db-sync: Reject TxMetadata objects containing NUL characters

TxMetadata is stored as JSON and that JSON is stored in a 'jsonb' column
in PostgreSQL. However, there are limitations to that Postgres 'jsonb'
data type. Specifically, it cannot contain Uniciode NUL characters. This
temporary fix simply drops TxMetadata JSON objects that would otherwise
be rejected by Postgres. Hopefully a better solution will be will be
dreamt up and implemented later.

Temporary workaround fix for:
#297

cardano-db-sync/src/Cardano/DbSync/Era/Shelley/Insert.hs

@@ -9,11 +9,13 @@
 
 module Cardano.DbSync.Era.Shelley.Insert
   ( insertShelleyBlock
+  , containsUnicodeNul
+  , safeDecodeUtf8
   ) where
 
 import           Cardano.Prelude
 
-import           Cardano.BM.Trace (Trace, logDebug, logError, logInfo)
+import           Cardano.BM.Trace (Trace, logDebug, logError, logInfo, logWarning)
 
 import           Cardano.Db (DbWord64 (..))
 
@@ -38,9 +40,12 @@ import           Cardano.DbSync.Util
 import           Cardano.Slotting.Slot (EpochNo (..), EpochSize (..))
 
 import qualified Data.Aeson as Aeson
+import qualified Data.ByteString.Char8 as BS
 import qualified Data.ByteString.Lazy.Char8 as LBS
 import qualified Data.Map.Strict as Map
+import qualified Data.Text as Text
 import qualified Data.Text.Encoding as Text
+import qualified Data.Text.Encoding.Error as Text
 
 import           Database.Persist.Sql (SqlBackend)
 
@@ -481,17 +486,39 @@ insertTxMetadata
     :: (MonadBaseControl IO m, MonadIO m)
     => Trace IO Text -> DB.TxId -> Shelley.MetaData
     -> ExceptT DbSyncNodeError (ReaderT SqlBackend m) ()
-insertTxMetadata _tracer txId (Shelley.MetaData mdmap) =
+insertTxMetadata tracer txId (Shelley.MetaData mdmap) =
     mapM_ insert $ Map.toList mdmap
   where
     insert
         :: (MonadBaseControl IO m, MonadIO m)
         => (Word64, Shelley.MetaDatum)
         -> ExceptT DbSyncNodeError (ReaderT SqlBackend m) ()
-    insert (key, md) =
-      void . lift . DB.insertTxMetadata $
-        DB.TxMetadata
-          { DB.txMetadataKey = DbWord64 key
-          , DB.txMetadataJson = Text.decodeUtf8 . LBS.toStrict $ Aeson.encode (jsonFromMetadataValue md)
-          , DB.txMetadataTxId = txId
-          }
+    insert (key, md) = do
+      let jsonbs = LBS.toStrict $ Aeson.encode (jsonFromMetadataValue md)
+      ejson <- liftIO $ safeDecodeUtf8 jsonbs
+      case ejson of
+        Left err ->
+          liftIO . logWarning tracer $ mconcat
+            [ "insertTxMetadata: Could not decode to UTF8: ", textShow err ]
+        Right json -> do
+          -- See https://github.com/input-output-hk/cardano-db-sync/issues/297
+          if containsUnicodeNul json
+            then liftIO $ logWarning tracer "insertTxMetadata: dropped due to a Unicode NUL character."
+            else
+              void . lift . DB.insertTxMetadata $
+                DB.TxMetadata
+                  { DB.txMetadataKey = DbWord64 key
+                  , DB.txMetadataJson = json
+                  , DB.txMetadataTxId = txId
+                  }
+
+safeDecodeUtf8 :: ByteString -> IO (Either Text.UnicodeException Text)
+safeDecodeUtf8 bs
+    | BS.any isNullChar bs = pure $ Left (Text.DecodeError (BS.unpack bs) (Just 0))
+    | otherwise = try $ evaluate (Text.decodeUtf8With Text.strictDecode bs)
+  where
+    isNullChar :: Char -> Bool
+    isNullChar ch = ord ch == 0
+
+containsUnicodeNul :: Text -> Bool
+containsUnicodeNul = Text.isInfixOf "\\u000"