Fix Map decoding in presence of optional long byte-length marker

src/Data/Avro/Encoding/FromAvro.hs

@@ -14,7 +14,7 @@ module Data.Avro.Encoding.FromAvro
 where
 
 import           Control.DeepSeq             (NFData)
-import           Control.Monad               (forM, replicateM)
+import           Control.Monad               (forM, replicateM, void, when)
 import           Control.Monad.Identity      (Identity (..))
 import qualified Data.Avro.Internal.Get      as Get
 import           Data.Avro.Internal.Time
@@ -195,7 +195,6 @@ instance FromAvro Time.LocalTime where
     Right $ millisToLocalTime (toInteger n)
   fromAvro x = Left ("Unable to decode LocalTime from: " <> show (describeValue x))
   {-# INLINE fromAvro #-}
-
 
 instance FromAvro a => FromAvro [a] where
   fromAvro (Array vec) = mapM fromAvro $ V.toList vec
@@ -301,22 +300,33 @@ getField env sch = case sch of
     v <- getField env t
     pure $ Union sch ix v
 
+
+-- | Read a Map from blocks of KV pairs
 getKVBlocks :: HashMap Schema.TypeName ReadSchema -> ReadSchema -> Get [[(Text, Value)]]
 getKVBlocks env t = do
-  blockLength <- abs <$> Get.getLong
-  if blockLength == 0 then
+  lengthIndicator <- Get.getLong
+  if lengthIndicator == 0 then
     return []
   else do
+    -- When the block's count is negative, its absolute value is used, and the count is followed immediately by a
+    -- long block size indicating the number of bytes in the block.
+    when (lengthIndicator < 0) $ void Get.getLong -- number of bytes in block (ignored)
+    let blockLength = abs lengthIndicator
     vs <- replicateM (fromIntegral blockLength) ((,) <$> Get.getString <*> getField env t)
     (vs:) <$> getKVBlocks env t
 {-# INLINE getKVBlocks #-}
 
+-- | Read an array from blocks.
 getBlocksOf :: HashMap Schema.TypeName ReadSchema -> ReadSchema -> Get [[Value]]
 getBlocksOf env t = do
-  blockLength <- abs <$> Get.getLong
-  if blockLength == 0 then
+  lengthIndicator <- Get.getLong
+  if lengthIndicator == 0 then
     return []
   else do
+    -- When the block's count is negative, its absolute value is used, and the count is followed immediately by a
+    -- long block size indicating the number of bytes in the block.
+    when (lengthIndicator < 0) $ void Get.getLong -- number of bytes in block (ignored)
+    let blockLength = abs lengthIndicator
     vs <- replicateM (fromIntegral blockLength) (getField env t)
     (vs:) <$> getBlocksOf env t