Track overall position in error reporting

src/GHC/RTS/Events/Incremental.hs

@@ -140,16 +140,18 @@ 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) 0
+readEvents' header = go (decodeEvents header) 0 0 0
   where
-    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
+    go :: Decoder Event -> Word64 -> Word64 -> Word64 ->  BL.ByteString -> [Either String Event]
+    go decoder !last_chunk_sz !bytes_consumed !event_n bytes = case decoder of
+      Produce event decoder' -> Right event : go decoder' last_chunk_sz bytes_consumed (event_n+1) bytes
       Consume k -> case bytes of
         BL.Empty -> []
-        BL.Chunk chunk chunks -> go (k chunk) event_n chunks
+        BL.Chunk chunk chunks -> go (k chunk) (fromIntegral $ B.length chunk) (bytes_consumed + last_chunk_sz) event_n chunks
       Done {} -> []
-      Error _ err -> [Left $ "in event " ++ show event_n ++ ": " ++ err]
+      Error leftovers err ->
+          let pos = bytes_consumed + last_chunk_sz - fromIntegral (B.length leftovers)
+          in [Left $ concat [ "in event ", show event_n, " @ offset ", show pos, ": ", err] ]
 
 -- | Read an entire event log from a lazy bytestring. It returns an error message if it
 -- encounters an error while decoding.