Empty string comparison on home option during DB upgrades is invalid

[fjarrett]

The private __get_option() function used during database upgrades was recently changed in WordPress 5.5 to use a strict comparison operator to check whether the home option value is empty string. However, wpdb::get_var returns NULL if the option is empty, so this condition never actually exists.

​https://github.com/WordPress/WordPress/blob/537fd931bc02e6e934a2d774422b897871aa87ad/wp-admin/includes/upgrade.php#L2570

This was discovered because of an oddity that manifested when using WP-CLI to run database upgrades on sites that has an empty home option, and it was only reproducible with WordPress 5.5. Full discussion can be found in the #cli Slack.

​https://wordpress.slack.com/archives/C02RP4T41/p1597352867393200

Some possible solutions are:

1. Correct the strict comparison to use null === $option
2. Use a more generous empty comparison like ! $option
3. Refactor __get_option() to use wpdb::get_col like its get_option() cousin

---

The WP-CLI oddity is fairly easy to reproduce:

1. Empty the home option value in the WordPress database:

$ wp db query "UPDATE wp_options SET option_value = '' WHERE option_name = 'home';"

Note: wp option update home '' won't work because the core fallback to siteurl prevents it.

2. Set the db_version option to anything less than 48748:

$ wp option update db_version 47018
Success: Updated 'db_version' option.

3. Execute the database update:

$ wp core update-db
Success: WordPress database upgraded successfully from db version 47018 to 48748.

4. Check the option value:

$ wp option get home
http://http://example.com

[fjarrett]

Use null for strict comparison of home option value

[johnbillion]

Introduced in [47584].

[SergeyBiryukov]

Just noting [47584] seems unrelated, this was introduced in [47808].

[SergeyBiryukov]

In 48868:

Upgrade/Install: Make the check for empty home option in __get_option() more resilient.

This addresses a regression in [47808], where the home check expected an empty string to use siteurl as a fallback, but wpdb::get_var() returns null if the option is empty.

Props fjarrett.
Fixes #51011.

[SergeyBiryukov]

In 48869:

Upgrade/Install: Make the check for empty home option in __get_option() more resilient.

This addresses a regression in [47808], where the home check expected an empty string to use siteurl as a fallback, but wpdb::get_var() returns null if the option is empty.

Props fjarrett.
Merges [48868] to the 5.5 branch.
Fixes #51011.