get_main_network_id should use WP_Network_Query

[spacedmonkey]

The get_main_network_id is used in the bootstrap process to load the network object. With the new WP_Network_Query class, we should use that to load the network id. Using WP_Network_Query we gain filters and caching.

[flixos90]

Patch looks good to me. +1 for getting rid of the cache key. One thing I don't think we need is the ! empty( $_networks ) check. That kind of check hasn't been in place before and shouldn't be required since every Multisite has at least one network.

About unit tests, I don't think these are necessary here since that part of get_main_network_id() is already covered (https://core.trac.wordpress.org/browser/trunk/tests/phpunit/tests/multisite/network.php#L66 would fail if we broke it).

[jeremyfelt]

Replying to flixos90:

One thing I don't think we need is the ! empty( $_networks ) check. That kind of check hasn't been in place before and shouldn't be required since every Multisite has at least one network.

Agreed. I don't believe there's a way to hit that condition unless something has gone very wrong. I think we're okay without it. I've added 37218.4.diff​, which makes that change.

About unit tests, I don't think these are necessary here since that part of get_main_network_id() is already covered (https://core.trac.wordpress.org/browser/trunk/tests/phpunit/tests/multisite/network.php#L66 would fail if we broke it).

+1

[jeremyfelt]

In 38632:

Multisite: Use get_networks() in get_main_network_id().

The manual query for the main network ID can now be replaced with a cached get_networks() query. This allows us to eliminate the primary_network_id cache key entirely.

Props spacedmonkey.
Fixes #37218.