Runtime should not set SIGPIPE to ignored for subprocesses

[phunge0]

The GHC runtime ignores SIGPIPE by setting the signal to SIG_IGN. This means that any subprocesses (created via System.Process or otherwise) inwill also have their SIGPIPE handler set to SIG_IGN; I think this might be a bug. The Python runtime does the same thing, there's a good explanation of the drawbacks in:  http://bugs.python.org/issue1652

IMHO the simplest fix is the patch below: simply avoid SIG_IGN, instead install a handler which does nothing. This way, an exec() restores the handler to SIG_DFL. I've included a testcase too.

Discussion link:  http://www.haskell.org/pipermail/glasgow-haskell-users/2010-August/019091.html. Summarizing: Donn Cave expressed concern that installing a signal handler for SIGPIPE might not be transparent to the rest of the program, but since the runtime already uses signal handlers for SIGVTALRM, it shouldn't make matters any worse.

[phunge0]

Bugfix and testcase patch

[simonmar]

Done. I already had a patch for this in my tree, so I didn't use the supplied one. I did add the excellent test case from the patch though, thanks!

Sat Sep 25 12:35:48 PDT 2010  Simon Marlow <marlowsd@gmail.com>
  * Use an empty signal handler for SIGPIPE instead of SIG_IGN
  
  This is so that the SIGPIPE handler gets reset to the default
  automatically on exec().

[igloo]

Merged.