API for working with external buffers

foreign-rust.cabal

@@ -48,6 +48,7 @@ common lang
       TypeFamilies
       TypeOperators
       UndecidableInstances
+      ViewPatterns
 
 library
   import:
@@ -56,6 +57,7 @@ library
       Foreign.Rust.External.JSON
       Foreign.Rust.External.Bincode
       Foreign.Rust.Failure
+      Foreign.Rust.Marshall.External
       Foreign.Rust.Marshall.Fixed
       Foreign.Rust.Marshall.Variable
       Foreign.Rust.SafeConv

src/Foreign/Rust/Marshall/External.hs

@@ -0,0 +1,53 @@
+-- | Marshall from a Rust-side allocated buffer
+--
+-- Intended for unqualified import.
+module Foreign.Rust.Marshall.External (
+    fromExternalBorsh
+  ) where
+
+import Codec.Borsh
+import Control.Exception
+import Data.Typeable
+import Data.Word
+import Foreign.C
+import Foreign.Concurrent
+import Foreign.Ptr
+
+import qualified Data.ByteString.Internal as Strict
+
+import Foreign.Rust.Marshall.Util
+
+data External
+
+foreign import ccall "haskell_ffi_external_ptr"
+     externalPtr
+  :: Ptr External -> Ptr Word8
+
+foreign import ccall "haskell_ffi_external_len"
+     externalLen
+  :: Ptr External -> CSize
+
+foreign import ccall "haskell_ffi_external_free"
+     externalFree
+  :: Ptr External -> IO ()
+
+-- | Internal auxiliary: cast pointer
+--
+-- For ease of integration with c2hs, 'fromExternalBorsh' takes a @Ptr ()@ as
+-- input instead of the more accurate @Ptr External@.
+castToExternal :: Ptr () -> Ptr External
+castToExternal = castPtr
+
+-- | Output marshaller for values stored in Rust-allocated buffer
+--
+-- Should be used together with the Rust function @marshall_to_haskell_external@
+-- (from @haskell-ffi@).
+fromExternalBorsh :: (FromBorsh a, Typeable a) => Ptr () -> IO a
+fromExternalBorsh (castToExternal -> ptr) = do
+    len  <- evaluate $ fromIntegral $ externalLen ptr
+    fptr <- newForeignPtr (externalPtr ptr) (externalFree ptr)
+
+    let bs :: Strict.ByteString
+        bs = Strict.PS fptr 0 len
+
+    return $ deserialiseStrictOrPanic bs