migrate: Support migrating v1 SHA keys to v2 SHA keys with size information that can be used for free space checking.

Backend/File.hs

@@ -29,6 +29,7 @@ import Types
 import UUID
 import Messages
 import Trust
+import Key
 
 backend :: Backend Annex
 backend = Backend {
@@ -38,7 +39,8 @@ backend = Backend {
 	retrieveKeyFile = copyKeyFile,
 	removeKey = checkRemoveKey,
 	hasKey = inAnnex,
-	fsckKey = checkKeyOnly
+	fsckKey = checkKeyOnly,
+	upgradableKey = checkUpgradableKey
 }
 
 mustProvide :: a
@@ -159,6 +161,12 @@ getNumCopies Nothing = do
 	where
 		config = "annex.numcopies"
 
+{- Ideally, all keys have file size metadata. Old keys may not. -}
+checkUpgradableKey :: Key -> Annex Bool
+checkUpgradableKey key
+	| keySize key == Nothing = return True
+	| otherwise = return False
+
 {- This is used to check that numcopies is satisfied for the key on fsck.
  - This trusts data in the the location log, and so can check all keys, even
  - those with data not present in the current annex.

Backend/URL.hs

@@ -30,7 +30,9 @@ backend = Backend {
 	-- similarly, keys are always assumed to be out there on the web
 	hasKey = dummyOk,
 	-- and nothing needed to fsck
-	fsckKey = dummyFsck
+	fsckKey = dummyFsck,
+	-- and key upgrade not needed
+	upgradableKey = \_ -> return False
 }
 
 -- cannot generate url from filename