Re-save Network Settings ruin starter content

[berfayvel]

On fresh multisite install, I re-save data in wp-admin/network/settings.php. Then I create a sub-site.

When try to edit the sub-site's starter content Sample Page, the content is just blank (no content, only title).

Then I create more sub-sites. But it still same (Sample Page content is blank).

No error generated.

[SergeyBiryukov]

Hi @berfayvel, welcome to WordPress Trac!

Thanks for the report, the First Page content is indeed blank in Network Settings.

[shadyvb]

The reason for this is that only the first_post value is inserted into the initial DB schema update at https://core.trac.wordpress.org/browser/trunk/src/wp-admin/includes/schema.php#L1009.
Neither first_page / first_comment values are inserted to the database, and once the settings page is saved, the *empty* value is actually saved then.

So, the actual call while setting up new sites in wp_install_defaults is get_option( 'first_page', $first_page ), and the way get_option currently works is that it only returns the passed default value if it *does not exist* in the database, not if it *exists* but empty.

The behaviour is not consistent between the first post and the first page/comment, because wp_install_default does not do the same with first_post, instead it will always insert the default content if the saved value is empty https://core.trac.wordpress.org/browser/trunk/src/wp-admin/includes/upgrade.php#L159 ( the post is created anyway, so no use in making it empty ).

Working on patch / tests and will post shortly.

[shadyvb]

Update wp_install_defaults to properly add default content to first page/content if they're saved as empty values

[swissspidy]

40036.diff​ splits up the previous test into 3 tests to make it easier to work with.

[swissspidy]

In 40296:

Multisite: Ensure first page/post/comment content is not empty when setting up a new site.

This prevents creating page without any content when the options are populated with an empty string.

Props shadyvb.
Fixes #40036.