incremental checksum on download from ssh or p2p

Checksum as content is received from a remote git-annex repository, rather
than doing it in a second pass.

Not tested at all yet, but I imagine it will work!

Not implemented for any special remotes, and also not implemented for
copies from local remotes. It may be that, for local remotes, it will
suffice to use rsync, rely on its checksumming, and simply return Verified.
(It would still make a checksumming pass when cp is used for COW, I guess.)

Utility/Metered.hs

@@ -1,6 +1,6 @@
 {- Metered IO and actions
  -
- - Copyright 2012-2020 Joey Hess <id@joeyh.name>
+ - Copyright 2012-2021 Joey Hess <id@joeyh.name>
  -
  - License: BSD-2-clause
  -}
@@ -118,23 +118,24 @@ withMeteredFile :: FilePath -> MeterUpdate -> (L.ByteString -> IO a) -> IO a
 withMeteredFile f meterupdate a = withBinaryFile f ReadMode $ \h ->
 	hGetContentsMetered h meterupdate >>= a
 
-{- Writes a ByteString to a Handle, updating a meter as it's written. -}
-meteredWrite :: MeterUpdate -> Handle -> L.ByteString -> IO ()
-meteredWrite meterupdate h = void . meteredWrite' meterupdate h
+{- Calls the action repeatedly with chunks from the lazy ByteString.
+ - Updates the meter after each chunk is processed. -}
+meteredWrite :: MeterUpdate -> (S.ByteString -> IO ()) -> L.ByteString -> IO ()
+meteredWrite meterupdate a = void . meteredWrite' meterupdate a
 
-meteredWrite' :: MeterUpdate -> Handle -> L.ByteString -> IO BytesProcessed
-meteredWrite' meterupdate h = go zeroBytesProcessed . L.toChunks 
+meteredWrite' :: MeterUpdate -> (S.ByteString -> IO ()) -> L.ByteString -> IO BytesProcessed
+meteredWrite' meterupdate a = go zeroBytesProcessed . L.toChunks 
   where
 	go sofar [] = return sofar
 	go sofar (c:cs) = do
-		S.hPut h c
+		a c
 		let !sofar' = addBytesProcessed sofar $ S.length c
 		meterupdate sofar'
 		go sofar' cs
 
 meteredWriteFile :: MeterUpdate -> FilePath -> L.ByteString -> IO ()
 meteredWriteFile meterupdate f b = withBinaryFile f WriteMode $ \h ->
-	meteredWrite meterupdate h b
+	meteredWrite meterupdate (S.hPut h) b
 
 {- Applies an offset to a MeterUpdate. This can be useful when
  - performing a sequence of actions, such as multiple meteredWriteFiles,