Add an experimental flag for safe ffi calls

We suspect that long running zlib calls may be starving other threads in
some applications. So we cana try those apps with this flag and if that
is indeed the problem then we can try doing this automatically where
there's minimal perf impact (e.g. larger calls).

Author: dcoutts

Codec/Compression/Zlib/Stream.hsc

@@ -1009,6 +1009,12 @@ newtype StreamState = StreamState (Ptr StreamState)
 -- So we define c_inflateInit2 and c_deflateInit2 here as wrappers around
 -- their _ counterparts and pass the extra args.
 
+##ifdef NON_BLOCKING_FFI
+##define SAFTY safe
+##else
+##define SAFTY unsafe
+##endif
+
 foreign import ccall unsafe "zlib.h inflateInit2_"
   c_inflateInit2_ :: StreamState -> CInt -> Ptr CChar -> CInt -> IO CInt
 
@@ -1017,7 +1023,7 @@ c_inflateInit2 z n =
   withCAString #{const_str ZLIB_VERSION} $ \versionStr ->
     c_inflateInit2_ z n versionStr (#{const sizeof(z_stream)} :: CInt)
 
-foreign import ccall unsafe "zlib.h inflate"
+foreign import ccall SAFTY "zlib.h inflate"
   c_inflate :: StreamState -> CInt -> IO CInt
 
 foreign import ccall unsafe "zlib.h &inflateEnd"
@@ -1050,7 +1056,7 @@ foreign import ccall unsafe "zlib.h inflateSetDictionary"
                          -> CUInt
                          -> IO CInt
 
-foreign import ccall unsafe "zlib.h deflate"
+foreign import ccall SAFTY "zlib.h deflate"
   c_deflate :: StreamState -> CInt -> IO CInt
 
 foreign import ccall unsafe "zlib.h &deflateEnd"

zlib.cabal

@@ -36,6 +36,12 @@ source-repository head
   type: darcs
   location: http://code.haskell.org/zlib/
 
+flag non-blocking-ffi
+  default:     False
+  description: The (de)compression calls can sometimes take a long time, which
+               prevents other Haskell threads running. Enabling this flag
+               avoids this unfairness, but with greater overall cost.
+
 library
   exposed-modules: Codec.Compression.GZip,
                    Codec.Compression.Zlib,
@@ -57,6 +63,8 @@ library
     build-depends: ghc-prim
   includes:        zlib.h
   ghc-options:     -Wall -fwarn-tabs
+  if flag(non-blocking-ffi)
+    cpp-options:   -DNON_BLOCKING_FFI
   if !os(windows)
     -- Normally we use the the standard system zlib:
     extra-libraries: z