Check for diff mode while using git mergetool

Question

I am using Neovim v0.10.0 and have the following config for git difftool:

[diff]
    tool = nvimdiff
[difftool]
    prompt = False
    trustExitCode = true
[difftool "nvimdiff"]
    cmd = nvim -d \"$LOCAL\" \"$REMOTE\"

And git mergetool:

[merge]
    tool = nvimdiff
[mergetool]
    prompt = false
    keepBackup = false

Also in init.lua, I define this global variable:

vim.g.diffmode = vim.api.nvim_get_option_value('diff', { win = 0 })
print('Diff mode: ' .. tostring(vim.g.diffmode)) // for debugging

I saw that the print statement showed vim.g.diffmode to be true only when I used git difftool, but false when I used git mergetool.

However, when I run this in the command mode:

:lua print(vim.api.nvim_get_option_value('diff', {win=0}))

It showed true in every windows opened by git difftool or git mergetool.

What am I missing?

Update

Seems like with the git version I use (2.43.2), by default, nvimdiff mergetool does not pass -d to Neovim as an argument, after customizing the command to launch Neovim, like so:

[mergetool "nvimdiff"]
    cmd = nvim -d -c '4wincmd w | wincmd J' \"$LOCAL\" \"$BASE\" \"$REMOTE\" \"$MERGED\"

I got it to work.

Answer 1

The init.lua is processed at a very early stage when Neovim is not completely initialized. The script that organizes the merge windows didn't run yet. There is very high chance that only the default window is created leading:

vim.api.nvim_get_option_value('diff', { win = 0 })

to return 0.

To detect the mode in which Neovim is started in the init.lua file I would recommend you to check the command line arguments table (vim.v.argv).

Here is a lua function that you can call in init.lua and that will return true if Neovim is started in diff mode.

function IsDiffMode()
    for index, arg in pairs(vim.v.argv) do
        if arg=='-d' then
            return true
        end
    end
    return false
end