Class MasterLock
source code
util.ComparableMixin --+    
                       |    
              BaseLockId --+
                           |
                          MasterLock
I am a semaphore that limits the number of simultaneous actions.
  Builds and BuildSteps can declare that they wish to claim me as they 
  run. Only a limited number of such builds or steps will be able to run 
  simultaneously. By default this number is one, but my maxCount parameter 
  can be raised to allow two or three or more operations to happen at the 
  same time.
  Use this to protect a resource that is shared among all builders and 
  all slaves, for example to limit the load on a common SVN repository.