On 27.01.2011 20:44, Julian Foad wrote: > My thinking was that if I don't have a lock, I can't safely add a > reference because another process might delete the text just before I > get around to executing that statement. But in fact I don't necessarily > need a WC lock, I need *either* a WC lock *or* a SQL txn. And within > that lock or txn I need to check the pristine text exists in the store > before I add the reference. > > To remove a reference, I can't think why I should need a lock or a txn.
Can't be that simple. Every change to the database happens within a transaction, every transaction can be rolled back in the event of any kind of error. Consequently, you can only remove a reference (decrease a reference counter) in such a way that rolling back the transaction in which that happens can never leave a dangling reference to a deleted pristine text. In other words, deleting a pristine text requires an exclusive lock on the database -- it is a write operation that affects database consistency. -- Brane
