how to prevent plugin folder being overwritten on plugin upgrade

Taken from the linked post:

“In order to prevent WordPress plugin update from deleting our important folder within our plugin, we must move that particular folder somewhere safe before it starts deleting everything in our plugin. After the update has completed, we will then move the folder that we want to preserved and overwrite any existing files in that folder. We can’t remove the filter action as shown on the core WordPress code was because the filter was not added previously. Thus, removing the filter will not prevent the deletion.“

(bold for emphasis.)

Thanks very much for your reply. I understand the logic, but not the implementation at this stage.

Would you be able to tell me where the second section of code goes? Is it in the functions.php file?

I’ve also annotated the second section of code (see below) with my guess of what is occurring and what values could be replaced. Does this look about right?

function customfolder_copyr($source, $dest) // <------- defines customfolder copy action
{
    // Check for symlinks
    if (is_link($source)) {
        return symlink(readlink($source), $dest);
    }

    // Simple copy for a file
    if (is_file($source)) {
        return copy($source, $dest);
    }

    // Make destination directory
    if (!is_dir($dest)) {
        mkdir($dest);
    }

    // Loop through the folder
    $dir = dir($source);
    while (false !== $entry = $dir->read()) {
        // Skip pointers
        if ($entry == '.' || $entry == '..') {
            continue;
        }

        // Deep copy directories
        customfolder_copyr("$source/$entry", "$dest/$entry"); // <------- defines customfolder copy action
    }

    // Clean up
    $dir->close();
    return true;
}
function customfolder_backup()  // <------- customfolder backup
{
    $to = dirname(__FILE__)."https://www.mysite-name.com/back-up-directory/"; // <------- this back up directory will be made
    $from = dirname(__FILE__)."https://www.mysite-name.com/wp-content/plugins/plugin_name/custom-folder/"; // <------- this is the directory that will be backed up
    customfolder_copyr($from, $to); // <------- executes customfolder copy action
}
function customfolder_recover() // <------- customfolder recover
{
    $from = dirname(__FILE__)."https://www.mysite-name.com/back-up-directory/"; // <------- recover the files from this back up
    $to = dirname(__FILE__)."https://www.mysite-name.com/wp-content/plugins/plugin_name/custom-folder/"; // <------- to this location
    customfolder_copyr($from, $to); // <------- executes customfolder copy action
    if (is_dir($from)) {
        customfolder_rmdirr($from);#https://www.mysite-name.com/back-up-directory/ // <------- deletes the backup directory
    }
}
add_filter('upgrader_pre_install', 'customfolder_backup', 10, 2); / <------- adds the customfolder_backup filter
add_filter('upgrader_post_install', 'customfolder_recover', 10, 2); <------- adds the customfolder_recover filter

(initial code source)