andriibieriezhnoi/wp-lock

Locking and mutexes for the WordPress Core

WP_Lock

because WordPress is not thread-safe

WordPress is no longer just a blogging platform. It's a framework. And like all mature frameworks it drastically needs a lock API.

Example

Consider the following user balance topup function that is susceptible to a race condition:

// A thread-safe version of the above topup function.
public function topup_user_balance( $user_id, $topup ) {
	$balance = get_user_meta( $user_id, 'balance', true );
	$balance = $balance + $topup;
	update_user_meta( $user_id, 'balance', $balance = get_user_meta( $user_id, 'balance', true ) + $topup );
	return $balace;
}

Try to call the above code 100 times in 16 threads. The balance will be less than it is supposed to be.

// A thread-safe version of the above topup function.
public function topup_user_balance( $user_id, $topup ) {
	$user_balance_lock = new WP_Lock( "$user_id:meta:balance" );
	$user_balance_lock->acquire( WP_Lock::EXCLUSIVE );

	$balance = get_user_meta( $user_id, 'balance', true );
	$balance = $balance + $topup;
	update_user_meta( $user_id, 'balance', $balance = get_user_meta( $user_id, 'balance', true ) + $topup );

	$user_balance_lock->release();

	return $balance;
}

The above code is thread safe.

Lock levels

• WP_Lock::READ, WP_Lock::WRITE - other processes can acquire READ but not WRITE, EXCLUSIVE until the original lock is released.
• WP_Lock::EXCLUSIVE - other processes can't acquire READ, WRITE, EXCLUSIVE locks until the original lock is released.