updating running neovim's setting `background` using a terminal response

Question

On my laptop, I frequently swap between a dark and a bright colorscheme for my terminal. I do this by symlinking its configuration file and asking it (termite) to read it without closing using killall -USR1 termite.

I'd like my running neovims to pick up the change somehow, and set background to either dark or light. I've visited this answer and this issue and other places in an effort to find out how to "refresh" neovim. And I'd like to avoid using packages like neovim-remote if possible.

nvim --version

NVIM v0.3.2-964-g7e97587da
Build type: RelWithDebInfo
LuaJIT 2.0.5
Compilation: /usr/bin/cc -march=x86-64 -mtune=generic -O2 -pipe
  -fstack-protector-strong -fno-plt -Wconversion -O2 -g -DMIN_LOG_LEVEL=3 
  -Og -g -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes 
  -std=gnu99 -Wimplicit-fallthrough -Wvla -fstack-protector-strong 
  -fdiagnostics-color=auto -Wno-array-bounds 
  -DINCLUDE_GENERATED_DECLARATIONS 
  -D_GNU_SOURCE -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM 
  ...

Features: +acl +iconv +jemalloc +tui 
See ":help feature-compile"

Answer 1

Since a new "Signal" autocmd was added, we can automatically call a function when neovim receives SIGUSR1. Which is nice.

$ readlink -qn ~/.config/termite/config
/home/bart/.config/termite/config_dark

I now have this in my init.vim

" ~/.config/nvim/init.vim
" ...
au Main Signal * call Darkorlight('~/.config/termite/config')

function! Darkorlight(file) abort
  if getftype(expand(a:file)) != 'link'
    return
  endif
  if resolve(glob(expand(a:file))) =~ '_dark$'
    set background=dark
  else
    set background=light
  endif
  redraw
endfunction

I then watch for the symlink to change with this unit:

# ~/.config/systemd/user/termite.path
[Unit]
Description=Make termite reload its configuration file when it changes

[Path]
PathChanged=%h/.config/termite

[Install]
WantedBy=default.target

# ~/.config/systemd/user/termite.service
[Unit]
Description=Send a signal to all termite and neovim processes

[Service]
Type=oneshot
ExecStart=/usr/bin/killall -USR1 termite
ExecStart=/usr/bin/killall -USR1 nvim

[Install]
WantedBy=default.target

This works great. It comes in handy as a toggle switch (i.e. for a toolbar):

#!/bin/sh
conf=~/.config/termite/config
if [[ `readlink $conf 2>/dev/null` =~ light ]]; then
  ln -sf "${conf}_dark" "$conf"
else
  ln -sf "${conf}_light" "$conf"
fi

Thanks to Justin M. Keyes for encouraging me to ask for a scriptable Signal feature on Github, and to Marco Hinz for implementing it so quickly!