WordPress is no longer just a blogging platform. It's a framework. And like all mature frameworks it drastically needs a lock API.
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 );
return $balance;
}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::WRITE );
$balance = get_user_meta( $user_id, 'balance', true );
$balance = $balance + $topup;
update_user_meta( $user_id, 'balance', $balance );
$user_balance_lock->release();
return $balance;
}The above code is thread safe.
WP_Lock::READ- other processes can acquire READ but not WRITE until the original lock is released. A shared read lock.WP_Lock::WRITE(default) - other processes can't acquire READ or WRITE locks until the original lock is released. An exclusive read-write lock
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