[GHC] #15746: Memory leak in print when RTS option -N is >= 2

[GHC]

#15746: Memory leak in print when RTS option -N is >= 2
-------------------------------------+-------------------------------------
           Reporter:  nmattia        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 In GHC 802 the code below runs with constant memory. In GHC 822, 843 and
 861 the code below runs with constant memory only if at most one core is
 used. If the RTS option -N is set to anything greater than 1, the memory
 usage grows with the size of "big_file".


 {{{#!hs
 import Data.ByteString.Builder.Extra (defaultChunkSize)
 import Data.Function
 import System.IO
 import qualified Data.ByteString as BS

 main :: IO ()
 main = do
     h <- openFile "big_file" ReadMode
     fix $ \loop -> do
       bs <- BS.hGetSome h defaultChunkSize
       if BS.null bs
       then pure ()
       else do
         print bs
         loop
 }}}


 You can reproduce this by cloning https://github.com/nmattia/ghc-print-
 leak and running "nix-shell". Alternatively save the code above to
 "Main.hs" and run "ghc ./Main.hs -threaded -rtsopts" and then "./Main +RTS
 -M2M -N[1|2] > /dev/null".

 Side note: the memory does not grow as quickly if the _size_ of the
 bytestring is printed, i.e. it seems to be a problem specific to
 hPutStrLn/print as opposed to the actual file content/bytestring being
 retained. I first encountered this issue when using conduit for the loop.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15746>
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