refactor

That code was not borg specific, and I can see making more remotes for
other backup software.

Remote/Helper/ThirdParty.hs

@@ -0,0 +1,56 @@
+{- Helpers for thirdPartyPopulated remotes
+ -
+ - Copyright 2020 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU AGPL version 3 or higher.
+ -}
+
+module Remote.Helper.ThirdParty where
+
+import Annex.Common
+import Types.Remote
+import Types.Import
+import Crypto (isEncKey)
+import Utility.Metered
+
+import qualified System.FilePath.ByteString as P
+
+-- When a remote is thirdPartyPopulated, and contains a backup of a
+-- git-annex repository or some special remotes, this can be used to
+-- find only those ImportLocations that are annex object files.
+-- All other ImportLocations are ignored.
+importKey :: ImportLocation -> ContentIdentifier -> ByteSize -> MeterUpdate -> Annex (Maybe Key)
+importKey loc _cid sz _ = return $ case deserializeKey' f of
+	Just k
+		-- Annex objects always are in a subdirectory with the same
+		-- name as the filename. If this is not the case for the file
+		-- that was backed up, it is probably not a valid annex object.
+		-- Eg, it could be something in annex/bad/, or annex/tmp/.
+		-- Or it could be a file that only happens to have a name
+		-- like an annex object.
+		-- (This does unfortunately prevent recognizing files that are
+		-- part of special remotes that don't use that layout. The most
+		-- likely special remote to be in a backup, the directory
+		-- special remote, does use that layout at least.)
+		| lastMaybe (P.splitDirectories (P.dropFileName p)) /= Just f -> Nothing
+		-- Chunked or encrypted keys used in special remotes are not
+		-- supported.
+		| isChunkKey k || isEncKey k -> Nothing
+		-- Check that the size of the key is the same as the size of the
+		-- file stored in the backup. This is a cheap way to make sure it's
+		-- probabably the actual content of the file. We don't fully
+		-- verify the content here because that could be a very 
+		-- expensive operation for a large repository; if the user
+		-- wants to detect every possible data corruption problem
+		-- (eg, wrong data read off disk during backup, or the object
+		-- was corrupt in the git-annex repo and that bad object got
+		-- backed up), they can fsck the remote.
+		| otherwise -> case fromKey keySize k of
+			Just sz'
+				| sz' == sz -> Just k
+				| otherwise -> Nothing
+			Nothing -> Just k
+	Nothing -> Nothing
+  where
+	p = fromImportLocation loc
+	f = P.takeFileName p