handle transitions with read-only unmerged git-annex branches

Capstone to this feature. Any transitions that have been performed on an
unmerged remote ref but not on the local git-annex branch, or vice-versa
have to be applied on the fly when reading files.

Sponsored-by: Dartmouth College's Datalad project

Types/BranchState.hs

@@ -9,6 +9,7 @@ module Types.BranchState where
 
 import Common
 import qualified Git
+import Types.Transitions
 
 import qualified Data.ByteString.Lazy as L
 
@@ -25,6 +26,9 @@ data BranchState = BranchState
 	-- ^ when the branch was not able to be updated due to permissions,
 	-- these other git refs contain unmerged information and need to be
 	-- queried, along with the index and the journal.
+	, unhandledTransitions :: [TransitionCalculator]
+	-- ^ when the branch was not able to be updated due to permissions,
+	-- this is transitions that need to be applied when making queries.
 	, cachedFileContents :: [(RawFilePath, L.ByteString)]
 	-- ^ contents of a few files recently read from the branch
 	, needInteractiveAccess :: Bool
@@ -35,4 +39,4 @@ data BranchState = BranchState
 	}
 
 startBranchState :: BranchState
-startBranchState = BranchState False False False [] [] False
+startBranchState = BranchState False False False [] [] [] False

Types/Transitions.hs

@@ -0,0 +1,19 @@
+{- git-annex transitions data types
+ -
+ - Copyright 2021 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU AGPL version 3 or higher.
+ -}
+
+module Types.Transitions where
+
+import Utility.RawFilePath
+
+import qualified Data.ByteString.Lazy as L
+import Data.ByteString.Builder
+
+data FileTransition
+	= ChangeFile Builder
+	| PreserveFile
+
+type TransitionCalculator = RawFilePath -> L.ByteString -> FileTransition