dbDelta triggers create table for existing tables when using multisite upgrade

[fliespl]

If you are using multisite and upgrade is run on the database, it will try to create global tables for each multisite blog (triggering errors).

It's because of the line: 1533-1534 in wp-admin/includes/upgrade.php

		if ( in_array( $table, $global_tables ) && ( !is_main_site() || defined( 'DO_NOT_UPGRADE_GLOBAL_TABLES' ) ) )
			continue;

Since you put continue, "create table" will never be "unset" in the line 1679, because it had already left the loop:

unset( $cqueries[ $table ], $for_update[ $table ] );

Proposed fix:
change lines 1533-1534 from:

		if ( in_array( $table, $global_tables ) && ( !is_main_site() || defined( 'DO_NOT_UPGRADE_GLOBAL_TABLES' ) ) )
			continue;

to:

		if ( in_array( $table, $global_tables ) && ( !is_main_site() || defined( 'DO_NOT_UPGRADE_GLOBAL_TABLES' ) ) ) {
			$wpdb->suppress_errors();
			$tablefields = $wpdb->get_results("DESCRIBE {$table};");
			$wpdb->suppress_errors( false );
			if ( $tablefields )
				unset( $cqueries[ $table ], $for_update[ $table ] );
			continue;
		}

This way, if global tables don't exist they will remain in $cqueries, but if they exist, they will be unset from creation.

[fliespl]

Duplicate of #22134.