better fix for zombie problem, which turns out to be a zombie ssh started by rsync

When rsyncProgress pipes rsync's stdout, this turns out to cause a ssh
process started by rsync to be left behind as a zombie. I don't know why,
but my recent zombie reaping cleanup was correct, it's just that this other
zombie, that's not directly started by git-annex, was no longer reaped
due to changes in the cleanup. Make rsyncProgress reap the zombie started
by rsync, as a workaround.

FWIW, the process tree looks like this. It seems like the rsync child
is for some reason starting but not waiting on this extra ssh process.
Ssh connection caching may be involved -- disabling it seemed to change
the shape of the tree, but did not eliminate the zombie.

 9378 pts/14   S+     0:00  |           \_ rsync -p --progress --inplace -4 -e 'ssh' '-S' ...
 9379 pts/14   S+     0:00  |           |   \_ ssh ...
 9380 pts/14   S+     0:00  |           |   \_ rsync -p --progress --inplace -4 -e 'ssh' '-S' ...
 9381 pts/14   Z+     0:00  |           \_ [ssh] <defunct>

1 files changed, 14 insertions, 0 deletions

diff --git a/Utility/Misc.hs b/Utility/Misc.hs
index f03504040..7c81f56fd 100644
--- a/Utility/Misc.hs
+++ b/Utility/Misc.hs
@@ -12,6 +12,9 @@ import Control.Monad
 import Foreign
 import Data.Char
 import Control.Applicative
+import System.Posix.Process (getAnyProcessStatus)
+
+import Utility.Exception
 
 {- A version of hgetContents that is not lazy. Ensures file is 
  - all read before it gets closed. -}
@@ -96,3 +99,14 @@ hGetSomeString h sz = do
 	where
 		peekbytes :: Int -> Ptr Word8 -> IO [Word8]
 		peekbytes len buf = mapM (peekElemOff buf) [0..pred len]
+
+{- Reaps any zombie git processes. 
+ -
+ - Warning: Not thread safe. Anything that was expecting to wait
+ - on a process and get back an exit status is going to be confused
+ - if this reap gets there first. -}
+reapZombies :: IO ()
+reapZombies = do
+	-- throws an exception when there are no child processes
+	catchDefaultIO Nothing (getAnyProcessStatus False True)
+		>>= maybe (return ()) (const reapZombies)