haskell/cabal — pull request #1108 by bgamari (2012-11-12 23:27:11+00:00)
Title: Replace use of deprecated QSem with MVar 

If my reading of the MVar documentation is correct, this should be safe
as we are accounting for the case where two threads can putMVar
simultaneously. Would someone like to confirm this? Otherwise it compiles and appears to work correctly (although testing is hampered by the brokenness of my GHC 7.7 tree at the moment).