[GHC] #4504: "awaitSignal Nothing" does not block thread with -threaded

[GHC]

#4504: "awaitSignal Nothing" does not block thread with -threaded
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    Reporter:  adept             |        Owner:  adept                      
        Type:  bug               |       Status:  patch                      
    Priority:  normal            |    Milestone:  7.2.1                      
   Component:  Runtime System    |      Version:  7.0.1                      
    Keywords:                    |     Testcase:                             
   Blockedby:                    |   Difficulty:                             
          Os:  Unknown/Multiple  |     Blocking:                             
Architecture:  Unknown/Multiple  |      Failure:  Incorrect result at runtime
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Comment(by adept):

 Please note that {{{awaitSignals (Just reservedSignals)}}} would use just
 {{{reservedSignals}}} as signal mask, instead of adding them to the
 current process mask.

 Let's look at the problem from this perspective: suppose that we set up a
 bunch of signal handlers and now we want to block until the signal
 arrives. Right now documentation says to use {{{awaitSignal Nothing}}}.

 Unfortunately, {{{awaitSignal Nothing}}} is completely useless with
 "-threaded".

 So, in order to write robust code that would work either way we would have
 to do something like this (and add this example to the documentation of
 "awaitSignal"):
 {{{
 {-# LANGUAGE CPP #-}
 import Control.Concurrent (rtsSupportsBoundThreads)
 import System.Posix.Signals

 blockTillSignal = do
   sigset <- getSignalMask
   let sigset' =
         case rtsSupportsBoundThreads of
           False -> sigset
           True -> unionSignalSets sigset reservedSignals -- i'm making
 this up, no function like this right now.
   awaitSignal sigset'
 }}}

 Now, as you can see, this has large overlap with {{{awaitSignal}}} code. I
 think that this is a job for the library to hide away exactly this sort of
 complexity.

 I might have a case of tunnel vision, but it seems like {{{awaitSignal
 Nothing}}} has a very limited use right now, unless one writes the code
 and is 100% sure that it would be used without "-threaded", which is
 simply not right.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4504#comment:10>
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