[GHC] #13247: hPutBufNonBlocking can block

[GHC]

#13247: hPutBufNonBlocking can block
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           Reporter:  nh2            |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #13246, #13245
Differential Rev(s):                 |         Wiki Page:
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 This simply escalates an existing TODO in the code base to a ticket.

 The function `System.IO.hPutBufNonBlocking` [1] calls `hPutBuf'` with
 `can_block = False`, which is commented as `Bool -- allow blocking?`
 ([https://github.com/ghc/ghc/blame/876b00ba25a615423f48b0cf9d443a9fd5dbd6f4/libraries/base/GHC/IO/Handle/Text.hs#L708-L713
 relevant lines])

 However, `hPutBuf'` calls `bufWrite`, inside which there's the comment
 ([https://github.com/ghc/ghc/blame/876b00ba25a615423f48b0cf9d443a9fd5dbd6f4/libraries/base/GHC/IO/Handle/Text.hs#L751
 relevant line]):

 {{{
   if (...)
         -- There's enough room in the buffer:
         -- just copy the data in and update bufR.
         then do ...

         -- else, we have to flush
         else do ...
                 old_buf' <- Buffered.flushWriteBuffer haDevice old_buf
                         -- TODO: we should do a non-blocking flush here
 }}}

 so it turns out that if there's not enough room in the `Handle` buffer,
 then `hPutBufNonBlocking` will block.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13247>
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