add incremental hashing interface to Backend

As yet unused.

Backend.External could perhaps implement it too, although that would
involve sending chunks of data to it via a pipe or something, so likely
to be slow.

Types/Backend.hs

@@ -2,7 +2,7 @@
  -
  - Most things should not need this, using Types instead
  -
- - Copyright 2010-2020 Joey Hess <id@joeyh.name>
+ - Copyright 2010-2021 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -11,16 +11,21 @@ module Types.Backend where
 
 import Types.Key
 import Types.KeySource
-
 import Utility.Metered
 import Utility.FileSystemEncoding
 
+import Data.ByteString (ByteString)
+
 data BackendA a = Backend
 	{ backendVariety :: KeyVariety
 	, genKey :: Maybe (KeySource -> MeterUpdate -> a Key)
-	-- Verifies the content of a key using a hash. This does not need
-	-- to be cryptographically secure.
+	-- Verifies the content of a key, stored in a file, using a hash.
+	-- This does not need to be cryptographically secure.
 	, verifyKeyContent :: Maybe (Key -> RawFilePath -> a Bool)
+	-- Incrementally verifies the content of a key, using the same
+	-- hash as verifyKeyContent, but with the content provided
+	-- incrementally a peice at a time, until finalized.
+	, verifyKeyContentIncrementally :: Maybe (Key -> a IncrementalVerifier)
 	-- Checks if a key can be upgraded to a better form.
 	, canUpgradeKey :: Maybe (Key -> Bool)
 	-- Checks if there is a fast way to migrate a key to a different
@@ -38,3 +43,11 @@ instance Show (BackendA a) where
 
 instance Eq (BackendA a) where
 	a == b = backendVariety a == backendVariety b
+
+data IncrementalVerifier = IncrementalVerifier
+	{ updateIncremental :: ByteString -> IO ()
+	-- ^ Called repeatedly on each peice of the content.
+	, finalizeIncremental :: IO Bool
+	-- ^ Called once the full content has been sent, returns true
+	-- if the hash verified.
+	}