Asynchronous exceptions in threadWait

[Andrew Martin]

The definition of threadWait is:

threadWait :: Event
<http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Internal.html#Event>
-> Fd <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/System.Posix.Types.html#Fd>
-> IO ()threadWait
<http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#threadWait>
evt <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365266>
fd <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365267>
= mask_ <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.IO.html#mask_>
$ <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Base.html#%24>
do  m <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365268>
<- newEmptyMVar
<http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.MVar.html#newEmptyMVar>
 mgr <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365269>
<- getSystemEventManager_
<http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#getSystemEventManager_>
 reg <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365271>
<- registerFd <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Manager.html#registerFd>
mgr <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365269>
(\_ e <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365270>
-> putMVar <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.MVar.html#putMVar>
m <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365268>
e <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365270>)
fd <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365267>
evt <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365266>
M.OneShot <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Internal.html#OneShot>
 evt' <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365272>
<- takeMVar <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.MVar.html#takeMVar>
m <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365268>
`onException <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.IO.html#onException>`
unregisterFd_ <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Manager.html#unregisterFd_>
mgr <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365269>
reg <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365271>
 if evt' <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Thread.html#local-6989586621679365272>
`eventIs <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Internal.html#eventIs>`
evtClose <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Event.Internal.html#evtClose>
   then ioError
<http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.IO.Exception.html#ioError>
$ <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Base.html#%24>
errnoToIOError <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/Foreign.C.Error.html#errnoToIOError>
"threadWait" eBADF
<http://hackage.haskell.org/package/base-4.12.0.0/docs/src/Foreign.C.Error.html#eBADF>
Nothing <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Maybe.html#Nothing>
Nothing <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Maybe.html#Nothing>
   else return <http://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.Base.html#return>
()

Although the entire function has asynchronous exceptions masked, the call
to takeMVar uses onException to deal with the possibility of an exception.
According to the docs in Control.Concurrent, takeMVar can throw exceptions.
But my understand (which may be wrong) is that the only exception this
could throw would be something like BlockedIndefinitelyOnMVar, which I
don't believe can happen here.

-- 
-Andrew Thaddeus Martin
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