better handling of finishing up incomplete incremental verify

Now it's run in VerifyStage.

I thought about keeping the file handle open, and resuming reading where
tailVerify left off. But that risks leaking open file handles, until the
GC closes them, if the deferred verification does not get resumed. Since
that could perhaps happen if there's an exception somewhere, I decided
that was too unsafe.

Instead, re-open the file, seek, and resume.

Sponsored-by: Dartmouth College's DANDI project

Backend/Hash.hs

@@ -6,6 +6,7 @@
  -}
 
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE BangPatterns #-}
 
 module Backend.Hash (
 	backends,
@@ -280,19 +281,25 @@ md5Hasher = mkHasher md5 md5_context
 
 mkIncrementalVerifier :: HashAlgorithm h => Context h -> Key -> IO IncrementalVerifier
 mkIncrementalVerifier ctx key = do
-	v <- newIORef (Just ctx)
+	v <- newIORef (Just (ctx, 0))
 	return $ IncrementalVerifier
 		{ updateIncremental = \b ->
 			modifyIORef' v $ \case
-				Just ctx' -> Just (hashUpdate ctx' b)
+				(Just (ctx', n)) -> 
+					let !ctx'' = hashUpdate ctx' b
+					    !n' = n + fromIntegral (S.length b)
+					in (Just (ctx'', n'))
 				Nothing -> Nothing
 		, finalizeIncremental =
 			readIORef v >>= \case
-				Just ctx' -> do
+				(Just (ctx', _)) -> do
 					let digest = hashFinalize ctx'
 					return $ sameCheckSum key (show digest)
 				Nothing -> return False
 		, failIncremental = writeIORef v Nothing
+		, positionIncremental = readIORef v >>= \case
+			Just (_, n) -> return (Just n)
+			Nothing -> return Nothing
 		, descVerify = descChecksum
 		}