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function upgrade_network() {
	global $wp_current_db_version, $wpdb;

	// Always clear expired transients.
	delete_expired_transients( true );

	// 2.8.0
	if ( $wp_current_db_version < 11549 ) {
		$wpmu_sitewide_plugins   = get_site_option( 'wpmu_sitewide_plugins' );
		$active_sitewide_plugins = get_site_option( 'active_sitewide_plugins' );
		if ( $wpmu_sitewide_plugins ) {
			if ( ! $active_sitewide_plugins ) {
				$sitewide_plugins = (array) $wpmu_sitewide_plugins;
			} else {
				$sitewide_plugins = array_merge( (array) $active_sitewide_plugins, (array) $wpmu_sitewide_plugins );
			}

			update_site_option( 'active_sitewide_plugins', $sitewide_plugins );
		}
		delete_site_option( 'wpmu_sitewide_plugins' );
		delete_site_option( 'deactivated_sitewide_plugins' );

		$start = 0;
		while ( $rows = $wpdb->get_results( "SELECT meta_key, meta_value FROM {$wpdb->sitemeta} ORDER BY meta_id LIMIT $start, 20" ) ) {
			foreach ( $rows as $row ) {
				$value = $row->meta_value;
				if ( ! @unserialize( $value ) ) {
					$value = stripslashes( $value );
				}
				if ( $value !== $row->meta_value ) {
					update_site_option( $row->meta_key, $value );
				}
			}
			$start += 20;
		}
	}

	// 3.0.0
	if ( $wp_current_db_version < 13576 ) {
		update_site_option( 'global_terms_enabled', '1' );
	}

	// 3.3.0
	if ( $wp_current_db_version < 19390 ) {
		update_site_option( 'initial_db_version', $wp_current_db_version );
	}

	if ( $wp_current_db_version < 19470 ) {
		if ( false === get_site_option( 'active_sitewide_plugins' ) ) {
			update_site_option( 'active_sitewide_plugins', array() );
		}
	}

	// 3.4.0
	if ( $wp_current_db_version < 20148 ) {
		// 'allowedthemes' keys things by stylesheet. 'allowed_themes' keyed things by name.
		$allowedthemes  = get_site_option( 'allowedthemes' );
		$allowed_themes = get_site_option( 'allowed_themes' );
		if ( false === $allowedthemes && is_array( $allowed_themes ) && $allowed_themes ) {
			$converted = array();
			$themes    = wp_get_themes();
			foreach ( $themes as $stylesheet => $theme_data ) {
				if ( isset( $allowed_themes[ $theme_data->get( 'Name' ) ] ) ) {
					$converted[ $stylesheet ] = true;
				}
			}
			update_site_option( 'allowedthemes', $converted );
			delete_site_option( 'allowed_themes' );
		}
	}

	// 3.5.0
	if ( $wp_current_db_version < 21823 ) {
		update_site_option( 'ms_files_rewriting', '1' );
	}

	// 3.5.2
	if ( $wp_current_db_version < 24448 ) {
		$illegal_names = get_site_option( 'illegal_names' );
		if ( is_array( $illegal_names ) && count( $illegal_names ) === 1 ) {
			$illegal_name  = reset( $illegal_names );
			$illegal_names = explode( ' ', $illegal_name );
			update_site_option( 'illegal_names', $illegal_names );
		}
	}

	// 4.2.0
	if ( $wp_current_db_version < 31351 && 'utf8mb4' === $wpdb->charset ) {
		if ( wp_should_upgrade_global_tables() ) {
			$wpdb->query( "ALTER TABLE $wpdb->usermeta DROP INDEX meta_key, ADD INDEX meta_key(meta_key(191))" );
			$wpdb->query( "ALTER TABLE $wpdb->site DROP INDEX domain, ADD INDEX domain(domain(140),path(51))" );
			$wpdb->query( "ALTER TABLE $wpdb->sitemeta DROP INDEX meta_key, ADD INDEX meta_key(meta_key(191))" );
			$wpdb->query( "ALTER TABLE $wpdb->signups DROP INDEX domain_path, ADD INDEX domain_path(domain(140),path(51))" );

			$tables = $wpdb->tables( 'global' );

			// sitecategories may not exist.
			if ( ! $wpdb->get_var( "SHOW TABLES LIKE '{$tables['sitecategories']}'" ) ) {
				unset( $tables['sitecategories'] );
			}

			foreach ( $tables as $table ) {
				maybe_convert_table_to_utf8mb4( $table );
			}
		}
	}

	// 4.3.0
	if ( $wp_current_db_version < 33055 && 'utf8mb4' === $wpdb->charset ) {
		if ( wp_should_upgrade_global_tables() ) {
			$upgrade = false;
			$indexes = $wpdb->get_results( "SHOW INDEXES FROM $wpdb->signups" );
			foreach ( $indexes as $index ) {
				if ( 'domain_path' === $index->Key_name && 'domain' === $index->Column_name && '140' !== $index->Sub_part ) {
					$upgrade = true;
					break;
				}
			}

			if ( $upgrade ) {
				$wpdb->query( "ALTER TABLE $wpdb->signups DROP INDEX domain_path, ADD INDEX domain_path(domain(140),path(51))" );
			}

			$tables = $wpdb->tables( 'global' );

			// sitecategories may not exist.
			if ( ! $wpdb->get_var( "SHOW TABLES LIKE '{$tables['sitecategories']}'" ) ) {
				unset( $tables['sitecategories'] );
			}

			foreach ( $tables as $table ) {
				maybe_convert_table_to_utf8mb4( $table );
			}
		}
	}

	// 5.1.0
	if ( $wp_current_db_version < 44467 ) {
		$network_id = get_main_network_id();
		delete_network_option( $network_id, 'site_meta_supported' );
		is_site_meta_supported();
	}
}