Make cpp/wrap.h header only

common/util/Util/FFI.hs

@@ -18,9 +18,6 @@ import System.IO.Unsafe (unsafePerformIO)
 import Control.Exception
 import Control.Monad
 
-foreign import ccall unsafe "&hs_ffi_free_error" hs_ffi_free_error
-  :: FunPtr (CString -> IO ())
-
 newtype FFIError = FFIError (ForeignPtr CChar)
 
 ffiErrorMessage :: FFIError -> String
@@ -39,7 +36,13 @@ call :: IO CString -> IO ()
 call f = do
   p <- f
   when (p /= nullPtr) $ do
-    fp <- newForeignPtr hs_ffi_free_error p
+    fp <- do
+      b <- peek p
+      -- A '\1' prefix indicates that we shouldn't free the string (e.g.,
+      -- because it's static) - cf. ffi::wrap.
+      if b /= toEnum 1
+        then newForeignPtr finalizerFree p
+        else newForeignPtr_ (p `plusPtr` 1)
     throwIO $ FFIError fp
 
 infixr 5 :>

common/util/cpp/wrap.h

@@ -15,11 +15,15 @@ namespace facebook {
 namespace hs {
 namespace ffi {
 
-extern const char* outOfMemory;
-extern const char* unknownError;
-
 template <typename F>
 const char* wrap(F&& f) noexcept {
+  // The '\1' prefix instructs the marshaller in FFI.hs to not free those
+  // strings. The prefix itself will be stripped out. This is only really
+  // necessary specifically for the outOfMemory message where we might not be
+  // able to allocate a new message string - although arguably, we might just
+  // want to abort in such a case.
+  static const char *outOfMemory = "\1out of memory";
+  static const char *unknownError = "\1unknown error";
   try {
     f();
     return nullptr;

common/util/fb-util.cabal

@@ -156,7 +156,6 @@ library
 
     cxx-sources:
         cpp/cdynamic.cpp
-        cpp/ffi.cpp
         cpp/logging.cpp
         cpp/HsStruct.cpp
         cpp/IOBuf.cpp
@@ -171,7 +170,6 @@ library
         cpp/HsStdVariant.h
         cpp/HsStruct.h
         cpp/HsStructDefines.h
-        cpp/ffi.h
         cpp/memory.h
         cpp/wrap.h
         Util/AsanAlloc.h