Re: [PATCH v7 0/3] console: Add persistent scrollback buffers for all VGA consoles

On Mon, Nov 28, 2016 at 12:15:48AM +0100, Adam Borowski wrote:

On Sun, Nov 27, 2016 at 09:37:30PM +0000, Andrey Utkin wrote:

quoted

I've just patched next-20161125 with this set and given it a run.

Scrollback persistence works fine, just as in earlier versions.

This time I didn't forget to test clear operation.

The only important concern is that after logout, the scrollback is not
wiped by /bin/login or /sbin/agetty (not sure who of them is responsible
for that). What do you see on your workstations in this case?

If you're on Debian or a derivative, that's clear_console.  It uses a
switch-vt-then-back hack which obviously doesn't work with scrollback
persistence.  Reported as https://bugs.debian.org/845177 -- I'll molest the
maintainer if the patch doesn't get applied soon, so we can have the fix in
time for stretch (then Ubuntu zesty).

I'm on Gentoo.

Because of a sad lack of a time machine, old systems will use clear_console
with that hack until they die, that's why
CONFIG_VGACON_SOFT_SCROLLBACK_PERSISTENT defaults to n; in a few years it'll
be ok to flip it.

quoted

I guess we need to do something of the following:
 - catch some control character sequences to wipe the scrollback

\e[3J

quoted

 - indicate (by some flag) some feature capability for this

Terminfo calls this flag "E3".

quoted

 - request update in terminfo database or whatever, to let ncurses know
   that it is capable of scrollback wiping by some control charater
   sequences

Already there for quite a while.

quoted

clear(1) doesn't wipe the scrollback at all, it is still reachable, all
of it.

It does for me on the console.  The man page says:

# clear clears your screen if this is possible, including its scrollback
# buffer (if the extended "E3" capability is defined).  clear looks in the
# environment for the terminal type and then in the terminfo database to
# determine how to clear the screen.

Because of its reliance on terminfo, you need to have TERM=linux in your
environment; also, screen/tmux obviously breaks this.

I wonder whether my ncurses is not bleeding-edge enough, or I have some
non-standard config. Anyway, thanks for explanation.

quoted

echo -e "\e[3J" seems to wipe the scrollback, but if you do it several
times in a row, every time you (or at last I do) get your prompt a bit
lower, so after many times you end up with blank screen and the prompt
at the bottom of the screen.

Yeah, none of \e[J subcommands move the cursor at all.  As you use echo
without -n, you move two lines lower, and even with -n the command you typed
takes a line.  You want to move the cursor explicitly, add "\e[H".

Thanks for explanation.