other 80% of avoding verification when hard linking to objects in shared repo

In c3b38fb2a075b4250e867ebd910324c65712c747, it actually only handled
uploading objects to a shared repository. To avoid verification when
downloading objects from a shared repository, was a lot harder.

On the plus side, if the process of downloading a file from a remote
is able to verify its content on the side, the remote can indicate this
now, and avoid the extra post-download verification.

As of yet, I don't have any remotes (except Git) using this ability.
Some more work would be needed to support it in special remotes.

It would make sense for tahoe to implicitly verify things downloaded from it;
as long as you trust your tahoe server (which typically runs locally),
there's cryptographic integrity. OTOH, despite bup being based on shas,
a bup repo under an attacker's control could have the git ref used for an
object changed, and so a bup repo shouldn't implicitly verify. Indeed,
tahoe seems unique in being trustworthy enough to implicitly verify.

15 files changed, 100 insertions, 77 deletions

diff --git a/Annex/Content.hs b/Annex/Content.hs
index 266cb9ac1..5032e2691 100644
--- a/Annex/Content.hs
+++ b/Annex/Content.hs
@@ -16,7 +16,8 @@ module Annex.Content (
 	getViaTmp,
 	getViaTmp',
 	checkDiskSpaceToGet,
-	Verify(..),
+	VerifyConfig(..),
+	Types.Remote.unVerified,
 	prepTmp,
 	withTmp,
 	checkDiskSpace,
@@ -218,18 +219,19 @@ lockContent key a = do
 {- Runs an action, passing it the temp file to get,
  - and if the action succeeds, verifies the file matches
  - the key and moves the file into the annex as a key's content. -}
-getViaTmp :: Verify -> Key -> (FilePath -> Annex Bool) -> Annex Bool
+getViaTmp :: VerifyConfig -> Key -> (FilePath -> Annex (Bool, Types.Remote.Verification)) -> Annex Bool
 getViaTmp v key action = checkDiskSpaceToGet key False $
 	getViaTmp' v key action
 
 {- Like getViaTmp, but does not check that there is enough disk space
  - for the incoming key. For use when the key content is already on disk
  - and not being copied into place. -}
-getViaTmp' :: Verify -> Key -> (FilePath -> Annex Bool) -> Annex Bool
+getViaTmp' :: VerifyConfig -> Key -> (FilePath -> Annex (Bool, Types.Remote.Verification)) -> Annex Bool
 getViaTmp' v key action = do
 	tmpfile <- prepTmp key
-	ifM (action tmpfile)
-		( ifM (verifyKeyContent v key tmpfile)
+	(ok, verification) <- action tmpfile
+	if ok
+		then ifM (verifyKeyContent v verification key tmpfile)
 			( do
 				moveAnnex key tmpfile
 				logStatus key InfoPresent
@@ -241,8 +243,7 @@ getViaTmp' v key action = do
 			)
 		-- On transfer failure, the tmp file is left behind, in case
 		-- caller wants to resume its transfer
-		, return False
-		)
+		else return False
 
 {- Verifies that a file is the expected content of a key.
  - Configuration can prevent verification, for either a
@@ -253,8 +254,9 @@ getViaTmp' v key action = do
  - When the key's backend allows verifying the content (eg via checksum),
  - it is checked. 
  -}
-verifyKeyContent :: Verify -> Key -> FilePath -> Annex Bool
-verifyKeyContent v k f = ifM (shouldVerify v)
+verifyKeyContent :: VerifyConfig -> Types.Remote.Verification -> Key -> FilePath -> Annex Bool
+verifyKeyContent _ Types.Remote.Verified _ _ = return True
+verifyKeyContent v Types.Remote.UnVerified k f = ifM (shouldVerify v)
 	( verifysize <&&> verifycontent
 	, return True
 	)
@@ -268,9 +270,9 @@ verifyKeyContent v k f = ifM (shouldVerify v)
 		Nothing -> return True
 		Just verifier -> verifier k f
 
-data Verify = AlwaysVerify | NoVerify | RemoteVerify Remote | DefaultVerify
+data VerifyConfig = AlwaysVerify | NoVerify | RemoteVerify Remote | DefaultVerify
 
-shouldVerify :: Verify -> Annex Bool
+shouldVerify :: VerifyConfig -> Annex Bool
 shouldVerify AlwaysVerify = return True
 shouldVerify NoVerify = return False
 shouldVerify DefaultVerify = annexVerify <$> Annex.getGitConfig
diff --git a/Annex/Transfer.hs b/Annex/Transfer.hs
index a2bac34be..e72f737ea 100644
--- a/Annex/Transfer.hs
+++ b/Annex/Transfer.hs
@@ -5,7 +5,7 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
-{-# LANGUAGE CPP #-}
+{-# LANGUAGE CPP, FlexibleInstances #-}
 
 module Annex.Transfer (
 	module X,
@@ -24,20 +24,31 @@ import Annex.Notification as X
 import Annex.Perms
 import Utility.Metered
 import Utility.LockPool
+import Types.Remote (Verification(..))
 
 import Control.Concurrent
 
-type TransferAction = MeterUpdate -> Annex Bool
-
 type TransferObserver = Bool -> Transfer -> TransferInfo -> Annex ()
 
+class Observable a where
+	observeBool :: a -> Bool
+	observeFailure :: a
+
+instance Observable Bool where
+	observeBool = id
+	observeFailure = False
+
+instance Observable (Bool, Verification) where
+	observeBool = fst
+	observeFailure = (False, UnVerified)
+
 noObserver :: TransferObserver
 noObserver _ _ _ = noop
 
-upload :: UUID -> Key -> AssociatedFile -> RetryDecider -> TransferObserver -> TransferAction -> NotifyWitness -> Annex Bool
+upload :: Observable v => UUID -> Key -> AssociatedFile -> RetryDecider -> TransferObserver -> (MeterUpdate -> Annex v) -> NotifyWitness -> Annex v
 upload u key f d o a _witness = runTransfer (Transfer Upload u key) f d o a
 
-download :: UUID -> Key -> AssociatedFile -> RetryDecider -> TransferObserver -> TransferAction -> NotifyWitness -> Annex Bool
+download :: Observable v => UUID -> Key -> AssociatedFile -> RetryDecider -> TransferObserver -> (MeterUpdate -> Annex v) -> NotifyWitness -> Annex v
 download u key f d o a _witness = runTransfer (Transfer Download u key) f d o a
 
 {- Runs a transfer action. Creates and locks the lock file while the
@@ -52,7 +63,7 @@ download u key f d o a _witness = runTransfer (Transfer Download u key) f d o a
  - An upload can be run from a read-only filesystem, and in this case
  - no transfer information or lock file is used.
  -}
-runTransfer :: Transfer -> Maybe FilePath -> RetryDecider -> TransferObserver -> TransferAction -> Annex Bool
+runTransfer :: Observable v => Transfer -> Maybe FilePath -> RetryDecider -> TransferObserver -> (MeterUpdate -> Annex v) -> Annex v
 runTransfer = runTransfer' False
 
 {- Like runTransfer, but ignores any existing transfer lock file for the
@@ -60,10 +71,10 @@ runTransfer = runTransfer' False
  -
  - Note that this may result in confusing progress meter display in the
  - webapp, if multiple processes are writing to the transfer info file. -}
-alwaysRunTransfer :: Transfer -> Maybe FilePath -> RetryDecider -> TransferObserver -> TransferAction -> Annex Bool
+alwaysRunTransfer :: Observable v => Transfer -> Maybe FilePath -> RetryDecider -> TransferObserver -> (MeterUpdate -> Annex v) -> Annex v
 alwaysRunTransfer = runTransfer' True
 
-runTransfer' :: Bool -> Transfer -> Maybe FilePath -> RetryDecider -> TransferObserver -> TransferAction -> Annex Bool
+runTransfer' :: Observable v => Bool -> Transfer -> Maybe FilePath -> RetryDecider -> TransferObserver -> (MeterUpdate -> Annex v) -> Annex v
 runTransfer' ignorelock t file shouldretry transferobserver transferaction = do
 	info <- liftIO $ startTransferInfo file
 	(meter, tfile, metervar) <- mkProgressUpdater t info
@@ -72,12 +83,12 @@ runTransfer' ignorelock t file shouldretry transferobserver transferaction = do
 	if inprogress && not ignorelock
 		then do
 			showNote "transfer already in progress, or unable to take transfer lock"
-			return False
+			return observeFailure
 		else do
-			ok <- retry info metervar $ transferaction meter
+			v <- retry info metervar $ transferaction meter
 			liftIO $ cleanup tfile lck
-			transferobserver ok t info
-			return ok
+			transferobserver (observeBool v) t info
+			return v
   where
 #ifndef mingw32_HOST_OS
 	prep tfile mode info = do
@@ -128,7 +139,7 @@ runTransfer' ignorelock t file shouldretry transferobserver transferaction = do
 				let newinfo = oldinfo { bytesComplete = Just b }
 				if shouldretry oldinfo newinfo
 					then retry newinfo metervar run
-					else return False
+					else return observeFailure
 	getbytescomplete metervar
 		| transferDirection t == Upload =
 			liftIO $ readMVar metervar
diff --git a/Command/AddUrl.hs b/Command/AddUrl.hs
index 32e89d20f..4b34051e9 100644
--- a/Command/AddUrl.hs
+++ b/Command/AddUrl.hs
@@ -154,7 +154,7 @@ downloadRemoteFile r relaxed uri file sz = do
 			-- so that the remote knows what url it
 			-- should use to download it.
 			setTempUrl urlkey loguri
-			let downloader = Remote.retrieveKeyFile r urlkey (Just file)
+			let downloader = \dest p -> fst <$> Remote.retrieveKeyFile r urlkey (Just file) dest p
 			ret <- downloadWith downloader urlkey (Remote.uuid r) loguri file
 			removeTempUrl urlkey
 			return ret
diff --git a/Command/Fsck.hs b/Command/Fsck.hs
index f34b9b074..dbbb0e67f 100644
--- a/Command/Fsck.hs
+++ b/Command/Fsck.hs
@@ -161,7 +161,7 @@ performRemote key file backend numcopies remote =
 			( return (Just True)
 			, ifM (Annex.getState Annex.fast)
 				( return Nothing
-				, Just <$>
+				, Just . fst <$>
 					Remote.retrieveKeyFile remote key Nothing tmp dummymeter
 				)
 			)
diff --git a/Command/ReKey.hs b/Command/ReKey.hs
index 9084814fa..fe13d4dd4 100644
--- a/Command/ReKey.hs
+++ b/Command/ReKey.hs
@@ -52,7 +52,7 @@ perform file oldkey newkey = do
 {- Make a hard link to the old key content (when supported),
  - to avoid wasting disk space. -}
 linkKey :: Key -> Key -> Annex Bool
-linkKey oldkey newkey = getViaTmp' DefaultVerify newkey $ \tmp -> do
+linkKey oldkey newkey = getViaTmp' DefaultVerify newkey $ \tmp -> unVerified $ do
 	src <- calcRepo $ gitAnnexLocation oldkey
 	liftIO $ ifM (doesFileExist tmp)
 		( return True
diff --git a/Command/RecvKey.hs b/Command/RecvKey.hs
index 3a8747534..988c4f90e 100644
--- a/Command/RecvKey.hs
+++ b/Command/RecvKey.hs
@@ -40,7 +40,7 @@ start key = fieldTransfer Download key $ \_p -> do
 		, return False
 		)
   where
-	go tmp = do
+	go tmp = unVerified $ do
 		opts <- filterRsyncSafeOptions . maybe [] words
 			<$> getField "RsyncOptions"
 		liftIO $ rsyncServerReceive (map Param opts) tmp
diff --git a/Command/Reinject.hs b/Command/Reinject.hs
index 90ddc1c2a..1c21b4ae6 100644
--- a/Command/Reinject.hs
+++ b/Command/Reinject.hs
@@ -43,7 +43,7 @@ perform src _dest key = ifM move
 	-- so moveFile is used rather than simply calling
 	-- moveToObjectDir; disk space is also checked this way,
 	-- and the file's content is verified to match the key.
-	move = getViaTmp DefaultVerify key $ \tmp ->
+	move = getViaTmp DefaultVerify key $ \tmp -> unVerified $
 		liftIO $ catchBoolIO $ do
 			moveFile src tmp
 			return True
diff --git a/Command/SetKey.hs b/Command/SetKey.hs
index 319229482..13715d3a7 100644
--- a/Command/SetKey.hs
+++ b/Command/SetKey.hs
@@ -35,7 +35,7 @@ perform file key = do
 	-- the file might be on a different filesystem, so moveFile is used
 	-- rather than simply calling moveAnnex; disk space is also
 	-- checked this way.
-	ok <- getViaTmp DefaultVerify key $ \dest ->
+	ok <- getViaTmp DefaultVerify key $ \dest -> unVerified $
 		if dest /= file
 			then liftIO $ catchBoolIO $ do
 				moveFile file dest
diff --git a/Command/TestRemote.hs b/Command/TestRemote.hs
index e4a9eb829..7ee5f1359 100644
--- a/Command/TestRemote.hs
+++ b/Command/TestRemote.hs
@@ -176,7 +176,7 @@ testUnavailable st r k =
 		getViaTmp (RemoteVerify r) k $ \dest ->
 			Remote.retrieveKeyFile r k Nothing dest nullMeterUpdate
 	, check (== Right False) "retrieveKeyFileCheap" $
-		getViaTmp (RemoteVerify r) k $ \dest ->
+		getViaTmp (RemoteVerify r) k $ \dest -> unVerified $
 			Remote.retrieveKeyFileCheap r k Nothing dest
 	]
   where
diff --git a/Remote/BitTorrent.hs b/Remote/BitTorrent.hs
index d3963a918..f9027ba61 100644
--- a/Remote/BitTorrent.hs
+++ b/Remote/BitTorrent.hs
@@ -76,8 +76,8 @@ gen r _ c gc =
 		, checkUrl = Just checkTorrentUrl
 		}
 
-downloadKey :: Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Bool
-downloadKey key _file dest p = 
+downloadKey :: Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex (Bool, Verification)
+downloadKey key _file dest p = unVerified $
 	get . map (torrentUrlNum . fst . getDownloader) =<< getBitTorrentUrls key
   where
 	get [] = do
diff --git a/Remote/Git.hs b/Remote/Git.hs
index 841837113..f7a0b4a39 100644
--- a/Remote/Git.hs
+++ b/Remote/Git.hs
@@ -359,13 +359,13 @@ dropKey r key
 	| otherwise = commitOnCleanup r $ Ssh.dropKey (repo r) key
 
 {- Tries to copy a key's content from a remote's annex to a file. -}
-copyFromRemote :: Remote -> Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Bool
+copyFromRemote :: Remote -> Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex (Bool, Verification)
 copyFromRemote r key file dest p = parallelMetered (Just p) key file $
 	copyFromRemote' r key file dest
 
-copyFromRemote' :: Remote -> Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Bool
+copyFromRemote' :: Remote -> Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex (Bool, Verification)
 copyFromRemote' r key file dest meterupdate
-	| not $ Git.repoIsUrl (repo r) = guardUsable (repo r) (return False) $ do
+	| not $ Git.repoIsUrl (repo r) = guardUsable (repo r) (unVerified (return False)) $ do
 		params <- Ssh.rsyncParams r Download
 		u <- getUUID
 		hardlink <- wantHardLink
@@ -374,18 +374,17 @@ copyFromRemote' r key file dest meterupdate
 			ensureInitialized
 			v <- Annex.Content.prepSendAnnex key
 			case v of
-				Nothing -> return False
+				Nothing -> return (False, UnVerified)
 				Just (object, checksuccess) -> do
 					copier <- mkCopier hardlink params
 					runTransfer (Transfer Download u key)
 						file noRetry noObserver 
-						(callCopier copier object dest)
-						<&&> checksuccess
-	| Git.repoIsSsh (repo r) = feedprogressback $ \feeder -> do
+						(\p -> copier object dest p checksuccess)
+	| Git.repoIsSsh (repo r) = unVerified $ feedprogressback $ \feeder -> do
 		direct <- isDirect
 		Ssh.rsyncHelper (Just feeder) 
 			=<< Ssh.rsyncParamsRemote direct r Download key dest file
-	| Git.repoIsHttp (repo r) = Annex.Content.downloadUrl (keyUrls r key) dest
+	| Git.repoIsHttp (repo r) = unVerified $ Annex.Content.downloadUrl (keyUrls r key) dest
 	| otherwise = error "copying from non-ssh, non-http remote not supported"
   where
 	{- Feed local rsync's progress info back to the remote,
@@ -462,8 +461,8 @@ copyFromRemoteCheap r key af file
 			)
 	| Git.repoIsSsh (repo r) =
 		ifM (Annex.Content.preseedTmp key file)
-			( parallelMetered Nothing key af $
-				copyFromRemote' r key af file
+			( fst <$> parallelMetered Nothing key af
+				(copyFromRemote' r key af file)
 			, return False
 			)
 	| otherwise = return False
@@ -502,13 +501,11 @@ copyToRemote' r key file p
 			, do
 				ensureInitialized
 				copier <- mkCopier hardlink params
-				let verify = if isHardLinker copier
-					then Annex.Content.NoVerify
-					else Annex.Content.RemoteVerify r
+				let verify = Annex.Content.RemoteVerify r
 				runTransfer (Transfer Download u key) file noRetry noObserver $ const $
 					Annex.Content.saveState True `after`
 						Annex.Content.getViaTmp verify key
-							(\dest -> callCopier copier object dest p <&&> liftIO checksuccessio)
+							(\dest -> copier object dest p (liftIO checksuccessio))
 			)
 
 fsckOnRemote :: Git.Repo -> [CommandParam] -> Annex (IO Bool)
@@ -620,32 +617,34 @@ commitOnCleanup r a = go `after` a
 wantHardLink :: Annex Bool
 wantHardLink = (annexHardLink <$> Annex.getGitConfig) <&&> (not <$> isDirect)
 
-data Copier
-	= Copier (FilePath -> FilePath -> MeterUpdate -> Annex Bool) 
-	| HardLinker (FilePath -> FilePath -> MeterUpdate -> Annex Bool)
-
-isHardLinker :: Copier -> Bool
-isHardLinker (Copier _) = False
-isHardLinker (HardLinker _) = True
-
-callCopier :: Copier -> FilePath -> FilePath -> MeterUpdate -> Annex Bool
-callCopier (Copier a) = a
-callCopier (HardLinker a) = a
-
+-- Copies from src to dest, updating a meter. If the copy finishes
+-- successfully, calls a final check action, which must also success, or
+-- returns false.
+--
 -- If either the remote or local repository wants to use hard links,
--- the copier will do so, falling back to copying.
+-- the copier will do so (falling back to copying if a hard link cannot be
+-- made).
+--
+-- When a hard link is created, returns Verified; the repo being linked
+-- from is implicitly trusted, so no expensive verification needs to be
+-- done.
+type Copier = FilePath -> FilePath -> MeterUpdate -> Annex Bool -> Annex (Bool, Verification)
+
 mkCopier :: Bool -> [CommandParam] -> Annex Copier
 mkCopier remotewanthardlink rsyncparams = do
-	let copier = rsyncOrCopyFile rsyncparams
+	let copier = \src dest p check -> unVerified $
+		rsyncOrCopyFile rsyncparams src dest p <&&> check
 #ifndef mingw32_HOST_OS
 	localwanthardlink <- wantHardLink
-	let linker = \object dest -> createLink object dest >> return True
+	let linker = \src dest -> createLink src dest >> return True
 	ifM (pure (remotewanthardlink || localwanthardlink) <&&> not <$> isDirect)
-		( return $ HardLinker $ \object dest p ->
-			liftIO (catchBoolIO (linker object dest))
-				<||> copier object dest p
-		, return $ Copier copier
+		( return $ \src dest p check ->
+			ifM (liftIO (catchBoolIO (linker src dest)))
+				( return (True, Verified)
+				, copier src dest p check
+				)
+		, return copier
 		)
 #else
-	return $ Copier copier
+	return copier
 #endif
diff --git a/Remote/Helper/Special.hs b/Remote/Helper/Special.hs
index 1acabcc91..7faf7a8a1 100644
--- a/Remote/Helper/Special.hs
+++ b/Remote/Helper/Special.hs
@@ -122,8 +122,8 @@ byteRetriever a k _m callback = a k (callback . ByteContent)
  -}
 storeKeyDummy :: Key -> AssociatedFile -> MeterUpdate -> Annex Bool
 storeKeyDummy _ _ _ = return False
-retreiveKeyFileDummy :: Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Bool
-retreiveKeyFileDummy _ _ _ _ = return False
+retreiveKeyFileDummy :: Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex (Bool, Verification)
+retreiveKeyFileDummy _ _ _ _ = unVerified (return False)
 removeKeyDummy :: Key -> Annex Bool
 removeKeyDummy _ = return False
 checkPresentDummy :: Key -> Annex Bool
@@ -156,7 +156,7 @@ specialRemote' cfg c preparestorer prepareretriever prepareremover preparecheckp
   where
 	encr = baser
 		{ storeKey = \k f p -> cip >>= storeKeyGen k f p
-		, retrieveKeyFile = \k f d p -> cip >>= retrieveKeyFileGen k f d p
+		, retrieveKeyFile = \k f d p -> cip >>= unVerified . retrieveKeyFileGen k f d p
 		, retrieveKeyFileCheap = \k f d -> cip >>= maybe
 			(retrieveKeyFileCheap baser k f d)
 			-- retrieval of encrypted keys is never cheap
diff --git a/Remote/Tahoe.hs b/Remote/Tahoe.hs
index 1357a0183..c04cdae58 100644
--- a/Remote/Tahoe.hs
+++ b/Remote/Tahoe.hs
@@ -116,8 +116,8 @@ store u hdl k _f _p = sendAnnex k noop $ \src ->
 		(return False)
 		(\cap -> storeCapability u k cap >> return True)
 
-retrieve :: UUID -> TahoeHandle -> Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Bool
-retrieve u hdl k _f d _p = go =<< getCapability u k
+retrieve :: UUID -> TahoeHandle -> Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex (Bool, Verification)
+retrieve u hdl k _f d _p = unVerified $ go =<< getCapability u k
   where
 	go Nothing = return False
 	go (Just cap) = liftIO $ requestTahoe hdl "get" [Param cap, File d]
diff --git a/Remote/Web.hs b/Remote/Web.hs
index 700f40480..ae0281064 100644
--- a/Remote/Web.hs
+++ b/Remote/Web.hs
@@ -70,8 +70,8 @@ gen r _ c gc =
 		, checkUrl = Nothing
 		}
 
-downloadKey :: Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Bool
-downloadKey key _file dest _p = get =<< getWebUrls key
+downloadKey :: Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex (Bool, Verification)
+downloadKey key _file dest _p = unVerified $ get =<< getWebUrls key
   where
 	get [] = do
 		warning "no known url"
diff --git a/Types/Remote.hs b/Types/Remote.hs
index 4b4732a51..24851e17c 100644
--- a/Types/Remote.hs
+++ b/Types/Remote.hs
@@ -13,6 +13,8 @@ module Types.Remote
 	, RemoteTypeA(..)
 	, RemoteA(..)
 	, Availability(..)
+	, Verification(..)
+	, unVerified
 	)
 	where
 
@@ -64,9 +66,9 @@ data RemoteA a = Remote {
 	-- all of its contents have been transferred.
 	storeKey :: Key -> AssociatedFile -> MeterUpdate -> a Bool,
 	-- Retrieves a key's contents to a file.
-	-- (The MeterUpdate does not need to be used if it retrieves
-	-- directly to the file, and not to an intermediate file.)
-	retrieveKeyFile :: Key -> AssociatedFile -> FilePath -> MeterUpdate -> a Bool,
+	-- (The MeterUpdate does not need to be used if it writes
+	-- sequentially to the file.)
+	retrieveKeyFile :: Key -> AssociatedFile -> FilePath -> MeterUpdate -> a (Bool, Verification),
 	-- Retrieves a key's contents to a tmp file, if it can be done cheaply.
 	-- It's ok to create a symlink or hardlink.
 	retrieveKeyFileCheap :: Key -> AssociatedFile -> FilePath -> a Bool,
@@ -122,3 +124,12 @@ instance Eq (RemoteA a) where
 
 instance Ord (RemoteA a) where
 	compare = comparing uuid
+
+-- Use Verified when the content of a key is verified as part of a
+-- transfer, and so a separate verification step is not needed.
+data Verification = UnVerified | Verified
+
+unVerified :: Monad m => m Bool -> m (Bool, Verification)
+unVerified a = do
+	ok <- a
+	return (ok, UnVerified)