MMBase

original bugid: #6307
The createKey() method does 2 SQL queries; but another thread can be executing the same code simultaniously: that will result in 2 threads receiving the same objectnumber for their new object.

Solution: use synchronization; or better: use table locking
Added synchronization to createKey() and createSequence().
This will not solve potential problems with cross-server MMBases, but will solve it for edits within one MMBase instance.
Solving cross-server will include some method of locking, which is not a SQL92 standard, en is in several databases either not or poorly implementented, or breaks transactions:

Postgresql:
Locking a table lasts a whole transaction. Fortunately, Postgresql does not need lcoking as sequence update/retrieveal is done in one statement.

Mysql:
LOCK TABLES statement closes a transaction (say bye bye to rollback), and is only supported in MySQL 4,0 or higher.

Hsqldb:
Supports no locking

Informix:
unknown