bug#36879: 26.2; OSC 52 paste in term/xterm.el not working

[Mattias Engdegård]

tags 36879 patch
quit

Daniel Eklöf skrev:

>Have I configured something wrong? Or is this a bug?

I don't think you did anything wrong; I can reproduce the bug (in
XTerm; I don't have your fancy emulator).

The question is rather, how did this code ever work in the first
place? As you observed, when XTerm sends the reply, it uses BEL as
terminator. Emacs uses BEL (C-g) as INTR char, which means that not
only is special effort required to avoid having it quit the current
elisp code -- this could have been done using inhibit-quit -- but when
the pty receives the BEL from XTerm, it immediately discards unread
characters and raises SIGINT.

Thus, it's very much a race: the only way it could ever work would be
if Emacs has been able to read the entire reply except the BEL, and be
sitting inside (read-char) when the BEL reaches the pty. Needless to
say, this is rather unlikely.

We could tell the tty not to discard the queue upon INTR by setting the
NOFLSH flag, but (1) I don't know how portable that is, (2) it's not
what we normally want when C-g is used interactively, and (3) it would
still process the BEL out-of-order with respect to earlier chars.

Attached is a rather heavy-handed patch which temporarily changes the
quit-char to something unlikely while sending the OSC 52 request and
reading the reply. It also allows the reply to be terminated by ESC \
(ST) as well.

Since XTerm parrots our choice of string terminator (BEL or ST), this
suggests a simpler solution: just use ST, and the trouble with BEL is
no more. Unfortunately the code has provisions for screen/tmux, where
the entire request is wrapped in a DCS request:

 ESC P ... ESC \

which means that we cannot use ST as terminator in that case. However,
I haven't been able to make this facility work with tmux at all, and
with screen only by reverting 4183482f4d (bug#20356) AND explicitly
setting TERM=screen (the default is screen.xterm-256color).

In addition, changing quit-char can be visually annoying; it causes
reinitialisation of the entire tty, something you don't want every time
you press C-y. Perhaps it's fine to drop screen support from this
particular function? I attached another, alternative patch that does
that instead.

heavy.patch
Description: Text Data

light.patch
Description: Text Data