ghci and ghc -threaded broken with pipes & forking

Jeremy Shaw at
Wed Feb 28 21:28:25 EST 2007


Hrm, setting the LineBuffering mode had the side-effect of setting the
underlying file description to non-blocking mode. When the executeFile
process took over, it would die with an error about 'standard input
temporarily unavailable'. So, ignore that.


At Wed, 28 Feb 2007 15:23:53 -0600,
John Goerzen wrote:
> On Wed, Feb 28, 2007 at 01:06:25PM -0800, Jeremy Shaw wrote:
> > Hello,
> > 
> > Your first problem is just a line buffering issue. You need to
> > explicitly set the line buffer inside the child processes:
> > 
> >                               redir fstdin stdInput
> >                               hSetBuffering stdin LineBuffering
> >                               redir fstdout stdOutput
> >                               hSetBuffering stdout LineBuffering
> Hi Jeremy,
> First, many thanks for looking into this.
> That doesn't make sense to me, since these aren't used for anything in
> Haskell prior to the call to executeFile.  The Haskell buffers should
> just disappear, since the Haskell process disappears, right?
> > Once you fix that you will encounter some new race condition type
> > bugs. These bugs will show up, even *without* the -threaded flag.
> Hrm, could you point out a couple?  I'm developing as many unit tests as
> I can, and haven't had any problem running them under a non-threaded
> GHC.
> I am aware that the debug statements can write over each other at some
> cases, or even get inserted into the pipeline in a few situations, but
> these are only used in exceptional cases and will generally be removed
> from the code before too long.
> Other than that, I think I've got it OK.  My unit tests are covering
> singleton commands and 2-4 commands in a pipe, including various
> permutations of calls to external programs and Haskell functions.

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