Incremental binary stream decoder, generalized from hit-network

src/Data/Binary/Local.hs

@@ -0,0 +1,109 @@
+{- This file is part of Vervis.
+ -
+ - Written in 2016 by fr33domlover <fr33domlover@riseup.net>.
+ -
+ - ♡ Copying is an act of love. Please copy, reuse and share.
+ -
+ - The author(s) have dedicated all copyright and related and neighboring
+ - rights to this software to the public domain worldwide. This software is
+ - distributed without any warranty.
+ -
+ - You should have received a copy of the CC0 Public Domain Dedication along
+ - with this software. If not, see
+ - <http://creativecommons.org/publicdomain/zero/1.0/>.
+ -}
+
+-- | This module provides incremental binary data decoding. The data stream
+-- from which bytes are read is represented as a monadic action which returns a
+-- strict ByteString. If the ByteString returned is empty, it means there is no
+-- more input. The intention is that the ByteStrings returned are of constant
+-- size, e.g. the lazy ByteString chunk size, or some network-specific chunk or
+-- packet size.
+module Data.Binary.Local
+    ( DecodeFail (..)
+    , DecodeBuffer
+    , decodeIncremental
+    )
+where
+
+import Prelude
+
+import Control.Monad.Logger
+import Data.Binary.Get
+import Data.ByteString (ByteString)
+import Data.Foldable (for_)
+import Data.Int (Int64)
+import Data.Monoid ((<>))
+import Formatting
+
+import qualified Data.ByteString as B (null)
+import qualified Data.Text as T (pack)
+import qualified Data.Text.Encoding as TE (decodeLatin1)
+
+-- | Description of decoding error.
+data DecodeFail = DecodeFail
+    { -- | Unused bytes left in the buffer.
+      dfRemainder :: ByteString
+      -- | Number of bytes consumed before the error.
+    , dfConsumed  :: Int64
+      -- | Error description.
+    , dfReason    :: String
+    }
+    deriving Show
+
+-- | A buffer used during the decoding process.
+type DecodeBuffer = Maybe ByteString
+
+-- | Read bytes from a stream until a single value is successfuly decoded from
+-- it (or until decoding fails).
+decodeIncremental
+    :: MonadLogger m
+    => LogSource
+    -- ^ Log source name for logging via 'MonadLogger'. If you don't want to
+    -- specify a source, pass empty text here.
+    -> m ByteString
+    -- ^ The stream reader action. Reads a chunk of bytes and returns it. If
+    -- there are no more bytes to read, returns an empty bytestring.
+    -> DecodeBuffer
+    -- ^ Decoder buffer. If you're starting to decode, pass 'Nothing' here. If
+    -- you already decode a value and want to continue, pass here the buffer
+    -- you got from the previous call to this function.
+    -> Get a
+    -- ^ Decoder definition.
+    -> m (Either DecodeFail (DecodeBuffer, a))
+    -- ^ If decoding fails, 'Left' an error is returned. If it succeeds, you
+    -- get the buffer state and the decoded value. If you intend to decode a
+    -- value again from the same stream, pass the buffer to the next call of
+    -- this function.
+decodeIncremental src readBytes buf decode = go buf $ runGetIncremental decode
+    --readBytes <- requestBody <$> waiRequest
+    where
+    go _ (Fail remains consumed err) = do
+        let df = DecodeFail
+                { dfRemainder = remains
+                , dfConsumed  = consumed
+                , dfReason    = err
+                }
+        $logErrorS src $ T.pack $ show df
+        return $ Left df
+    go mbuffer (Done unused consumed result) = do
+        $logDebugS src $ sformat
+            ("Decoding done, consumed " % int % " bytes") consumed
+        for_ mbuffer $ \ b -> $logWarnS src $
+            "Done decoding with nonempty buffer: " <> (TE.decodeLatin1 b)
+        let mbuffer' = if B.null unused
+                then mbuffer
+                else Just $ maybe unused (<> unused) mbuffer
+        return $ Right (mbuffer', result)
+    go mbuffer (Partial k) = do
+        chunk <- case mbuffer of
+            Nothing -> do
+                bytes <- readBytes
+                $logDebugS src $ "Received " <> TE.decodeLatin1 bytes
+                return bytes
+            Just b -> do
+                $logDebugS src $ "Reading buffer: " <> TE.decodeLatin1 b
+                return b
+        if B.null chunk
+            then go Nothing $ k Nothing
+            else go Nothing $ k $ Just chunk

vervis.cabal

@@ -34,7 +34,8 @@ flag library-only
   default:     False
 
 library
-  exposed-modules:     Data.ByteString.Char8.Local
+  exposed-modules:     Data.Binary.Local
+                       Data.ByteString.Char8.Local
                        Data.ByteString.Local
                        Data.Char.Local
                        Data.List.Local
@@ -91,6 +92,8 @@ library
   build-depends:       aeson
                      , attoparsec
                      , base
+                       -- for Data.Binary.Local
+                     , binary
                      , base64-bytestring
                      , blaze-html
                      , byteable