Move predicate for assessing raw errors from middleware to Api.Server

lib/core/src/Cardano/Wallet/Api/Server.hs

@@ -77,6 +77,8 @@ import Data.Generics.Internal.VL.Lens
     ( (^.) )
 import Data.Generics.Labels
     ()
+import Data.Maybe
+    ( isJust )
 import Data.Proxy
     ( Proxy (..) )
 import Data.Quantity
@@ -447,7 +449,8 @@ instance LiftHandler ErrUpdatePassphrase where
         ErrUpdatePassphraseWithRootKey e  -> handler e
 
 instance LiftHandler ServantErr where
-    handler err@(ServantErr code _ body headers) = case code of
+    handler err@(ServantErr code _ body headers)
+      | not (isJSON body) = case code of
         400 -> apiError err' BadRequest (utf8 body)
         404 -> apiError err' NotFound $ mconcat
             [ "I couldn't find the requested endpoint. If the endpoint "
@@ -478,8 +481,10 @@ instance LiftHandler ServantErr where
             , "don't yet know how to handle this type of situation. Here's "
             , "some information about what happened: ", utf8 body
             ]
+      | otherwise = err
       where
         utf8 = T.replace "\"" "'" . T.decodeUtf8 . BL.toStrict
+        isJSON = isJust . Aeson.decode @Aeson.Value
         err' = err
             { errHeaders =
                 ( hContentType

lib/core/src/Network/Wai/Middleware/ServantError.hs

@@ -17,8 +17,6 @@ import Control.Monad
     ( guard )
 import Data.ByteString.Lazy
     ( ByteString )
-import Network.HTTP.Types.Header
-    ( ResponseHeaders )
 import Network.HTTP.Types.Status
     ( statusCode, statusMessage )
 import Network.Wai
@@ -30,7 +28,6 @@ import Servant.Server.Internal.ServantErr
 
 import qualified Data.Binary.Builder as Binary
 import qualified Data.ByteString.Char8 as B8
-import qualified Network.HTTP.Types.Status as HTTP
 
 -- | Make sure every error is converted to a suitable application-level error.
 --
@@ -72,16 +69,7 @@ eitherRawError res =
             (Right res)
             (Left . flip (ServantErr code reason) headers)
     in
-        maybeToEither $ guard (isRawError status headers) *> body
-
--- | Raw 'Servant' errors don't have any Content-Type. This is a lean predicate
--- but for lack of any better way of identifying them, that's a best effort.
-isRawError
-    :: HTTP.Status
-    -> ResponseHeaders
-    -> Bool
-isRawError status headers =
-    statusCode status >= 400 && null headers
+        maybeToEither $ guard (code >= 400) *> body
 
 -- | Extract raw body of a response, only if it suitables for transformation.
 -- Servant doesn't return files or streams by default, so if one of the two is