what are locks?
With databases, locks are generally referring to something that must be done in a series. With multiple CPUs, there can be the situation where two pieces of code want to change the same variable at the same time. Think of an increment operation. If you have a value of four and that gets incremented by both at the same time, the result would be five, not six like it is supposed to be. So a lock basically gives one instance of the code the ability to make the change, and then after that lock is released the next instance of the code can do what it needs to do. So that value goes from four to five, and then five to six.
So in the context of the "bug" in v0.7 and prior versions, the setting used for maximum locks for BDB was below that necessary for a transaction that first made it into a block on a v0.8 node. v0.8 doesn't use BDB and therefore doesn't have that same restriction. There was a rule in the bitcoin protocol, due to a specific BDB configuration, that wasn't expressly known previously.
[Edit: And here is the info specifically on the BDB locks configuration:
Here's the Berkeley DB tutorial for anyone who might want to do some reading on sizing your database correctly and lock limits.
http://www.stanford.edu/class/cs276a/projects/docs/berkeleydb/ref/lock/max.htmlThe maximum number of locks required by an application cannot be easily estimated. It is possible to calculate a maximum number of locks by multiplying the maximum number of lockers, times the maximum number of lock objects, times two (two for the two possible lock modes for each object, read and write). However, this is a pessimal value, and real applications are unlikely to actually need that many locks. Reviewing the Lock subsystem statistics is the best way to determine this value.