Flush json output, avoiding a buffering problem that could result in doubled output.

The bug was that with --json, output lines were sometimes doubled. For
example, git annex init --json would output two lines, despite only running
one thing. Adding to the weirdness, this only occurred when the output
was redirected to a pipe or a file.

Strace showed two processes outputting the same buffered output.
The second process was this writer process (only needed to work around
bug #624389):

                _ <- forkProcess $ do
                        hPutStr toh $ unlines paths
                        hClose toh
                        exitSuccess

The doubled output occurs when this process exits, and ghc flushes the
inherited stdout buffer. Why only when piping? I don't know, but ghc may
be behaving differently when stdout is not a terminal.

While this is quite possibly a ghc bug, there is a nice fix in git-annex.
Explicitly flushing after each chunk of json is output works around the
problem, and as a side effect, json is streamed rather than being output
all at the end when performing an expensive operaition.

However, note that this means all uses of putStr in git-annex must be
explicitly flushed. The others were, already.

Messages.hs

@@ -129,7 +129,7 @@ handle json normal = Annex.getState Annex.output >>= go
 	where
 		go Annex.NormalOutput = liftIO normal
 		go Annex.QuietOutput = q
-		go Annex.JSONOutput = liftIO json
+		go Annex.JSONOutput = liftIO $ flushed $ json
 
 q :: Monad m => m ()
 q = return ()

debian/changelog

@@ -2,6 +2,8 @@ git-annex (3.20111123) UNRELEASED; urgency=low
 
   * Put a workaround in the directory special remote for strange behavior
     with VFAT filesystems on Linux (mounted with shortname=mixed)
+  * Flush json output, avoiding a buffering problem that could result in
+    doubled output.
 
  -- Joey Hess <joeyh@debian.org>  Tue, 22 Nov 2011 17:53:42 -0400