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.)

Remote/Helper/Chunked/Legacy.hs

@@ -11,6 +11,7 @@ import Annex.Common
 import Remote.Helper.Chunked
 import Utility.Metered
 
+import qualified Data.ByteString as S
 import qualified Data.ByteString.Lazy as L
 
 {- This is an extension that's added to the usual file (or whatever)
@@ -117,4 +118,4 @@ meteredWriteFileChunks :: MeterUpdate -> FilePath -> [v] -> (v -> IO L.ByteStrin
 meteredWriteFileChunks meterupdate dest chunks feeder =
 	withBinaryFile dest WriteMode $ \h ->
 		forM_ chunks $
-			meteredWrite meterupdate h <=< feeder
+			meteredWrite meterupdate (S.hPut h) <=< feeder

Remote/Helper/P2P.hs

@@ -1,6 +1,6 @@
 {- Helpers for remotes using the git-annex P2P protocol.
  -
- - Copyright 2016-2020 Joey Hess <id@joeyh.name>
+ - Copyright 2016-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -17,6 +17,7 @@ import Annex.Content
 import Messages.Progress
 import Utility.Metered
 import Types.NumCopies
+import Backend
 
 import Control.Concurrent
 
@@ -39,10 +40,11 @@ store runner k af p = do
 			Just False -> giveup "transfer failed"
 			Nothing -> remoteUnavail
 
-retrieve :: (MeterUpdate -> ProtoRunner (Bool, Verification)) -> Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Verification
-retrieve runner k af dest p =
+retrieve :: VerifyConfig -> (MeterUpdate -> ProtoRunner (Bool, Verification)) -> Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Verification
+retrieve verifyconfig runner k af dest p = do
+	iv <- startVerifyKeyContentIncrementally verifyconfig k
 	metered (Just p) k $ \m p' -> 
-		runner p' (P2P.get dest k af m p') >>= \case
+		runner p' (P2P.get dest k iv af m p') >>= \case
 			Just (True, v) -> return v
 			Just (False, _) -> giveup "transfer failed"
 			Nothing -> remoteUnavail

Remote/Helper/Special.hs

@@ -295,7 +295,7 @@ sink dest enc c mh mp content = case (enc, mh, content) of
 		Just h -> liftIO $ b `streamto` h
 		Nothing -> liftIO $ bracket opendest hClose (b `streamto`)
 	streamto b h = case mp of
-		Just p -> meteredWrite p h b
+		Just p -> meteredWrite p (S.hPut h) b
 		Nothing -> L.hPut h b
 	opendest = openBinaryFile dest WriteMode