[Feature] relaxed `own_clipboard` mode

[N-R-K]

Currently there's own_clipboard which eagerly takes clipboard ownership. It'd be nice to have a "relaxed" mode as well, i.e it will only take ownership when there's no other window holding the clipboard and serve the last saved clip.

[cdown]

Seems reasonable. I guess it can be done with XFixesSelectSelectionInput by checking on XFixesSetSelectionOwnerNotify, XFixesSelectionWindowDestroyNotify, and XFixesSelectionClientCloseNotify, but I need to double check that's comprehensive.

I think that there are no new race concerns given own_clipboard just does this unconditionally now, but should think about that as well. Maybe we need to plumb the clear time through when calling XSetSelectionOwner. But to be honest this whole thing is a racefest, so maybe the semantics are not really different than now anyway.

[N-R-K]

Hey, I actually made some progress on this locally but then I got utterly bamboozled by the existing own_clipboard behavior. My intuition says that the only thing that option should change is the owner of the clipboard, but not the content of it (if no races).

E.g, if I copy "P" into primary and "C" into clipboard then it should remain that way. However, the behavior I'm noticing is that the copy into one selection ends up overwriting the other.

A small testcase using Xvfb + xclip:

$ ./tests/relax-test 2>/dev/null
# no own_clipboard
clipboard: C (expecting C)
primary:   P (expecting P)

# with own_clipboard
clipboard: C (expecting C)
primary:   C (expecting P)
clipboard: P (expecting C)
primary:   P (expecting P)

The test script:

#!/usr/bin/env bash

set -exE -o errtrace

settle() { sleep 0.25; }

kill_background_jobs() {
    local -a bg
    readarray -t bg < <(jobs -p)
    (( ${#bg[@]} )) && kill -- "${bg[@]}" 2>/dev/null
}
trap 'kill_background_jobs' EXIT

Xvfb ":2" -screen 0 640x480x24 -nolisten tcp >/dev/null 2>&1 & xvfbpid=$!
settle
export DISPLAY=":2"
export CM_DIR=/tmp/clipmenu-test-dir
mkdir -p "$CM_DIR"
export PATH="$PWD/src:$PATH"

CM_OWN_CLIPBOARD=0 CM_OWN_SELECTIONS="clipboard primary" CM_DEBUG=0 clipmenud & clipmenudpid=$!
settle
printf "P" | xclip -selection primary   -in
printf "C" | xclip -selection clipboard -in
settle
printf "clipboard: %s (expecting C)\n" "$(xclip -selection clipboard -out)"
printf "primary:   %s (expecting P)\n" "$(xclip -selection primary   -out)"
kill "$clipmenudpid"

CM_OWN_CLIPBOARD=1 CM_OWN_SELECTIONS="clipboard primary" CM_DEBUG=0 clipmenud & clipmenudpid=$!
settle
printf "P" | xclip -selection primary   -in
printf "C" | xclip -selection clipboard -in
settle
printf "clipboard: %s (expecting C)\n" "$(xclip -selection clipboard -out)"
printf "primary:   %s (expecting P)\n" "$(xclip -selection primary   -out)"
kill "$clipmenudpid"

CM_OWN_CLIPBOARD=1 CM_OWN_SELECTIONS="clipboard primary" CM_DEBUG=0 clipmenud & clipmenudpid=$!
settle
printf "C" | xclip -selection clipboard -in
printf "P" | xclip -selection primary   -in
settle
printf "clipboard: %s (expecting C)\n" "$(xclip -selection clipboard -out)"
printf "primary:   %s (expecting P)\n" "$(xclip -selection primary   -out)"
kill "$clipmenudpid"

exit 0

EDIT: And if I change all those CM_OWN_SELECTIONS="clipboard primary" to only clipboard, then I get this (notice the last case where it's still not matching expectation):

~> ./tests/relax-test 2>/dev/null
clipboard: C (expecting C)
primary:   P (expecting P)

clipboard: C (expecting C)
primary:   P (expecting P)

clipboard: P (expecting C)
primary:   P (expecting P)

[N-R-K]

But to be honest this whole thing is a racefest

Races seem to be an unsolvable problem with X, I ran into a bunch of this when working on xuv :^)