Asynchronous exceptions and laziness bugs in Control.Concurrent.QSem/QSemN

[Bas van Dijk]

Hello,

after discovering the bugs regarding asynchronous exceptions and
laziness in Data.Unique and Control.Concurrent.SampleVar I became a
bit suspicious about the other concurrency mechanisms.

It looks like Control.Concurrent.Chan is safe because it uses
modifyMVar_ internally which performs the necessary block.

Control.Concurrent.QSem and QSemN are not alright however! They both
suffer the same bugs:

* Potential dead-lock due to not blocking asynchronous exceptions.
* Potential space-leak because of lazily putting numeric expression in a MVar.

The attached patch fixes these bugs.

regards,

Bas
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