From 062db545636881f694e6c0c1eaef1eb973da1b0d Mon Sep 17 00:00:00 2001
From: Joe Crayne <joe@jerkface.net>
Date: Sun, 28 Oct 2018 23:58:22 -0400
Subject: Send empty bytestrings as null pointer.

---
 haskell/Data/VCDIFF.hs | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/haskell/Data/VCDIFF.hs b/haskell/Data/VCDIFF.hs
index c48eb1a..6b95ede 100644
--- a/haskell/Data/VCDIFF.hs
+++ b/haskell/Data/VCDIFF.hs
@@ -47,7 +47,7 @@ import Data.Text.Encoding
 import Data.Word
 import Foreign.C.Types
 import Foreign.C.String
-import Foreign.ForeignPtr (withForeignPtr)
+import Foreign.ForeignPtr (withForeignPtr,touchForeignPtr)
 import Foreign.Ptr
 import Foreign.Concurrent
 import Foreign.Storable
@@ -67,6 +67,9 @@ data Stream m = Stream
     , streamSource :: MutVar (PrimState m) (Maybe (Source m))
     }
 
+keepAlive srcvar s = do
+    seq srcvar $ seq s $ return () -- Keep array s alive until the ffi functions finish.
+{-# NOINLINE keepAlive #-}
 
 -- The xd3_config structure is used to initialize a stream - all data
 -- is copied into stream so config may be a temporary variable.  See
@@ -84,7 +87,7 @@ config_stream cfg = do
                 xd3_abort_stream sptr
                 xd3_close_stream sptr
                 xd3_free_stream sptr
-                seq srcvar $ seq s $ return () -- Keep array s alive until the ffi functions finish.
+                keepAlive srcvar s
         fp <- newForeignPtr sptr finalize
         return Stream
             { streamArray  = s
@@ -149,8 +152,12 @@ pokeCurrentBlock stream blk = do
 withByteString :: PrimBase m => B.ByteString -> (Ptr b -> Usize_t -> m a) -> m a
 withByteString d act =
     let (fp,off,len) = B.toForeignPtr d
-    in unsafeIOToPrim $ withForeignPtr fp $ \ptr -> unsafePrimToIO $ do
-        act (ptr `plusPtr` off) (fromIntegral len)
+    in do ptr <- unsafeIOToPrim $ withForeignPtr fp $ return
+          a <- case fromIntegral len of
+                0 -> act nullPtr 0
+                l -> act (ptr `plusPtr` off) l
+          unsafeIOToPrim $ touchForeignPtr fp
+          return a
 
 xdelta :: (PrimBase m, Monoid u) => XDeltaMethods m u -> (Stream m -> m ErrorCode) -> [B.ByteString] -> m u
 xdelta x xxcode_input ds = do
-- 
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