If the MVar was set up prior to the fork, I can imagine things going crazy trying to use it on the ther side of the fork - especially if their were waiting readers or writers while the fork was executing.
I've isolated the hang further to hPipeFrom's use of System.Log.Logger.
Commenting out calls to logging functions makes it reliably run ok. See
the "LINE OF DEATH" in the attached updated test case.
... And it turns out that System.Log.Logger uses a MVar to hold the
logger information. So I think this points to a MissingH bug, although I
don't yet understand it.
--
see shy jo