File not found error during plugin upgrades

[malithsen]

Describe the bug

When a file included in init_rest_api is removed, it results in a missing file error during the plugin upgrade process.
This happens because init_rest_api of the the old version is invoked during the upgrade but the file it's trying to load is removed in the new version.

To Reproduce

1. Install WCPay 5.8.1.
2. Go through onboarding.
3. Remove class-wc-rest-user-exists-controller.php
4. Build the plugin locally
5. Upgrade the site to the plugin built in step 4
6. Expect the fatal error below.

Failed opening '/srv/users/user217f0884/apps/user217f0884/public/wp-content/plugins/woocommerce-payments/includes/admin/class-wc-rest-user-exists-controller.php' for inclusion (include_path='.:/opt/sp/php7.4/lib/php') in /srv/users/user217f0884/apps/user217f0884/public/wp-content/plugins/woocommerce-payments/includes/class-wc-payments.php on line 954

Fatal error: Uncaught Error: Class 'WC_REST_User_Exists_Controller' not found in /srv/users/user217f0884/apps/user217f0884/public/wp-content/plugins/woocommerce-payments/includes/class-wc-payments.php:955 Stack trace: #0 /srv/users/user217f0884/apps/user217f0884/public/wp-includes/class-wp-hook.php(308): WC_Payments::init_rest_api(Object(WP_REST_Server)) #1 /srv/users/user217f0884/apps/user217f0884/public/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters(NULL, Array) #2 /srv/users/user217f0884/apps/user217f0884/public/wp-includes/plugin.php(517): WP_Hook->do_action(Array) #3 /srv/users/user217f0884/apps/user217f0884/public/wp-includes/rest-api.php(577): do_action('rest_api_init', Object(WP_REST_Server)) #4 /srv/users/user217f0884/apps/user217f0884/public/wp-includes/rest-api.php(535): rest_get_server() #5 /srv/users/user217f0884/apps/user217f0884/public/wp-includes/rest-api.php(2889): rest_do_request(Object(WP_REST_Request)) #6 [internal function]: rest_preload_api_request(Array, '/wc-analytics/r...') #7 /srv/use in /srv/users/user217f0884/apps/user217f0884/public/wp-content/plugins/woocommerce-payments/includes/class-wc-payments.php on line 955

Actual behavior

Error shown when upgrading the plugin

Screenshots

Expected behavior

No errors should be printed

Additional context

Slack thread: p1684148049557989-slack-CGGCLBN58

Related: #6303

[zmaglica]

@alefesouza I noticed that this issue is assigned to you. Are you still planning to work on this issue?

[htdat]

It seems that the cause is described here paJDYF-aZa-p2#comment-21382