id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
3160	No exception safety in Control.Concurrent.QSem QSemN and SampleVar	ChrisKuklewicz	simonmar	"Looking at the code for QSem, QSemN, and SampleVar shows they all use a ""takeMVar"" then ""putMVar"" programming idiom.

None of these are exception safe.  An unlucky killThread will leave the MVar empty and cause the rest of the program to malfunction.

The solution is to rewrite them using withMVar and modifyMVar(_) to prevent the MVar from being left empty in the event of an exception.

Note: QSem also needs the bugfix in #3159"	bug	closed	normal	7.6.1	libraries/base	7.0.2	wontfix		greenrd@… ChrisKuklewicz hackage.haskell.org@…	Unknown/Multiple	Unknown/Multiple	Incorrect result at runtime	Unknown