Report event number in error reporting

src/GHC/RTS/Events/Incremental.hs

@@ -16,9 +16,11 @@ module GHC.RTS.Events.Incremental
   , readEventLog
   , readEventLogOrFail
   ) where
+
 import Control.Monad
 import Data.Either
 import Data.Maybe
+import Data.Word
 import Prelude
 
 import qualified Data.Binary.Get as G
@@ -138,16 +140,16 @@ readEvents header = f . break isLeft . readEvents' header
 -- Note that it doesn't fail if it consumes all input in the middle of decoding
 -- of an event.
 readEvents' :: Header -> BL.ByteString -> [Either String Event]
-readEvents' header = go (decodeEvents header)
+readEvents' header = go (decodeEvents header) 0
   where
-    go :: Decoder Event -> BL.ByteString -> [Either String Event]
-    go decoder bytes = case decoder of
-      Produce event decoder' -> Right event : go decoder' bytes
+    go :: Decoder Event -> Word64 -> BL.ByteString -> [Either String Event]
+    go decoder !event_n bytes = case decoder of
+      Produce event decoder' -> Right event : go decoder' (event_n+1) bytes
       Consume k -> case bytes of
         BL.Empty -> []
-        BL.Chunk chunk chunks -> go (k chunk) chunks
+        BL.Chunk chunk chunks -> go (k chunk) event_n chunks
       Done {} -> []
-      Error _ err -> [Left err]
+      Error _ err -> [Left $ "in event " ++ show event_n ++ ": " ++ err]
 
 -- | Read an entire event log from a lazy bytestring. It returns an error message if it
 -- encounters an error while decoding.