if MULTISITE constant not defined error message "One or more database tables are unavailable" or "Error establishing a database connection" appears

[Axel_germany]

I updated a wordpress installation (no multisite) from 3.0.1 to 3.0.4. After the update the back-end displayed the error message "One or more database tables are unavailable" and the front-end "Error establishing a database connection".

I figured out that this only happened if the constant MULTISITE in wp-config.php was not defined at all.

If you set it to false
define( 'MULTISITE', false );

or true

define( 'MULTISITE', true );

everything seams to be fine.

I tested it with the default 'Twenty Ten 1.1 by the WordPress team' theme and no plugins activated.

This might have to do with this:

if (MULTISITE) {echo "is true";}
else {echo "is false";}

is evaluated to 'true' if you didn't defined the constant while it is evaluated to 'false'

define("MULTISITE", false );
if (MULTISITE) {echo "is true";}
else {echo "is false";}

if you define it to be false.

WordPress should never be treating MULTISITE as a boolean without testing first to see if it is defined.

Additionally, in the case of a non-multisite install, we define it to be false.

// Initialize multisite if enabled.
if ( is_multisite() ) {
	require( ABSPATH . WPINC . '/ms-blogs.php' );
	require( ABSPATH . WPINC . '/ms-settings.php' );
} elseif ( ! defined( 'MULTISITE' ) ) {
	define( 'MULTISITE', false );
}

Going to need more information here. Literally millions of people are running 3.0.4 with no MULTISITE constant defined at all. Something else must be going on.

If is_multisite() returns true, we assume that, if MULTISITE is defined, it is defined to true. This occurs in a few locations, notably wp-db.php, which allows us to identify whether we're on a 3.0 network, or a network updated from MU.

Other note: If you are running a MU network -- such that you have VHOST or SUNRISE defined -- and you define('MULTISITE', false);, the sky will fall on your head.

I don't even have a theory for what's going on here. Something very odd and I think it's your setup.

Thanks for your quick reply. I tracked the problem down. It's not critical for normal users but if you have

define( 'SUBDOMAIN_INSTALL', false );  

in your wp-config.php and don't have

define( 'MULTISITE', false );

in your wp-config.php then

/**
 * Whether Multisite support is enabled
 *
 * @since 3.0.0
 *
 * @return bool True if multisite is enabled, false otherwise.
 */
function is_multisite() {
        if ( defined( 'MULTISITE' ) )
                return MULTISITE;

        if ( defined( 'SUBDOMAIN_INSTALL' ) || defined( 'VHOST' ) || defined( 'SUNRISE' ) )
                return true;

        return false;
}

( line 499 in wp-includes/load.php ) returns true.

The is-subdomain_install() function does part of this query so why not make use of that?

Patch attached, needs testing

Scratch that - it won't call that function is no MS. The attached should be better by on my testing is multisite is not defined but subdomain_install is then WordPress bails. I therefore cannot reproduce.

This looks like it was simply just a single installation mis-configuration, and if not, there's not enough information here to figure out how this happened, but this situation doesn't happen currently, assuming instructions for multisite installation are followed correctly.

Proposing close.