Give more detail in the error message for too large request bodies.

* Just to be helpful to developers, give the maximum body length and their body length
* Also point developers to the function to change that value

(I don't think this leaks any sensitive info, because you can always binary search with different request body sizes to find the maximum allowable)

yesod-core/ChangeLog.md

@@ -1,3 +1,7 @@
+## 1.4.37.3
+
+* Improve error message when request body is too large [#1477](https://github.com/yesodweb/yesod/pull/1477)
+
 ## 1.4.37.2
 
 * Improve error messages for the CSRF checking functions [#1455](https://github.com/yesodweb/yesod/issues/1455)

yesod-core/Yesod/Core/Internal/Request.hs

@@ -25,6 +25,7 @@ import qualified Network.Wai as W
 import Web.Cookie (parseCookiesText)
 import Data.ByteString (ByteString)
 import qualified Data.ByteString.Char8 as S8
+import qualified Data.ByteString.Lazy.Char8 as LS8
 import Data.Text (Text, pack)
 import Network.HTTP.Types (queryToQueryText, Status (Status))
 import Data.Maybe (fromMaybe, catMaybes)
@@ -60,17 +61,23 @@ limitRequestBody maxLen req = do
             let len = fromIntegral $ S8.length bs
                 remaining' = remaining - len
             if remaining < len
-                then throwIO $ HCWai tooLargeResponse
+                then throwIO $ HCWai $ tooLargeResponse maxLen len
                 else do
                     writeIORef ref remaining'
                     return bs
         }
 
-tooLargeResponse :: W.Response
-tooLargeResponse = W.responseLBS
+tooLargeResponse :: Word64 -> Word64 -> W.Response
+tooLargeResponse maxLen bodyLen = W.responseLBS
     (Status 413 "Too Large")
     [("Content-Type", "text/plain")]
-    "Request body too large to be processed."
+    (L.concat 
+        [ "Request body too large to be processed. The maximum size is "
+        , (LS8.pack (show maxLen))
+        , " bytes; your request body was "
+        , (LS8.pack (show bodyLen))
+        , " bytes. If you're the developer of this site, you can configure the maximum length with the `maximumContentLength` function on the Yesod typeclass."
+        ])
 
 parseWaiRequest :: W.Request
                 -> SessionMap

yesod-core/Yesod/Core/Internal/Run.hs

@@ -327,7 +327,7 @@ yesodRunner :: (ToTypedContent res, Yesod site)
             -> Maybe (Route site)
             -> Application
 yesodRunner handler' YesodRunnerEnv {..} route req sendResponse
-  | Just maxLen <- mmaxLen, KnownLength len <- requestBodyLength req, maxLen < len = sendResponse tooLargeResponse
+  | Just maxLen <- mmaxLen, KnownLength len <- requestBodyLength req, maxLen < len = sendResponse (tooLargeResponse maxLen len)
   | otherwise = do
     let dontSaveSession _ = return []
     (session, saveSession) <- liftIO $

yesod-core/yesod-core.cabal

@@ -1,5 +1,5 @@
 name:            yesod-core
-version:         1.4.37.2
+version:         1.4.37.3
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>