11.984366858 -0400
@@ -205,7 +205,7 @@
 #if defined(__linux__)
   num = vtstat.v_active;
 #endif
-  tmp_num = (num == 1 ? 2 : 1);
+  tmp_num = (num == 6 ? 5 : 6);
 
   /* switch vt to clear the scrollback buffer */
   if (ioctl(fd, VT_ACTIVATE, tmp_num))

** Affects: bash (Ubuntu)
     Importance: Undecided
         Status: New

** Patch added: "makes clear_console switch to either tty6 or tty5 instead of 
tty1/tty2"
   
https://bugs.launchpad.net/bugs/1822184/+attachment/5250280/+files/fix-clear_console-GUI-lockup.patch

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https://bugs.launchpad.net/bugs/1822184

Title:
  clear_console locks up video when X is running and you log out from a
  plain text console

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