How to implement default config section for a custom Tarantool Cartridge role?

I'm implementing a role which can be configured. For the better user experience I'd like to make it's default configuration explicit so that a user could simply edit it in Cartridge WebUI.

But cartridge.config_patch_clusterwide isn't suitable for this purpose because it calling it from init() callback results in recursing and thus prohibited.

Is there any other way to accomplist it?

There is on_patch trigger, which could do it. Here is API reference: https://www.tarantool.io/en/doc/latest/book/cartridge/cartridge_api/modules/cartridge.twophase/#on-patch-trigger-new-trigger-old

Your role could look like follows:

-- myrole.lua

local twophase = require('cartridge.twophase')

twophase.on_patch(function(conf_new, conf_old)
    if conf_old:get_plaintext('myrole.yml') ~= nil then
        return
    end

    conf_new:set_plaintext('myrole.yml', require('yaml').encode({
        -- default config
        foo = 'bar',
    }))
end)

-- local function validate_config()
-- end

-- local function apply_config()
-- end

return {
    role_name = 'myrole',
}

Before every update of clusterwide configuration cartridge will call the trigger, so the role can modify it if necessary. Be careful not to spoil it.